ConspectusAdaptable crystalline frameworks are important in modern solid-state chemistry as they are able to accommodate a wide range of elements, oxidation states, and stoichiometries. Owing to this ability, such adaptable framework structures are emerging as the prototypes for technologically important advanced functional materials. In this Account, the idea of cosubstitution is explored as a useful “pairing” concept that can potentially lead to the creation of many new members of one particular framework structure. Cosubstitution as practiced is the simultaneous replacement of two or more cations, anions, complex anions, other fundamental building units, or vacancies. Although the overall sum of the oxidation states is constant, each component is not necessarily isovalent. This methodology is typically inspired by either mineral-type structural prototypes found in nature or those discovered in the laboratory. Either path leads to the appearance of new phases and the discovery of new materials. In addition, the chemical cosubstitution approach can be successfully adopted to improve physical properties associated with structures.This Account is structured as follows: first, we illustrate the significance and background of chemical cosubstitution by reviewing mineral-inspired structures, such as perovskite and lyonsite, and the structural unit discovered in some selected solid state compounds. With time, the number of lyonsite related phases should rival or even surpass the perovskite family. Several members of the lyonsite-type have been identified as Li-ion conductors and photocatalysts. There is also a noncentrosymmetric structure-type, and therefore the other properties associated with the loss of inversion symmetry should be anticipated. Next, we illustrate recent advances in the synthesis of the new cosubstituted solid state materials from our two groups including (1) nonlinear optical materials, (2) luminescent materials, (3) transparent conducting oxides, and (4) photocatalyst and photovoltaic materials. We emphasize that a concerted and rigorous theoretical and experimental approach will be required to define thermodynamic stability of the complex cosubstitution chemistries, structures, and properties that are yet unknown. We conclude by summarizing the topic and suggesting other possible adaptable framework structures where cosubstitution can be expected.

Local structure modification in solid solution is an essential part of photoluminescence tuning of rare earth doped solid state phosphors. Herein we report a new solid solution phosphor of Eu2+-doped xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7 (0 ≤ x ≤ 1), which share the same β-Ca3(PO4)2 type structure in the full composition range. Depending on the x parameter variation in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+, the vacancies generated in the M(4) site enable the nonlinear variation of cell parameters and volume, and this increases the magnitude of M(4)O6 polyhedra distortion. The local structure modulation around the Eu2+ ions causes different luminescent behaviors of the two-peak emission and induces the photoluminescence tuning. The shift of the emission peaks in the solid solution phosphors with different compositions has been discussed. It remains invariable at x ≤ 0.5, but the red-shift is observed at x > 0.5 which is attributed to combined effect of the crystal field splitting, Stokes shift, and energy transfer between Eu2+ ions. The temperature-dependent luminescence measurements are also performed, and it is shown that the photoionization process is responsible for the quenching effect.

The union of temperature-dependent phase transition and relating structural transformation via modification of chemical compositions is of fundamental importance for the discovery of new materials or their functional properties optimization. Herein, the synthesis temperature and Eu2+-doping content induced phase selection and variations of the local structures in nepheline, low-carnegieite and high-carnegieite types of NaAlSiO4 polymorphs were studied in detail. The luminescence of Eu2+ in low-carnegieite and nepheline phases shows blue (460 nm) and yellow (540 nm) broad-band emissions, respectively, under near-ultraviolet excitation. The photoluminescence evolution can be triggered by the different synthesis temperatures in relation to the Eu2+-doping concentration, as corroborated by density functional theory calculations on the local coordination structures and corresponding mechanical stabilities in terms of the Debye temperature. The fabricated white light-emitting diode device with high color rendering index demonstrates that the multicolor phosphors from one system provides a new gateway for the photoluminescence tuning.

A strategy to design step by step an inorganic single-doped white phosphor is demonstrated. The method consists in tuning different contributions of the emission by successively controlling the chemical compositions of the solid solution or nanosegregated host matrix and the oxidation states of the single dopant. We use this approach to design a white phosphor Na4CaMgSc4Si10O30:Eu with excellent color rendering (CRI > 90) that is similar to common mixed-phosphor light sources but for a single-phase. We show that this methodology can also be extended to other phosphors for use in diverse applications such as biomedicine or telecommunications.

The discovery of lead-free double perovskites provides a feasible way of searching for air-stable and environmentally benign solar cell absorbers. Herein we report the design and hydrothermal crystal growth of double perovskite Cs2AgInCl6. The crystal structure, morphology related to the crystal growth habit, band structure, optical properties, and stability are investigated in detail. This perovskite crystallized in a cubic unit cell with the space group Fm3m and is composed of [AgCl6] and [InCl6] octahedra alternating in a ordered rock-salt structure, and the as-obtained crystal size is dependent on the hydrothermal reaction time. Cs2AgInCl6 is a direct gap semiconductor with a wide band gap of 3.23 eV obtained experimentally and 3.33 eV obtained by DFT calculation. This theoretically predicted and experimentally confirmed optical gap is a prototype of the band gaps that are direct and optically allowed except at the single high-symmetry k-point, which didn't raise interest before but have potential applications in future technologies. Cs2AgInCl6 material with excellent moisture, light and heat stability shows great potential for photovoltaic and other optoelectronic applications via further band gap engineering.

A design principle for the chemical unit cosubstitution of [Al3+–F−] for [Si4+–O2−] has been adopted to establish the phase relations between isostructural (Sr,Ba)3SiO5 and (Sr,Ba)3AlO4F, and solid solutions of (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ with tunable luminescence properties have been prepared based on the comparative “one-step” and “two-step” methods. The structures and photoluminescence tuning of the (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ phosphors were investigated, and it was found that enhanced emission intensities could be obtained using the two-step method with high crystalline quality. The emission peaks of the (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ phosphors were blue-shifted from 520 to 490 nm upon increasing the content from x = 0 to x = 1.0, and the corresponding mechanism has been discussed. The results of the temperature-dependent luminescence properties further verified that the formation of a solid solution can improve the thermal stability of the photoluminescence. White light-emitting diode (w-LED) lamps were fabricated based on the as-prepared bluish-yellow emitting phosphors, and commercial red phosphors, combined with 415 nm NUV-emitting InGaN chips. The best packaged w-LED lamp gave CIE chromaticity coordinates of (0.38, 0.42) with a warm color temperature of 4277 K and a color rendering index of 86.3.

The development of new luminescent materials for anticounterfeiting is of great importance, owing to their unique physical, chemical, and optical properties. The authors report the use of color-tunable colloidal CdS/ZnS/ZnS:Mn2+/ZnS core/multishell quantum dots (QDs)-functionalized luminescent polydimethylsiloxane film (LPF) for anticounterfeiting applications. Both luminescent QDs and as-fabricated, stretchable, and transparent LPF show blue and orange emission simultaneously, which are ascribed to CdS band-edge emission and the 4T1 6A1 transition of Mn2+, respectively; their emission intensity ratios are dependent on the power-density of a single-wavelength excitation source. Additionally, photoluminescence tuning of CdS/ZnS/ZnS:Mn2+/ZnS QDs in hexane or embedded in LPF can also be realized under fixed excitation power due to a resonance energy transfer effect. Tunable photoluminescence of these flexible LPF grafted doped core/shell QDs can be finely controlled and easily realized, depending on outer excitation power and intrinsic QD concentration, which is intriguing and inspires the fabrication of many novel applications.

The emission colors of cesium lead halide perovskite nanocrystals (NCs) can be controlled by dynamic ion exchange and Mn2+ doping. Herein we report a novel strategy for the synthesis of Cs(PbxMn1−x)(ClyBr1−y)3 NCs via a post-synthetic cation–anion cosubstitution exchange reaction to tailor their optical properties in a wide range. Colloidal CsPbBr3 and CsPb1−xMnxCl3 NCs are firstly prepared, separately, and then mixed together in hexane solution. The Pb2+ and Br− ions in the CsPbBr3 NCs are simultaneously exchanged for the Mn2+ and Cl− ions in the CsPb1−xMnxCl3 NCs, resulting in homogeneous Cs(PbxMn1−x)(ClyBr1−y)3, with preservation of the shape and crystal structure of the initial NCs. This in situ ion exchange method avoids the degradation of photoluminescence originating from the additional purification process of the NCs, and in situ Mn2+ substitution also greatly enhances the emission intensity and quantum yield of the as-obtained NCs, which is found to be related to the bound exciton effect.

Thermal quenching is one of the most crucial challenges for photoluminescence and high-power applications, in which the phosphor suffers from emission loss due to non-radiative transitions with increasing temperature. Herein, we report new isostructural solid solution phosphors, Eu2+-doped KxCs1−xAlSi2O6 (0 ≤ x ≤ 0.6), with abnormal thermal quenching behavior, and their asymmetric emission spectra and decay curves for two emission centers verified the presence of two kinds of Eu2+ centers (Eu1 and Eu2) in the lattices. The maxima of both the Eu1 and Eu2 bands exhibit a blue-shift and the emission color was tuned from blue-green with color coordinates of (0.222, 0.324) to bright blue with color coordinates of (0.172, 0.132) upon 365 nm UV excitation as a function of x. These phosphors do not exhibit thermal quenching at high temperature, and this behavior is proposed to relate to the energy transfer from defect levels to the Eu2+ 5d band as also verified by thermoluminescence measurements, and the result demonstrated that the reported KxCs1−xAlSi2O6:Eu2+ phosphors may be candidates for blue-green components in UV-pumped high power white light emitting diodes (WLEDs).

Garnets have the general formula of A3B2C3O12 and form a wide range of inorganic compounds, occurring both naturally (gemstones) and synthetically. Their physical and chemical properties are closely related to the structure and composition. In particular, Ce3+-doped garnet phosphors have a long history and are widely applied, ranging from flying spot cameras, lasers and phosphors in fluorescent tubes to more recent applications in white light LEDs, as afterglow materials and scintillators for medical imaging. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sublattice. The flexibility in phosphor composition and the tunable luminescence properties rely on design and synthesis strategies for new garnet compositions with tailor-made luminescence properties. It is the aim of this review to discuss the variation in luminescence properties of Ce3+-doped garnet materials in relation to the applications. This review will provide insight into the relation between crystal chemistry and luminescence for the important class of Ce3+-doped garnet phosphors. It will summarize previous research on the structural design and optical properties of garnet phosphors and also discuss future research opportunities in this field.

Phosphor materials enable the optical frequency conversion to realize the full-color white emission light-emitting diodes (LEDs). So far much effort has been devoted to the design and discovery of novel LED phosphors for solid state lighting. In this review, firstly, we briefly describe several representative families of LED phosphors. Secondly, we propose the design methodology aimed at discovery of new phosphors with focus on the crystal structural considerations. Thirdly, we review the results of our work and other researchers on the recent advances in discovery and structural design of LED phosphors that exemplify the adopted strategies, including (1) design of the novel phosphors from the existed structural models, (2) discovery of novel phosphors from new crystal materials by doping and (3) structural modification of the known phosphors. The importance on the structure-property relations and recently reported methodologies involved in the crystal chemistry analysis for the discovery of LED phosphors, including mineral-inspired structural model design, exploratory crystal growth via single particle diagnostic approach, chemical unit cosubstitution, and so on, have been summarized in this review. We finally discuss the topics of structure-related active investigations and future opportunities for new and improved host materials for the color conversion applied in LEDs.

Controlled photoluminescence tuning is important for the optimization and modification of phosphor materials. Herein we report an isostructural solid solution of (CaMg)x(NaSc)1–xSi2O6 (0 < x < 1) in which cation nanosegregation leads to the presence of two dilute Eu2+ centers. The distinct nanodomains of isostructural (CaMg)Si2O6 and (NaSc)Si2O6 contain a proportional number of Eu2+ ions with unique, independent spectroscopic signatures. Density functional theory calculations provided a theoretical understanding of the nanosegregation and indicated that the homogeneous solid solution is energetically unstable. It is shown that nanosegregation allows predictive control of color rendering and therefore provides a new method of phosphor development.

This paper reports the development of new phosphors using the chemical unit cosubstituting solid solution design strategy. Starting from Lu3Al5O12, the Al3+–Al3+ couple in respective octahedral and tetrahedral coordination was simultaneously substituted by a Mg2+–Si4+ pair forming the Lu3(Al2−xMgx)(Al3−xSix)O12:Ce3+ (x = 0.5–2.0) series; as a result, the CeO8 polyhedrons were compressed and the emission got red-shifted from green to yellow together with the broadening. The evolution of, the unit cell, the local structural geometry as well as the optical properties of Ce3+ in these garnet creations, in response to the gradual Mg–Si substitution for Al–Al, were studied by combined techniques of structural refinement and luminescence measurements. The new composition Lu2.97Ce0.03Mg0.5Al4Si0.5O12 was comprehensively evaluated regarding its potential application in blue LED-driven solid state white lighting: the maximum emission is at 550 nm under λex = 450 nm; the internal and external quantum efficiencies can reach 85% and 49%, respectively; a 1-phosphor-converted wLED lamp fabricated using the as-prepared phosphor exhibits the luminous efficacy of 105 lm W−1, the correlated color temperature of 6164 K and the color rendering index (Ra) of 75.6. The new solid solution composition series is open for further optimization to enhance the competence for commercial consideration.

The design scheme of the chemical unit cosubstitution of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ has been put into practice to discover the new phosphor systems with tunable luminescence properties, and the structures and photoluminescence tuning of yellow-emitting LuxSr2−xSiNxO4−x:Eu2+ phosphors have been investigated. Crystal structures of LuxSr2−x−ySiNxO4−x:yEu2+ samples were resolved using the Rietveld method, suggesting that the as-prepared Sr2SiO4 belonged to monoclinic symmetry (P21/n) of β-phase Sr2SiO4, while Sr1.97Eu0.03SiO4 and Sr1.965Eu0.03Lu0.005SiO3.995N0.005 belonged to orthorhombic symmetry (Pnma) of α-Sr2SiO4. The emission peaks of LuxSr1.97−xSiNxO4−x:0.03Eu2+ phosphors were red-shifted from 563 to 583 nm upon increasing the [Lu3+–N3−] substitution content from x = 0 to x = 0.005, furthermore, the PL emission peaks of Lu0.005Sr1.965−ySiN0.005O3.995:yEu2+ also showed a red-shift from 583 nm to 595 nm with increasing Eu2+ concentration (y = 0.03, 0.07, 0.10 and 0.15), and their corresponding red-shift mechanism has been discussed. The temperature dependent luminescence results further verified that the introduction of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ can improve the thermal stability of the photoluminescence, which indicated that the LuxSr2−x−ySiNxO4−x:yEu2+ phosphors have potential applications in white light-emitting diodes (wLEDs).

This paper demonstrates a facile hydrothermal method using ethylenediamine (EDA) as a “shape modifier” for the controlled synthesis of rod bunch, decanedron, spindle, flakiness, and flowerlike NaCaSiO3OH microarchitectures. The set of experimental conditions is important to obtain adjustable shape and size of NaCaSiO3OH particles, as the change in either the amount of EDA/H2O or reaction time, or the amount of NaOH. Accordingly, the crystal growth mechanism during the synthesis process is proposed, and it is found that the EDA, acting as the chelating agent and shape modifier, plays a crucial role in fine-tuning the NaCaSiO3OH morphology. Morphology evolution process of flowerlike NaCaSiO3OH as a function of NaOH is also explained in detail. Eu3+/Tb3+ doped NaCaSiO3OH samples exhibit strong red and green emission under ultraviolet excitation, corresponding to the characteristic electronic transitions of Eu3+ and Tb3+. These results imply that the morphology-tunable NaCaSiO3OH:Eu3+/Tb3+ microarchitectures with tunable luminescence properties are expected to have promising applications for micro/nano optical functional devices.

Stable and efficient phosphor systems for white light-emitting diodes (LEDs) are highly important with respect to their application in solid-state lighting beyond the technical limitations of traditional lighting technologies. Therefore, inorganic solid-state conversion phosphors must be precisely selected and evaluated with regard to their special material properties and synergistic optical parameters. In this perspective, we present an overview of the recent developments of LED phosphors; firstly, general photoluminescence-controlling strategies for phosphors to match LED applications have been evaluated; secondly, state-of-the-art and emerging new LED phosphors have been demonstrated. Then, methodologies for the discovery of new LED phosphors by mineral-inspired prototype evolution and new phase construction, as well as combinatorial optimization screening, and the single-particle-diagnosis approach, have been analyzed and exemplified. Finally, future developments of LED phosphors have been proposed.

We report on the phase formation of a Ba2(Gd,Tb)2Si4O13 solid-solution, and the coexistence of Eu2+/Eu3+ was identified after Eu ion doping although the samples were prepared in a reducing atmosphere. Under 377 nm near-ultraviolet (UV) light excitation, Ba2Tb2Si4O13 exhibits the characteristic emission originating from Tb3+ corresponding to 5D4–7F6,5,4,3 transitions; whereas Ba2Tb2Si4O13:Eu emits bright red emission from Eu3+ with peaks around 594, 613 and 623 nm. Accordingly, photoluminescence tuning of Eu-doped Ba2(Gd,Tb)2Si4O13 phosphors has been realized from green, yellow, orange, to red emission light. Decay time and time-resolved luminescence results revealed that the tunable luminescence behavior should be ascribed to the existence of energy migration in the terbium subset, and successive energy transfer processes Eu2+–Eu3+(Tb3+) and Tb3+–Eu3+ appear to occur in the Ba2Tb2−ySi4O13:yEu (y = 0–0.12) solid-solution phosphors under investigation.

A series of color-tunable and white light emitting phosphors BaY2Si3O10:Tm3+,Dy3+ were synthesized by a high temperature solid-state reaction, and their phase structure, photoluminescence properties, and energy transfer processes between rare-earth ions were investigated in detail. Upon UV excitation, white light emission depending on dopant concentrations could be achieved by integrating a blue emission band located at 458 nm and an orange one located at 576 nm attributed to Tm3+ and Dy3+ ions, respectively. In addition, the energy transfer process between Tm3+ and Dy3+ ions was demonstrated to be a resonant type via a dipole–quadrupole mechanism. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for UV chip pumped white-light LEDs.

It was recently reported that Ce-doped Ca6BaP4O17 displayed blue-green emission under excitation in the near-ultraviolet (UV) region and that luminescence intensities can be greatly improved by codoping with Si. Here, a combination of hybrid density functional theory (DFT) and wave function-based CASSCF/CASPT2 calculations at the spin–orbit level has been performed on geometric and electronic structures of the material to gain insights into effects of Si codoping on its optical properties. It is found that the observed luminescence arises from 4f–5d transitions of Ce3+ occupying the two crystallograhically distinct Ca1 and Ca2 sites of the host compound with comparable probabilities, with the energy of the lowest 4f → 5d transition of CeCa1 being slightly higher than that of Ceca2. The codopant Si prefers to substitute for the nearest-neighbor (NN) P1 atom over the NN P2 atom around Ce3+, and this preference induces a blueshift of the lowest-energy 4f → 5d transition, consistent with experimental observations. The blueshift originates from a reduction in 5d crystal field splitting of Ce3+ associated mainly with electronic effects of the NN SiP1 substitution, while the contribution from the change in 5d centroid energy is negligible. On the basis of calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Ca2+ sites of Ca6BaP4O17 is constructed and discussed in connection with experimental findings.

Controlled synthesis of absorber materials Cu2ZnGeS4 (CZGS) has been performed using different Ge precursors, including GeCl4 and the self-synthesized Ge complexes with Ge-glycolic acid (denoted as Ge-Gly), Ge-tartaric acid (denoted as Ge-Tar), and Ge-citric acid (denoted as Ge-Cit). The grain size of as-prepared CZGS nanocrystals (NCs) is dependent on the Ge precursors. All four Ge precursors enabled the wurtzstannite CZGS phase formation. The Ge-Cit precursor led to the formation of monodispersed NCs owing to the fact that the undissolved metal-Cit complex in OLA absorbed the small CZSG NCs and avoided the irregular crystalline behavior. The other three precursors induced two different sizes, and the corresponding reaction mechanism has been proposed. Moreover, the Cu2ZnGe1–xSnxS4 NCs with different Ge/Sn ratios were prepared using the Ge-Cit precursor, verifying the general effect on the phase formation and selective grain sizes. The compositional effect on the band gap variation and morphologies of Cu2ZnGe1–xSnxS4 was also studied.

The union of structural and spectroscopic modeling can accelerate the discovery and improvement of phosphor materials if guided by an appropriate principle. Herein, we describe the concept of “chemical unit cosubstitution” as one such potential design scheme. We corroborate this strategy experimentally and computationally by applying it to the Ca2(Al1–xMgx)(Al1–xSi1+x)O7:Eu2+ solid solution phosphor. The cosubstitution is shown to be restricted to tetrahedral sites, which enables the tuning of luminescent properties. The emission peaks shift from 513 to 538 nm with a decreasing Stokes shift, which has been simulated by a crystal-field model. The correlation between the 5d crystal-field splitting of Eu2+ ions and the local geometry structure of the substituted sites is also revealed. Moreover, an energy decrease of the electron–phonon coupling effect is explained on the basis of the configurational coordinate model.

A series of single-component blue-, white- and orange-emitting phosphors have been developed based on the BaY2Si3O10 host via the Ce3+/Eu3+, Tb3+/Eu3+, Ce3+/Tb3+, and Ce3+/Tb3+/Eu3+ couples and the energy transfer among them has been discussed in detail. A terbium bridge model via Ce3+–Tb3+–Eu3+ energy transfer has been studied and verified. The phase structures, photoluminescence (PL) properties, decay curves, PL thermal stability, and the energy transfer mechanism were investigated in this paper. The emission colors of the phosphors can be tuned from blue (0.1501, 0.0805) to white (0.3230, 0.3140) and eventually to orange (0.4496, 0.3537) through the corresponding Ce3+–Tb3+–Eu3+ energy transfer. Moreover, thermal quenching luminescence results reveal that BaY2Si3O10:Ce3+,Tb3+,Eu3+ exhibits good thermal stability. The above results indicate that the phosphors may be potentially used as single-component multi-color or white emission phosphors for application in white light emitting diodes (w-LEDs).

A series of yellow-emitting La0.975−xGdxSr2AlO5:0.025Ce3+ phosphors have been synthesized by a solid-state reaction. All the samples retain the same tetragonal crystal structure, and the phosphors emit yellow emission with a peak maximum shifting from 540 nm to 556 nm, depending on the Gd/La ratio, when excited by 440 nm blue light. Their temperature dependent photoluminescence, fluorescence decay curves, and CIE values were also discussed. With increasing Gd/La ratio, the red-shift behaviors can be ascribed to the enhancing covalence, and the emission intensities first increased, then decreased, and finally increased to the maximum at x = 0.6. A white light-emitting diode (w-LED) lamp was fabricated based on the selected yellow-emitting La0.375Gd0.6Sr2AlO5:0.025Ce3+ phosphors combined with a 460 nm blue-emitting InGaN chip. The packaged w-LED lamp gave CIE chromaticity coordinates of (0.3562, 0.3660) with a warm color temperature of 4664 K and a color rendering index of 83.

Blue-emitting CaSrSiO4:Ce3+,Li+ phosphors were prepared by a high temperature solid-state method, and the effect of substituting Al3+ for Si4+ in CaSrSiO4:Ce3+,Li+ has been studied. Crystal structures of the as-prepared Ca1−ySr1−ySi1−xAlxO4:yCe3+,yLi+ phosphors were resolved by the Rietveld method, which suggested that all the samples belonged to the orthorhombic symmetry (Pnma) group of α-CaSrSiO4. The photoluminescence (PL) emission and excitation spectra, the lifetime, and the effect of Al3+ concentration on the PL properties were investigated in detail. The emission peaks of the CaSrSi1−xAlxO4:Ce3+,Li+ (x = 0–0.10) phosphors were red-shifted from 452 to 472 nm with increasing Al/Si ratio. The red-shift of the Ce3+ emission is ascribed to the polyhedra distortion of the cations, originating from the variation in the neighboring [(Si,Al)O4] polyhedra, and the detailed mechanism has been discussed.

A series of Ca6−x−yBa(PO4)4−x−y(SiO4)x+yO:xCe3+,yTb3+ phosphors was synthesized via a high-temperature solid-state method. Charge balance was realized via the collaborative substitution of a Ca2+/Ce3+(Tb3+)–P5+/Si4+ couple with the pure phase. On excitation at 365 nm, the emission spectra of the as-obtained phosphor contained both the asymmetrical broad band Ce3+ emissions and the line-type Tb3+ emissions. Energy transfer occurred from Ce3+ to Tb3+ and the mechanism was ascribed to a resonant type via a non-radiative dipole–dipole interaction. The critical distance of the energy transfer was calculated via the concentration quenching method. The hues of the studied phosphors could be adjusted by the relative ratios of the Ce3+ and Tb3+ ions. The phosphors have good thermal stability and a high quantum efficiency and therefore may have potential applications in near-UV pumped white light-emitting diodes.

Ce3+/Li+,Eu2+ singly doped and Ce3+/Li+/Eu2+-co-doped CaSrAl2SiO7 phosphors have been prepared using the conventional solid-state reaction method. The crystal structure of the melilite-type CaSrAl2SiO7 phase and the preferred crystallographic positions of the doped ions were refined using the Rietveld method. The luminescence properties and energy transfer of CaSrAl2SiO7:Ce3+,Li+,Eu2+ were studied in detail. The Ce3+/Li+ activated CaSrAl2SiO7 phosphor has a strong absorption band in the range of 200–450 nm and shows a blue emission centered at 477 nm. When Eu2+ ions are co-doped with Ce3+/Li+, the emission color of CaSrAl2SiO7:Ce3+,Li+,Eu2+ phosphors under the irradiation of 365 nm can be tuned from blue to green via the energy transfer from Ce3+ to Eu2+ ions. Also the involved energy transfer process and the corresponding mechanism between Ce3+ and Eu2+ have been discussed in detail. These results indicate that the as-reported CaSrAl2SiO7:Ce3+,Li+,Eu2+ phosphors have potential applications in near-UV chip pumped white LEDs.

The versatile polymorphism and chemical compositions of barium silicates have been studied for a long time and their crystal structures have been established. Herein, we focused on the understanding of the crystal structure of the Ba4Si6O16 phase and the structural correlation of Ba4Si6O16 and Ba2Si3O8; moreover, the luminescence properties of Ce3+,Eu2+-co-activated Ba4Si6O16 phosphors have been discussed. Ba4Si6O16:Ce3+,Eu2+ phosphors show tunable blue-green emission upon excitation with 365 nm ultraviolet (UV) light. The blue emission originates from Ce3+, whereas the bluish-green emission is ascribed to Eu2+, and variation in the emission peak wavelength from 442 to 497 nm can be achieved by properly tuning the Ce3+/Eu2+ ratio. Energy transfer from Ce3+ to Eu2+ in the Ba4Si6O16 host has been validated by the variation of emission spectra as well as the variation of Ce3+ decay lifetimes with increasing Eu2+ concentration, and the energy transfer mechanism is demonstrated to be a resonant type via a dipole–dipole process. The results suggest that Ba4Si6O16:Ce3+,Eu2+ phosphors are potential candidates as a blue-green component for UV-excited white light-emitting diodes.

The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 107 Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals ∼19 μm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λex = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.Keywords: garnet; luminescence; phosphor; pressure; structure; synthesis;

The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure–property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca2–xSrxSiO4:Ce3+ phosphor by Sr2+ substituting for Ca2+ within 0 ≤ x < 2. The crystal structures of Ca2–xSrxSiO4:Ce3+ are divided into two groups, namely, β phase (0 ≤ x < 0.15) and α′ phase (0.18 ≤ x < 2), and the phase transition (β → α′) mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition Pnma (α′-phase) ↔ P21/n (β-phase) can be ascribed to the second-order type, and Sr2+ ions in Ca2–xSrxSiO4 preferentially occupy the seven-coordinated Ca2+ sites rather than the eight-coordinated sites with increasing Sr2+ content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca–O bond length in the two polymorphs of Ca2SiO4. The emission peaks of Ce3+ shift from 417 to 433 nm in the composition range of 0 ≤ x ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca2–xSrxSiO4:Ce3+ samples were evaluated, and the results show that the integral emission intensities at 200 °C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors.

Cr3+-activated mullite-type Bi2Ga(4-x)AlxO9 (x = 0, 1, 2, 3, and 4) solid solutions were prepared by the solid state reaction, and their spectroscopic properties were investigated in conjunction with the structural evolution. Under excitation at 610 nm, Bi2[Ga(4-y)Aly]3.97O9:0.03Cr3+ (y = 0, 1, 2, 3, and 4) phosphors exhibited broad-band near-infrared (NIR) emission peaking at ∼710 nm in the range 650–850 nm, and the optimum Cr3+ concentrations and concentration quenching mechanism were determined. Except for the interesting NIR emission, the body color changed from white (at x = 0) to green (at x = 0.08) for Bi2Ga4–xO9:xCr3+, and from light yellow (at x = 0) to deep brown (at x = 0.08) for Bi2Al4–xO9:xCr3+, respectively. Moreover, as a result of variable Al/Ga ratio, the observed body color for Bi2[Ga(4-y)Aly]3.97O9:0.03Cr3+ (y = 0, 1, 2, 3, and 4) varied from deep brown to green. The relationship between the observed colors and their diffuse reflectance spectra were also studied for the understanding of the different absorption bands. The results indicated that Cr3+-doped Bi2Ga(4-x)AlxO9 solid solutions appeared as the bifunctional materials with NIR phosphors and color-tunable pigments.

Eu2+ and Ce3+/Li+ singly doped and Eu2+/Ce3+/Li+-codoped Ca1.65Sr0.35SiO4 phosphors have been synthesized by a solid-state reaction method. The crystal structure was determined by Rietveld refinement to verify the formation of the αL′-Ca2SiO4 phase with the Sr addition into Ca2SiO4, and the preferred crystallographic positions of the Eu2+ and Ce3+/Li+ ions in Ca1.65Sr0.35SiO4 were analyzed based on a comparison of the unit-cell volumes and the designed chemical compositions of undoped isostructural compounds Ca(2–x)SrxSiO4 (x = 0.25, 0.35, 0.45, 0.55 and 0.65). Ce3+/Li+ singly activated Ca1.65Sr0.35SiO4 phosphors exhibit strong absorption in the range of 250–450 nm and a blue emission peak centered at about 465 nm. When Eu2+ ions are codoped, the emission colors of Ca1.65Sr0.35SiO4:Ce3+,Li+,Eu2+ phosphors under the irradiation of 365 nm can be finely tuned from blue to green through the energy transfer from Ce3+ to Eu2+. The involved energy-transfer process between Ce3+ and Eu2+ and the corresponding mechanism are discussed in detail. The reported Ca1.65Sr0.35SiO4:Ce3+,Li+,Eu2+ phosphor might be a candidate for color-tunable blue-green components in the fabrication of near-ultraviolet-pumped white-light-emitting diodes (WLEDs).

Multifunctional materials based on rare earth ion doped ferro/piezoelectrics have attracted considerable attention in recent years. In this work, new lead-free multifunctional ceramics of Ca1−x(LiHo)x/2Bi4Ti4O15 were prepared by a conventional solid-state reaction method. The great multi-improvement in ferroelectricity/piezoelectricity, down/up-conversion luminescence and temperature stability of the multifunctional properties is induced by the partial substitution of (Li0.5Ho0.5)2+ for Ca2+ ions in CaBi4Ti4O15. All the ceramics possess a bismuth-layer structure, and the crystal structure of the ceramics is changed from a four layered bismuth-layer structure to a three-layered structure with the level of (Li0.5Ho0.5)2+ increasing. The ceramic with x = 0.1 exhibits simultaneously, high resistivity (R = 4.51 × 1011 Ω cm), good piezoelectricity (d33 = 10.2 pC N−1), high Curie temperature (TC = 814 °C), strong ferroelectricity (Pr = 9.03 μC cm−2) and enhanced luminescence. These behaviours are greatly associated with the contribution of (Li0.5Ho0.5)2+ in the ceramics. Under the excitation of 451 nm light, the ceramic with x = 0.1 exhibits a strong green emission peak centered at 545 nm, corresponding to the transition of the 5S2 → 5I8 level in Ho3+ ions, while a strong red up-conversion emission band located at 660 nm is observed under the near-infrared excitation of 980 nm at room temperature, arising from the transition of 5F5 → 5I8 levels in Ho3+ ions. Surprisingly, the excellent temperature stability of ferroelectricity/piezoelectricity/luminescence and superior water-resistance behaviors of piezoelectricity/luminescence are also obtained in the ceramic with x = 0.1. Our study suggests that the present ceramics may have potential applications in advanced multifunctional devices at high temperature.

A series of double molybdate scheelite-type phosphors LixAg1−xYb0.99(MoO4)2:0.01Er3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were synthesized by the solid state reaction method, and their crystal structures and upconversion (UC) luminescence properties were investigated in detail. The phase structure evolution of this series samples was discussed and the selected Li0.5Ag0.5Yb0.99(MoO4)2:0.01Er3+ was analyzed based on the Rietveld refinement. The UC emission properties and the related UC mechanism were also studied. With an increasing Li/Ag ratio in this host, the UC emission intensities of LixAg1−xYb0.99(MoO4)2:0.01Er3+ increased obviously, and the enhancement could be attributed to the coupling effect and the nonradiative transition between two energy levels of LixAg1−xYb(MoO4)2 matrices and the activator Er3+, which have also been analyzed based on the results of the ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS) and Raman spectroscopy.

We comparatively investigated the effect of different charge compensators (Li+, Na+, K+ and Si4+) via the charge balance strategies of (i) 2Ca2+ → Ce3+ + M+ (M = Li, Na, K) and (ii) Ca2+ + P5+ → Ce3+ + Si4+ on the photoluminescence (PL) properties of blue-emitting Ca6Ba(PO4)4O:Ce3+ phosphors. The optimized emission intensity was realized in Ca6Ba(PO4)4O:Ce3+ with codoping Li+. The different charge compensation mechanisms and PL enhancement were analysed. Moreover, tunable luminescence can be further achieved through the introduction of Eu2+ into the Ca6Ba(PO4)4O:Ce3+, Li+ phosphor, and charge compensation also plays an important role in the optimization of luminescence behaviours. It is proved that the emission spectra of Ca6Ba(PO4)4O:Ce3+, Li+, Eu2+ can be tuned from 463 to 528 nm through Ce3+–Eu2+ energy transfer (ET). The intrinsic ET mechanism and process have also been studied.

In this study, the Ba3Eu(PO4)3 and Sr3Eu(PO4)3 compounds were synthesized and the crystal structures were determined for the first time by Rietveld refinement using powder X-ray diffraction (XRD) patterns. Ba3Eu(PO4)3 crystallizes in cubic space group I3d, with cell parameters of a = 10.47996(9) Å, V = 1151.01(3) Å3 and Z = 4; Ba2+ and Eu3+ occupy the same site with partial occupancies of 3/4 and 1/4, respectively. Besides, in this structure, there exists two distorted kinds of the PO4 polyhedra orientation. Sr3Eu(PO4)3 is isostructural to Ba3Eu(PO4)3 and has much smaller cell parameters of a = 10.1203(2) Å, V = 1036.52(5) Å3. The bandgaps of Ba3Eu(PO4)3 and Sr3Eu(PO4)3 are determined to be 4.091 eV and 3.987 eV, respectively, based on the UV–Vis diffuse reflectance spectra. The photoluminescence measurements reveal that, upon 396 nm n-UV light excitation, Ba3Eu(PO4)3 and Sr3Eu(PO4)3 exhibit orange-red emission with two main peaks at 596 nm and prevailing 613 nm, corresponding to the 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+, respectively. The dynamic disordering in the crystal structures contributes to the broadening of the luminescence spectra. The electronic structure of the phosphates was calculated by the first-principles method. The analysis elucidats that the band structures are mainly governed by the orbits of phosphorus, oxygen and europium, and the sharp peaks of the europium f-orbit occur at the top of the valence bands.

Double molybdate scheelite-type solid-solution phosphors Li1−xAgxLu1−y(MoO4)2:yEu3+ were synthesized by the solid state reaction method, and their crystal structures and luminescence properties were investigated in detail. The composition modulation and structural evolution of this series of samples were studied and the selected AgEu(MoO4)2, AgLu(MoO4)2, LiLu(MoO4)2 and LiEu(MoO4)2 phases were analyzed based on the Rietveld refinement. Depending on the variation of the Li/Ag ratio in Li1−xAgxLu1−y(MoO4)2:yEu3+ phosphors, the difference in the luminescence properties of Li1−xAgxLu1−y(MoO4)2:yEu3+ phosphors was ascribed to two factors, one reason could be assigned to the coupling effect and the nonradiative transition between the energy levels of LixAg1−xLu(MoO4)2 matrices and the activator Eu3+, another could be due to the near ultraviolet energy absorption and transmission efficiency between the charge-transfer (CT) band of O2−–Mo6+ and the 4f → 4f emissive transitions of Eu3+. The ultraviolet-visible diffuse reflection spectra (UV-vis DRS) and Raman spectra analysis were also used to verify the above mechanism.

Novel self-activated yellow-emitting BaLuAlxZn4−xO7−(1−x)/2 photoluminescent materials were investigated by a combined experimental and theoretical analysis. The effects of Al/Zn composition modulation, calcination atmosphere and temperature on the crystal structure and photoluminescence properties have been studied via engineering oxygen vacancies. Accordingly, BaLuAl0.91Zn3.09O7 prepared in an air atmosphere was found to be the stable crystalline phase with optimal oxygen content and gave a broad yellow emission band with a maximum at 528 nm. The self-activated luminescence mechanism is ascribed to the O-vacancies based on the density functional theory (DFT) calculation. A theoretical model originating from the designed oxygen vacancies has been proposed in order to determine the influence of O-vacancies on the band structure and self-activated luminescence. Therefore, the appearance of a new local energy level in the band gap will cause the wide-band optical transitions in the studied BaLuAlxZn4−xO7−(1−x)/2 materials.

Crystal structures of the series of double perovskites Ca(2−x)BaxLaNbO6:Eu3+ phosphors have been examined by powder X-ray diffraction and Rietveld refinements. Ca2LaNbO6 has a monoclinic (P21/n) and Ba2LaNbO6 has a monoclinic (C2/m) structure. The structural phases of Ca(2−x)BaxLaNbO6:Eu3+ samples are divided into three sections depending on different Ca/Ba ratios: (1) monoclinic phase (P21/n) as Ca2LaNbO6 in the range of x = 0–0.1, (2) mixed phases containing Ca2LaNbO6 and Ba2LaNbO6 between 0.15 and 1.2, and (3) monoclinic phase (C2/m) as Ba2LaNbO6 for x = 1.4–2. Eu3+ ions act as the structural probes to study the structural phase transitions, and the evolution of the photoluminescence properties and thermal stability behaviours has been also comparatively investigated depending on different structural symmetries from Ca2LaNbO6 to Ba2LaNbO6 phase. The strong red emission from 5D0–7F2 peaking at 618 nm can be found in Ca2LaNbO6:Eu3+ phosphors, which is attributed to the low crystal field effect of the activator ions located in the highly distorted [LaO8] polyhedra sites. The composition-optimized phosphors can find applications in white light emitting diodes (LEDs).

•CaSrSiO4:Ce3+,Li+,Mn2+ were synthesized by a solid-state method.•Energy transfer from Ce3+ to Mn2+ ions has been discussed.•Blue/red dual-wavelength emission was realized.A series of color tunable phosphors CaSrSiO4:Ce3+,Li+,Mn2+ were synthesized by the traditional high temperature solid-state method. The as-prepared CaSrSiO4:Ce3+,Li+ phosphors exhibit a broad excitation band ranging from 250 to 400 nm and give a blue emission band centered at 450 nm. When Mn2+ ions are codoped for the fabrication of CaSrSiO4:Ce3+,Li+,Mn2+ phosphors, another new red-emitting peak appears at near 650 nm corresponding to the spin-forbidden 4T1(4G)–6A1(6S) transition of the Mn2+ ions. By adjusting the relative concentration of the Ce3+ and Mn2+ ions, the CIE chromaticity coordinate can be modulated from (0.156, 0.106) to (0.224, 0.211) owing to the energy transfer from Ce3+ to Mn2+ ions. The dipole–quadrupole interaction predominates in the energy transfer mechanism. The temperature dependent photoluminescence properties prove that the CaSrSiO4 host has a good thermal stability. These results indicate that CaSrSiO4:Ce3+,Li+,Mn2+ samples show tunable blue/red dual-wavelength emission depending on the codoped ions, which might have potential applications in the white light emitting diodes (wLEDs) and functional sunlight spectrum conversion device as greenhouse for green agriculture.

•Gd2−xO2S:xTb3+ phosphors were prepared by the microwave solid state method.•The composition-optimized Gd1.85O2S:15%Tb3+ exhibited green emission peaking at 546 nm.•The PL and CL emission properties have been comparatively investigated.Gd2−xO2S:xTb3+ phosphors were prepared by the microwave solid state method, and its phase formation and morphologies were studied by the X-ray powder diffraction (XRD) and scanning electron microscope (SEM) techniques. The photoluminescence (PL) properties, cathodoluminescence (CL) properties and PL thermal stability of the samples were investigated, which indicated that better luminescence properties can be obtained via the microwave method compared to the conventional high temperature solid-state method. The composition-optimized Gd1.85O2S:15%Tb3+ exhibited strong green emission peaking at 546 nm upon excitation at 254 nm with the CIE coordinates of (0.238, 0.382). Different electric voltage and current dependent CL spectra investigations of Gd1.85O2S:15%Tb3+ phosphor shows similar green spectral profile as PL emission and it also demonstrates the good luminescence stability suggesting its potential application as green emission component in cathode ray tube (CRT).

•Apatite-type Ca5[(P,V)O4)]3F:Eu3+ red-emitting phosphors have been synthesized.•Ca5[(P,V)O4)]3F:Eu3+ pure phase can be obtained depending on the [PO4]3−/[VO4]3− substitution.•The variations of the emission and excitation spectra of Ca5[(P,V)O4)]3F:Eu3+ phosphors have been discussed.Ca5[(P,V)O4)]3F:Eu3+ red-emitting phosphors were synthesized by a solid-state method. The effect of [PO4]3−/[VO4]3− substitution on the structures and luminescence properties have been discussed. Apatite-type Ca5[(P,V)O4)]3F:Eu3+ phosphors are obtained, and the lattice parameters a, c values and cell volume V increase with the increasing content of [VO4]3− substitution for [PO4]3−. The optimum Eu3+ doping concentration in a selected system has been studied. The variations of the emission and excitation spectra of Ca5[(P,V)O4)]3F:Eu3+ phosphors depending on the varied [PO4]3−/[VO4]3− substitution amounts have been discussed. The mechanism on the broadening of the excitation band with increasing [VO4]3− contents has been analyzed.The variation of the emission and excitation spectra of Ca5[(P,V)O4)]3F:Eu3+ phosphors depending on the varied [PO4]3−/[VO4]3− substitution amounts have been also discussed.

A series of iso-structural La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors with apatite structure have been prepared. A combination of powder X-ray diffraction and neutron scattering technique was employed to explore the crystal structural evolution and the rigid nature from oxy- to oxynitride-based apatites, and some local structures were also characterized by HRTEM and 29Si NMR data, respectively. The new La5(Si2+xB1–x)(O13–xNx):Ce3+ solid solution phosphors gave continuously controlled emission from 421 nm [La5Si2BO13:Ce3+, end-member (x = 0)] to 463 nm (La5Si3O12N:Ce3+, end-member (x = 1)). Substitution of B3+ and O2– by Si4+ and N3– in La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors produced more covalency into the crystal field environment around the Ce3+ ions inducing the red-shifted emission, further improving the thermal stability of the oxynitride-based apatite phosphors. The proposed approach from oxy- to oxynitride based iso-structural phases could significantly contribute to future research in designing complex solid solution phosphors.

The cation substitution-dependent phase transition was used as a strategy to discover new solid solution phosphors and to efficiently tune the luminescence property of divalent europium (Eu2+) in the M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors including the greenish-white SrCa2(PO4)2:Eu2+, the yellow Sr2Ca(PO4)2:Eu2+, and the cyan Ba2Ca(PO4)2:Eu2+ were reported, and the drastic red shift of the emission toward the phase transition point was discussed. Different behavior of luminescence evolution in response to structural variation was verified among the three M3(PO4)2:Eu2+ joins. Sr3(PO4)2 and Ba3(PO4)2 form a continuous isostructural solid solution set in which Eu2+ exhibits a similar symmetric narrow-band blue emission centered at 416 nm, whereas Sr2+ substituting Ca2+ in Ca3(PO4)2 induces a composition-dependent phase transition and the peaking emission gets red shifted to 527 nm approaching the phase transition point. In the Ca3–xBax(PO4)2:Eu2+ set, the validity of crystallochemical design of phosphor between the phase transition boundary was further verified. This cation substitution strategy may assist in developing new phosphors with controllably tuned optical properties based on the phase transition.

A series of color-tunable green-red emitting La3GaGe5O16:Tb3+,Eu3+ phosphors were prepared by a high temperature solid-state reaction, and their phase structure and photoluminescence (PL) properties were investigated in detail. A series of characteristic emission lines originated from the f–f transitions of Tb3+ and Eu3+ can be observed from the PL spectra and the emission intensities' variation of the two strong emission lines peaking at around 544 nm (green) and 617 nm (red) induced the multi-color emission evolution via the fine tuning of the Tb3+/Eu3+ content ratio. The energy transfer process between Tb3+ and Eu3+ was demonstrated to be a resonant type via a dipole–dipole mechanism, and the crystal distance Rc calculated by the quenching concentration method and the spectral overlap method was 15.24 and 17.32 Å, respectively. Thermal quenching luminescence results reveal that La3GaGe5O16:Tb3+,Eu3+ exhibits good thermal stability.

The orthophosphate phosphors ABaPO4:Eu2+ (A = K, Rb) have been studied in this paper. The continuous solid-solution phases in (Kx,Rb1−x)BaPO4 have been determined and the phase structure evolution and its dependence on the K:Rb ratio have been discussed, and all the Eu2+-doped (Kx,Rb1−x)BaPO4 phosphors show a blue emission, peaking at approximately 420 nm, originating from the 4f–5d transition of Eu2+. The differences in the fine microstructure have been investigated and the effect on the photoluminescence behavior, especially the thermally stable luminescence, has been studied for this series of solid-solution phosphors. The relationship between the activation energy and thermally stable luminescence is also discussed. The compositionally optimized (Kx,Rb1−x)BaPO4:Eu2+ phosphor can be potentially applied in phosphor-converted white light-emitting diodes (w-LEDs).

We demonstrated that a new intermediate composition of Ba1.55Ca0.45SiO4 between the orthosilicates Ca2SiO4 and Ba2SiO4 yields the best phosphor hosts, and interesting luminescence properties can be found from the Eu2+ singly doped and/or Eu2+/Mn2+ codoped Ba1.55Ca0.45SiO4 phosphors. The phosphors can be excited by near-ultraviolet (nUV) light at wavelengths ranging from 200 to 450 nm matching well with the nUV light-emitting diode (LED) chips. As a result of fine-tuning the activators of different Eu2+ content and Eu2+/Mn2+ couples with different ratios, tunable full-color emission under UV light excitation can be realized by combining the blue emission (460 nm) and green emission (520 nm) originating from Eu2+ with the red emission (595 nm) from Mn2+ in the Ba1.55Ca0.45SiO4 host lattice. Energy-transfer efficiency between Eu2+ and Mn2+ increases and tunable emission can be obtained with increasing Mn2+ doping content. These results indicate that the Ba1.55Ca0.45SiO4:Eu2+,Mn2+ phosphor will have potential use in nUV chip pumped white LED devices.

Cation substitution dependent tunable bimodal photoluminescence behavior was observed in the Ca3–xSrx(PO4)2:Eu2+ (0 ≤ x ≤ 2) solid solution phosphors. The Rietveld refinements verified the phase purity and whitlockite type crystal structure of the solid solutions. The tunable photoluminescence evolution was studied as a function of strontium content, over the composition range 0.1 ≤ x ≤ 2. In addition to the emission band peak at 416 nm in Ca3(PO4)2:Eu2+, the substitution of Ca2+ by Sr2+ induced the emerging broad-band peak at 493–532 nm. A dramatic red shift of the emission peak located in the green-yellow region was observed on an increase of x in the samples with 0.75 ≤ x ≤ 2.00. The two emission bands could be related to the EuOn–Ca9 and EuOn–Ca9–xSrx emitting blocks, respectively. The values for the two kinds of emitting blocks in the solid solutions can be fitted well with the observed intensity evolution of the two emission peaks.

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr1.75Ca1.25(PO4)2 newly found by composition design in the Sr3(PO4)2–Ca3(PO4)2 join was reported. Crystal structure and luminescence properties of Sr1.75Ca1.25(PO4)2:Eu2+ were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu2+ in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr1.75Ca1.25(PO4)2:Eu2+ phosphors absorb in the UV–vis spectral region of 250–430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λex = 365 nm which is ascribed to the 5d–4f allowed transition of Eu2+. The luminescence properties and mechanism are also investigated as a function of Eu2+ concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties.

A series of iso-structural eulytite-type (Ba,Sr)3Lu(PO4)3:Eu2+ solid-solution phosphors with different Sr/Ba ratios were synthesized by a solid-state reaction. Crystal structures of (Ba,Sr)3Lu(PO4)3:Eu2+ were resolved by the Rietveld method, which shows an eulytite-type cubic Bi4(SiO4)3 structure with cations disordered in a single C3 site while the oxygen atoms were distributed over two partially occupied sites. The emission peaks of Ba(3−x)SrxLu(PO4)3:Eu2+ (0 ≤ x ≤ 3) phosphors were blue-shifted, from 506 to 479 nm, with increasing Sr/Ba ratio upon the same excitation wavelength of 365 nm, and such interesting luminescence behaviours can also be found in other eulytite-type (Ba,Sr)3Ln(PO4)3:Eu2+ (Ln = Y, Gd) solid-solution phosphors. The blue-shift of the Eu2+ emission with increasing Sr/Ba ratio was ascribed to the variation of the crystal field strength that the 5d orbital of Eu2+ ion experiences, and a new model based on the Eu–O bond length and released neighboring-cation stress in disordered Ba2+/Sr2+/Ln3+ sites is proposed.

A novel blue-emitting double-phosphate phosphor Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ was synthesized by the sol–gel method, and the structure and luminescence properties were investigated in detail. The crystal structure and chemical composition of Cs0.72Ca0.72Gd1.28(PO4)2 matrix was analyzed and determined based on Rietveld refinements and phase and chemical composition analysis. The composition-optimized Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ exhibited strong blue light, peaking at 462 nm upon excitation at 365 nm with the CIE coordinates of (0.139, 0.091). The quenching concentration of Eu2+ in the Cs0.72Ca0.72Gd1.28(PO4)2 phase was about 0.01 and attributed to the dipole–quadrupole interaction. The thermally stable luminescence properties, fluorescence decay curves and diffuse reflectance spectra of Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ phosphors are also discussed, all of which indicate that the Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ phosphor is a promising phosphor for application in white-light UV LEDs.

A kind of novel blue-emitting chloro-germanate phosphor Ca3GeO4Cl2:Eu2+ has been synthesized via a high temperature solid-state method. The crystal structure of the as-prepared phosphor was discussed from the viewpoint of the doping behaviors of the activators. The luminescence properties and thermal stability of Ca3GeO4Cl2:Eu2+ was investigated in detail. Ca3GeO4Cl2:Eu2+ phosphors exhibit a broad-band excitation band in the near ultra-violet region and a blue emission peak at 428 nm, which are both ascribed to the 4f–5d transitions of the Eu2+ ions. The optimum concentration of Eu2+ in the Ca3GeO4Cl2 phosphor was determined to be 3 mol%, and the concentration quenching mechanism was considered to be the dipole–dipole interaction with a critical distance of Rc = 22.11 Å. Thermal stability studies show that the photoluminescence intensity of the Ca3GeO4Cl2:Eu2+ phosphor at 150 °C was 77% of the initial value at room temperature. The activation energy, E, was calculated to be 0.163 eV suggesting good thermal stability. The variation in lifetime of Ca3GeO4Cl2:Eu2+ phosphor was also discussed to verify different luminescence centers in the lattice.

Scheelite related alkali-metal rare-earth double molybdate compounds with a general formula of ALn(MoO4)2 can find wide application as red phosphors. The crystal chemistry and luminescence properties of red-emitting CsGd1−xEux(MoO4)2 solid-solution phosphors have been evaluated in the present paper. A detailed analysis of the structural properties indicates the formation of isostructural scheelite-type CsGd1−xEux(MoO4)2 solid-solutions over the composition range of 0 ≤ x ≤ 1. The photoluminescence emission (PL) and excitation (PLE) spectra, and the decay curves were measured for this series of compounds. The critical doping concentration of Eu3+ is determined to be x = 0.6 in order to realize the maximum emission intensity. The emission spectra of the as-obtained CsGd(1−x)Eux(MoO4)2 phosphors show narrow high intensity red lines at 592 and 615 nm upon excitation at 394 or 465 nm, revealing great potential for applications in white light-emitting diode devices.

CaMoO4:Eu3+, Li+ red phosphors have been successfully achieved by a microwave-assisted solid-state reaction, and their morphologies and luminescence properties have been studied in this paper. The as-prepared Ca0.9MoO4:0.05Eu3+, 0.05Li+ powders occur as uniform spherical particles with a size distribution of 1–2 μm via the optimized reaction temperature at 600 °C and reaction time of 1 h. The phosphors showed an intense red emission with maxima at 615 nm, which was ascribed to the Eu3+ electric dipole transition of 5D0 → 7F2. The comparison on the morphology and luminescence property between the as-prepared samples and the samples obtained by the conventional method was discussed in detail. A possible microwave-assisted solid-state reaction mechanism was proposed, and the effect of the microwave sintering temperature and time was also discussed.

A novel Cr3+-doped La3GaGe5O16 phosphor was synthesized by a solid-state reaction, and the phase formation and microstructure, near-infrared (NIR) photoluminescence (PL) properties and PL thermal stability were investigated in detail. The excitation spectrum of La3GaGe5O16:Cr3+ at 270, 415 and 573 nm corresponded to three spin-allowed Cr3+ d–d intra-transitions of 4A2–4T1 (4P), 4A2–4T1 (4F), and 4A2–4T2 (4F), respectively, among which the main red emission peak observed at 700 nm is identified, which is due to the 2E–4A2 transition from Cr3+ ions. It is further proved that the dipole–dipole interactions result in the concentration quenching of Cr3+ in La3GaGe5O16:Cr3+ phosphors, and the energy-transfer distance at the quenching concentration was around 26.90 Å. Moreover, thermal quenching luminescence results reveal that La3GaGe5O16:Cr3+ exhibits good thermal stability. The above results indicate that La3GaGe5O16:Cr3+ has potential practical applications in luminescence solar concentrators with broad-band absorption.

•Ce3+/Dy3+ co-doped GdOBr phosphors have been synthesized.•Color-tunable emission can be realized in GdOBr:Ce3+,Dy3+ phosphors.•Energy transfer between Ce3+ and Dy3+ in GdOBr has been discussed.Color-tunable blue to bluish white-emitting Ce3+/Dy3+ co-doped GdOBr phosphors have been synthesized by the conventional solid-state method. The phase structures, luminescent properties and energy transfer process were discussed in detail. Broad-band absorption originating from the f-d transition of Ce3+ can be found for the as-prepared GdOBr:Ce3+,Dy3+ phosphor, and color-tunable blue to bluish white emission can be realized owing to the energy transfer between Ce3+ and Dy3+. The energy transfer mechanism is demonstrated to be the dipole–dipole process. The energy transfer efficiency increases with increasing Dy3+ concentrations. The results indicate that Ce3+/Dy3+-activated GdOBr phosphors may be potential for phosphor-converted white-light UV-LEDs.

•Ca1.97Al2−xSi1+xO7−xNx:0.03Eu2+ (x = 0–0.4) phosphors have been prepared.•The introduction of partial Si–N bonds is verified.•The activation energy becomes large along with the nitridation process.Ca1.97Al2−xSi1+xO7−xNx:0.03Eu2+ (x = 0–0.4) phosphors have been prepared by using the high temperature solid-state reaction. The effect of phase structures, photoluminescence (PL) properties and the thermal stabilities have been investigated based on the substitution of Al–O bond in Ca2Al2SiO7:Eu2+ phosphor with Si–N bond. The XRD Rietveld refinement and 29Si NMR analysis results verify the introduction of partial Si–N bonds. It is found that the PL spectra shift to the blue region abnormally from 530 to 515 nm, and the possible mechanism has been proposed. The activation energy becomes large along with the nitridation process, which coincides with the explanation of configuration coordinate diagram.Graphical abstract

Garnet-type Y2.96Sc2Ga3–xAlxO12:0.04Ce3+ (x = 0–3) phosphors have been prepared by using the high temperature solid-state reaction. Al/Ga ratio dependent Y3Sc2(Ga,Al)3O12 phase structures, photoluminescence (PL) properties, and long-lasting phosphorescence (LLP) properties for the Ce3+-doped phosphors have been investigated in detail. The PL emission bands of Y2.96Sc2Ga3–xAlxO12:0.04Ce3+ showed a red-shift tendency gradually from 503 to 520 nm with increasing Al content (x value), and the emission intensities increased first, maximized at x = 1, and then decreased. Y3Sc2Ga3O12:Ce3+ can show the green LLP emission, and afterglow can be obviously enhanced when (1) Al ions replaced Ga ions for a small amount and (2) the reaction atmosphere was varied from reducing to oxidation one. The afterglow emission, decay curves, and thermoluminescence (TL) of Y2.96Sc2Ga3–xAlxO12:0.04Ce3+ (x = 0–0.7) phosphors have been investigated, and related luminescence mechanisms are systematically discussed as well.

The versatile polymorphism and chemical compositions of barium silicates have been studied for a long time and their crystal structures have been established. Herein, we focused on the understanding of the crystal structure of the Ba4Si6O16 phase and the structural correlation of Ba4Si6O16 and Ba2Si3O8; moreover, the luminescence properties of Ce3+,Eu2+-co-activated Ba4Si6O16 phosphors have been discussed. Ba4Si6O16:Ce3+,Eu2+ phosphors show tunable blue-green emission upon excitation with 365 nm ultraviolet (UV) light. The blue emission originates from Ce3+, whereas the bluish-green emission is ascribed to Eu2+, and variation in the emission peak wavelength from 442 to 497 nm can be achieved by properly tuning the Ce3+/Eu2+ ratio. Energy transfer from Ce3+ to Eu2+ in the Ba4Si6O16 host has been validated by the variation of emission spectra as well as the variation of Ce3+ decay lifetimes with increasing Eu2+ concentration, and the energy transfer mechanism is demonstrated to be a resonant type via a dipole–dipole process. The results suggest that Ba4Si6O16:Ce3+,Eu2+ phosphors are potential candidates as a blue-green component for UV-excited white light-emitting diodes.

This paper reports the development of new phosphors using the chemical unit cosubstituting solid solution design strategy. Starting from Lu3Al5O12, the Al3+–Al3+ couple in respective octahedral and tetrahedral coordination was simultaneously substituted by a Mg2+–Si4+ pair forming the Lu3(Al2−xMgx)(Al3−xSix)O12:Ce3+ (x = 0.5–2.0) series; as a result, the CeO8 polyhedrons were compressed and the emission got red-shifted from green to yellow together with the broadening. The evolution of, the unit cell, the local structural geometry as well as the optical properties of Ce3+ in these garnet creations, in response to the gradual Mg–Si substitution for Al–Al, were studied by combined techniques of structural refinement and luminescence measurements. The new composition Lu2.97Ce0.03Mg0.5Al4Si0.5O12 was comprehensively evaluated regarding its potential application in blue LED-driven solid state white lighting: the maximum emission is at 550 nm under λex = 450 nm; the internal and external quantum efficiencies can reach 85% and 49%, respectively; a 1-phosphor-converted wLED lamp fabricated using the as-prepared phosphor exhibits the luminous efficacy of 105 lm W−1, the correlated color temperature of 6164 K and the color rendering index (Ra) of 75.6. The new solid solution composition series is open for further optimization to enhance the competence for commercial consideration.

A novel blue-emitting double-phosphate phosphor Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ was synthesized by the sol–gel method, and the structure and luminescence properties were investigated in detail. The crystal structure and chemical composition of Cs0.72Ca0.72Gd1.28(PO4)2 matrix was analyzed and determined based on Rietveld refinements and phase and chemical composition analysis. The composition-optimized Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ exhibited strong blue light, peaking at 462 nm upon excitation at 365 nm with the CIE coordinates of (0.139, 0.091). The quenching concentration of Eu2+ in the Cs0.72Ca0.72Gd1.28(PO4)2 phase was about 0.01 and attributed to the dipole–quadrupole interaction. The thermally stable luminescence properties, fluorescence decay curves and diffuse reflectance spectra of Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ phosphors are also discussed, all of which indicate that the Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ phosphor is a promising phosphor for application in white-light UV LEDs.

In this study, the Ba3Eu(PO4)3 and Sr3Eu(PO4)3 compounds were synthesized and the crystal structures were determined for the first time by Rietveld refinement using powder X-ray diffraction (XRD) patterns. Ba3Eu(PO4)3 crystallizes in cubic space group I3d, with cell parameters of a = 10.47996(9) Å, V = 1151.01(3) Å3 and Z = 4; Ba2+ and Eu3+ occupy the same site with partial occupancies of 3/4 and 1/4, respectively. Besides, in this structure, there exists two distorted kinds of the PO4 polyhedra orientation. Sr3Eu(PO4)3 is isostructural to Ba3Eu(PO4)3 and has much smaller cell parameters of a = 10.1203(2) Å, V = 1036.52(5) Å3. The bandgaps of Ba3Eu(PO4)3 and Sr3Eu(PO4)3 are determined to be 4.091 eV and 3.987 eV, respectively, based on the UV–Vis diffuse reflectance spectra. The photoluminescence measurements reveal that, upon 396 nm n-UV light excitation, Ba3Eu(PO4)3 and Sr3Eu(PO4)3 exhibit orange-red emission with two main peaks at 596 nm and prevailing 613 nm, corresponding to the 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+, respectively. The dynamic disordering in the crystal structures contributes to the broadening of the luminescence spectra. The electronic structure of the phosphates was calculated by the first-principles method. The analysis elucidats that the band structures are mainly governed by the orbits of phosphorus, oxygen and europium, and the sharp peaks of the europium f-orbit occur at the top of the valence bands.

A kind of novel blue-emitting chloro-germanate phosphor Ca3GeO4Cl2:Eu2+ has been synthesized via a high temperature solid-state method. The crystal structure of the as-prepared phosphor was discussed from the viewpoint of the doping behaviors of the activators. The luminescence properties and thermal stability of Ca3GeO4Cl2:Eu2+ was investigated in detail. Ca3GeO4Cl2:Eu2+ phosphors exhibit a broad-band excitation band in the near ultra-violet region and a blue emission peak at 428 nm, which are both ascribed to the 4f–5d transitions of the Eu2+ ions. The optimum concentration of Eu2+ in the Ca3GeO4Cl2 phosphor was determined to be 3 mol%, and the concentration quenching mechanism was considered to be the dipole–dipole interaction with a critical distance of Rc = 22.11 Å. Thermal stability studies show that the photoluminescence intensity of the Ca3GeO4Cl2:Eu2+ phosphor at 150 °C was 77% of the initial value at room temperature. The activation energy, E, was calculated to be 0.163 eV suggesting good thermal stability. The variation in lifetime of Ca3GeO4Cl2:Eu2+ phosphor was also discussed to verify different luminescence centers in the lattice.

A series of iso-structural eulytite-type (Ba,Sr)3Lu(PO4)3:Eu2+ solid-solution phosphors with different Sr/Ba ratios were synthesized by a solid-state reaction. Crystal structures of (Ba,Sr)3Lu(PO4)3:Eu2+ were resolved by the Rietveld method, which shows an eulytite-type cubic Bi4(SiO4)3 structure with cations disordered in a single C3 site while the oxygen atoms were distributed over two partially occupied sites. The emission peaks of Ba(3−x)SrxLu(PO4)3:Eu2+ (0 ≤ x ≤ 3) phosphors were blue-shifted, from 506 to 479 nm, with increasing Sr/Ba ratio upon the same excitation wavelength of 365 nm, and such interesting luminescence behaviours can also be found in other eulytite-type (Ba,Sr)3Ln(PO4)3:Eu2+ (Ln = Y, Gd) solid-solution phosphors. The blue-shift of the Eu2+ emission with increasing Sr/Ba ratio was ascribed to the variation of the crystal field strength that the 5d orbital of Eu2+ ion experiences, and a new model based on the Eu–O bond length and released neighboring-cation stress in disordered Ba2+/Sr2+/Ln3+ sites is proposed.

Double molybdate scheelite-type solid-solution phosphors Li1−xAgxLu1−y(MoO4)2:yEu3+ were synthesized by the solid state reaction method, and their crystal structures and luminescence properties were investigated in detail. The composition modulation and structural evolution of this series of samples were studied and the selected AgEu(MoO4)2, AgLu(MoO4)2, LiLu(MoO4)2 and LiEu(MoO4)2 phases were analyzed based on the Rietveld refinement. Depending on the variation of the Li/Ag ratio in Li1−xAgxLu1−y(MoO4)2:yEu3+ phosphors, the difference in the luminescence properties of Li1−xAgxLu1−y(MoO4)2:yEu3+ phosphors was ascribed to two factors, one reason could be assigned to the coupling effect and the nonradiative transition between the energy levels of LixAg1−xLu(MoO4)2 matrices and the activator Eu3+, another could be due to the near ultraviolet energy absorption and transmission efficiency between the charge-transfer (CT) band of O2−–Mo6+ and the 4f → 4f emissive transitions of Eu3+. The ultraviolet-visible diffuse reflection spectra (UV-vis DRS) and Raman spectra analysis were also used to verify the above mechanism.

Crystal structures of the series of double perovskites Ca(2−x)BaxLaNbO6:Eu3+ phosphors have been examined by powder X-ray diffraction and Rietveld refinements. Ca2LaNbO6 has a monoclinic (P21/n) and Ba2LaNbO6 has a monoclinic (C2/m) structure. The structural phases of Ca(2−x)BaxLaNbO6:Eu3+ samples are divided into three sections depending on different Ca/Ba ratios: (1) monoclinic phase (P21/n) as Ca2LaNbO6 in the range of x = 0–0.1, (2) mixed phases containing Ca2LaNbO6 and Ba2LaNbO6 between 0.15 and 1.2, and (3) monoclinic phase (C2/m) as Ba2LaNbO6 for x = 1.4–2. Eu3+ ions act as the structural probes to study the structural phase transitions, and the evolution of the photoluminescence properties and thermal stability behaviours has been also comparatively investigated depending on different structural symmetries from Ca2LaNbO6 to Ba2LaNbO6 phase. The strong red emission from 5D0–7F2 peaking at 618 nm can be found in Ca2LaNbO6:Eu3+ phosphors, which is attributed to the low crystal field effect of the activator ions located in the highly distorted [LaO8] polyhedra sites. The composition-optimized phosphors can find applications in white light emitting diodes (LEDs).

Blue-emitting CaSrSiO4:Ce3+,Li+ phosphors were prepared by a high temperature solid-state method, and the effect of substituting Al3+ for Si4+ in CaSrSiO4:Ce3+,Li+ has been studied. Crystal structures of the as-prepared Ca1−ySr1−ySi1−xAlxO4:yCe3+,yLi+ phosphors were resolved by the Rietveld method, which suggested that all the samples belonged to the orthorhombic symmetry (Pnma) group of α-CaSrSiO4. The photoluminescence (PL) emission and excitation spectra, the lifetime, and the effect of Al3+ concentration on the PL properties were investigated in detail. The emission peaks of the CaSrSi1−xAlxO4:Ce3+,Li+ (x = 0–0.10) phosphors were red-shifted from 452 to 472 nm with increasing Al/Si ratio. The red-shift of the Ce3+ emission is ascribed to the polyhedra distortion of the cations, originating from the variation in the neighboring [(Si,Al)O4] polyhedra, and the detailed mechanism has been discussed.

Ce3+/Li+,Eu2+ singly doped and Ce3+/Li+/Eu2+-co-doped CaSrAl2SiO7 phosphors have been prepared using the conventional solid-state reaction method. The crystal structure of the melilite-type CaSrAl2SiO7 phase and the preferred crystallographic positions of the doped ions were refined using the Rietveld method. The luminescence properties and energy transfer of CaSrAl2SiO7:Ce3+,Li+,Eu2+ were studied in detail. The Ce3+/Li+ activated CaSrAl2SiO7 phosphor has a strong absorption band in the range of 200–450 nm and shows a blue emission centered at 477 nm. When Eu2+ ions are co-doped with Ce3+/Li+, the emission color of CaSrAl2SiO7:Ce3+,Li+,Eu2+ phosphors under the irradiation of 365 nm can be tuned from blue to green via the energy transfer from Ce3+ to Eu2+ ions. Also the involved energy transfer process and the corresponding mechanism between Ce3+ and Eu2+ have been discussed in detail. These results indicate that the as-reported CaSrAl2SiO7:Ce3+,Li+,Eu2+ phosphors have potential applications in near-UV chip pumped white LEDs.

The orthophosphate phosphors ABaPO4:Eu2+ (A = K, Rb) have been studied in this paper. The continuous solid-solution phases in (Kx,Rb1−x)BaPO4 have been determined and the phase structure evolution and its dependence on the K:Rb ratio have been discussed, and all the Eu2+-doped (Kx,Rb1−x)BaPO4 phosphors show a blue emission, peaking at approximately 420 nm, originating from the 4f–5d transition of Eu2+. The differences in the fine microstructure have been investigated and the effect on the photoluminescence behavior, especially the thermally stable luminescence, has been studied for this series of solid-solution phosphors. The relationship between the activation energy and thermally stable luminescence is also discussed. The compositionally optimized (Kx,Rb1−x)BaPO4:Eu2+ phosphor can be potentially applied in phosphor-converted white light-emitting diodes (w-LEDs).

The design scheme of the chemical unit cosubstitution of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ has been put into practice to discover the new phosphor systems with tunable luminescence properties, and the structures and photoluminescence tuning of yellow-emitting LuxSr2−xSiNxO4−x:Eu2+ phosphors have been investigated. Crystal structures of LuxSr2−x−ySiNxO4−x:yEu2+ samples were resolved using the Rietveld method, suggesting that the as-prepared Sr2SiO4 belonged to monoclinic symmetry (P21/n) of β-phase Sr2SiO4, while Sr1.97Eu0.03SiO4 and Sr1.965Eu0.03Lu0.005SiO3.995N0.005 belonged to orthorhombic symmetry (Pnma) of α-Sr2SiO4. The emission peaks of LuxSr1.97−xSiNxO4−x:0.03Eu2+ phosphors were red-shifted from 563 to 583 nm upon increasing the [Lu3+–N3−] substitution content from x = 0 to x = 0.005, furthermore, the PL emission peaks of Lu0.005Sr1.965−ySiN0.005O3.995:yEu2+ also showed a red-shift from 583 nm to 595 nm with increasing Eu2+ concentration (y = 0.03, 0.07, 0.10 and 0.15), and their corresponding red-shift mechanism has been discussed. The temperature dependent luminescence results further verified that the introduction of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ can improve the thermal stability of the photoluminescence, which indicated that the LuxSr2−x−ySiNxO4−x:yEu2+ phosphors have potential applications in white light-emitting diodes (wLEDs).

A series of yellow-emitting La0.975−xGdxSr2AlO5:0.025Ce3+ phosphors have been synthesized by a solid-state reaction. All the samples retain the same tetragonal crystal structure, and the phosphors emit yellow emission with a peak maximum shifting from 540 nm to 556 nm, depending on the Gd/La ratio, when excited by 440 nm blue light. Their temperature dependent photoluminescence, fluorescence decay curves, and CIE values were also discussed. With increasing Gd/La ratio, the red-shift behaviors can be ascribed to the enhancing covalence, and the emission intensities first increased, then decreased, and finally increased to the maximum at x = 0.6. A white light-emitting diode (w-LED) lamp was fabricated based on the selected yellow-emitting La0.375Gd0.6Sr2AlO5:0.025Ce3+ phosphors combined with a 460 nm blue-emitting InGaN chip. The packaged w-LED lamp gave CIE chromaticity coordinates of (0.3562, 0.3660) with a warm color temperature of 4664 K and a color rendering index of 83.

Scheelite related alkali-metal rare-earth double molybdate compounds with a general formula of ALn(MoO4)2 can find wide application as red phosphors. The crystal chemistry and luminescence properties of red-emitting CsGd1−xEux(MoO4)2 solid-solution phosphors have been evaluated in the present paper. A detailed analysis of the structural properties indicates the formation of isostructural scheelite-type CsGd1−xEux(MoO4)2 solid-solutions over the composition range of 0 ≤ x ≤ 1. The photoluminescence emission (PL) and excitation (PLE) spectra, and the decay curves were measured for this series of compounds. The critical doping concentration of Eu3+ is determined to be x = 0.6 in order to realize the maximum emission intensity. The emission spectra of the as-obtained CsGd(1−x)Eux(MoO4)2 phosphors show narrow high intensity red lines at 592 and 615 nm upon excitation at 394 or 465 nm, revealing great potential for applications in white light-emitting diode devices.

Stable and efficient phosphor systems for white light-emitting diodes (LEDs) are highly important with respect to their application in solid-state lighting beyond the technical limitations of traditional lighting technologies. Therefore, inorganic solid-state conversion phosphors must be precisely selected and evaluated with regard to their special material properties and synergistic optical parameters. In this perspective, we present an overview of the recent developments of LED phosphors; firstly, general photoluminescence-controlling strategies for phosphors to match LED applications have been evaluated; secondly, state-of-the-art and emerging new LED phosphors have been demonstrated. Then, methodologies for the discovery of new LED phosphors by mineral-inspired prototype evolution and new phase construction, as well as combinatorial optimization screening, and the single-particle-diagnosis approach, have been analyzed and exemplified. Finally, future developments of LED phosphors have been proposed.

A series of single-component blue-, white- and orange-emitting phosphors have been developed based on the BaY2Si3O10 host via the Ce3+/Eu3+, Tb3+/Eu3+, Ce3+/Tb3+, and Ce3+/Tb3+/Eu3+ couples and the energy transfer among them has been discussed in detail. A terbium bridge model via Ce3+–Tb3+–Eu3+ energy transfer has been studied and verified. The phase structures, photoluminescence (PL) properties, decay curves, PL thermal stability, and the energy transfer mechanism were investigated in this paper. The emission colors of the phosphors can be tuned from blue (0.1501, 0.0805) to white (0.3230, 0.3140) and eventually to orange (0.4496, 0.3537) through the corresponding Ce3+–Tb3+–Eu3+ energy transfer. Moreover, thermal quenching luminescence results reveal that BaY2Si3O10:Ce3+,Tb3+,Eu3+ exhibits good thermal stability. The above results indicate that the phosphors may be potentially used as single-component multi-color or white emission phosphors for application in white light emitting diodes (w-LEDs).

A series of double molybdate scheelite-type phosphors LixAg1−xYb0.99(MoO4)2:0.01Er3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were synthesized by the solid state reaction method, and their crystal structures and upconversion (UC) luminescence properties were investigated in detail. The phase structure evolution of this series samples was discussed and the selected Li0.5Ag0.5Yb0.99(MoO4)2:0.01Er3+ was analyzed based on the Rietveld refinement. The UC emission properties and the related UC mechanism were also studied. With an increasing Li/Ag ratio in this host, the UC emission intensities of LixAg1−xYb0.99(MoO4)2:0.01Er3+ increased obviously, and the enhancement could be attributed to the coupling effect and the nonradiative transition between two energy levels of LixAg1−xYb(MoO4)2 matrices and the activator Er3+, which have also been analyzed based on the results of the ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS) and Raman spectroscopy.

The discovery of lead-free double perovskites provides a feasible way of searching for air-stable and environmentally benign solar cell absorbers. Herein we report the design and hydrothermal crystal growth of double perovskite Cs2AgInCl6. The crystal structure, morphology related to the crystal growth habit, band structure, optical properties, and stability are investigated in detail. This perovskite crystallized in a cubic unit cell with the space group Fm3m and is composed of [AgCl6] and [InCl6] octahedra alternating in a ordered rock-salt structure, and the as-obtained crystal size is dependent on the hydrothermal reaction time. Cs2AgInCl6 is a direct gap semiconductor with a wide band gap of 3.23 eV obtained experimentally and 3.33 eV obtained by DFT calculation. This theoretically predicted and experimentally confirmed optical gap is a prototype of the band gaps that are direct and optically allowed except at the single high-symmetry k-point, which didn't raise interest before but have potential applications in future technologies. Cs2AgInCl6 material with excellent moisture, light and heat stability shows great potential for photovoltaic and other optoelectronic applications via further band gap engineering.

A facile and controllable ethanol/water aided hydrothermal process was developed to prepare the NaCaSiO3OH:Tb3+/Eu3+ phosphor. The morphologies were in situ constructed with the phase transformation from NaCaSiO3OH to Na2Ca2Si2O7, and the intrinsic crystal structural transformation mechanism and the dependence of their photoluminescence tuning on the Tb3+/Eu3+ ratio have been discussed.

A series of Ca6−x−yBa(PO4)4−x−y(SiO4)x+yO:xCe3+,yTb3+ phosphors was synthesized via a high-temperature solid-state method. Charge balance was realized via the collaborative substitution of a Ca2+/Ce3+(Tb3+)–P5+/Si4+ couple with the pure phase. On excitation at 365 nm, the emission spectra of the as-obtained phosphor contained both the asymmetrical broad band Ce3+ emissions and the line-type Tb3+ emissions. Energy transfer occurred from Ce3+ to Tb3+ and the mechanism was ascribed to a resonant type via a non-radiative dipole–dipole interaction. The critical distance of the energy transfer was calculated via the concentration quenching method. The hues of the studied phosphors could be adjusted by the relative ratios of the Ce3+ and Tb3+ ions. The phosphors have good thermal stability and a high quantum efficiency and therefore may have potential applications in near-UV pumped white light-emitting diodes.

A series of color-tunable green-red emitting La3GaGe5O16:Tb3+,Eu3+ phosphors were prepared by a high temperature solid-state reaction, and their phase structure and photoluminescence (PL) properties were investigated in detail. A series of characteristic emission lines originated from the f–f transitions of Tb3+ and Eu3+ can be observed from the PL spectra and the emission intensities' variation of the two strong emission lines peaking at around 544 nm (green) and 617 nm (red) induced the multi-color emission evolution via the fine tuning of the Tb3+/Eu3+ content ratio. The energy transfer process between Tb3+ and Eu3+ was demonstrated to be a resonant type via a dipole–dipole mechanism, and the crystal distance Rc calculated by the quenching concentration method and the spectral overlap method was 15.24 and 17.32 Å, respectively. Thermal quenching luminescence results reveal that La3GaGe5O16:Tb3+,Eu3+ exhibits good thermal stability.

A new single phase based on the substitution of a Sr cation by a Ca cation in the apatite-type Sr5(PO4)2(SiO4) has been fabricated with the nominal chemical composition of Sr4Ca(PO4)2(SiO4), which appears as a definite compound rather than a solid solution between (Sr,Ca)3(PO4)2 and (Sr,Ca)2SiO4. The crystal structure of Sr4Ca(PO4)2(SiO4) has been firstly analysed by the difference electron map, and further resolved by the Rietveld refinement, and the final composition has been determined as Sr4Ca(PO4)(2+x)(SiO4)(1−x)(OH)x (x = 0.64) with a hexagonal cell (P63/m). The Ce3+/Eu2+ codoped Sr4Ca(PO4)2SiO4 phosphors have been designed and prepared by the solid state method, and the photoluminescence tuning from blue to green upon 365 nm ultraviolet (UV) radiation can be realized, which is ascribed to the energy transfer from Ce3+ to Eu2+ ions. The luminescence properties and the energy transfer mechanism in Ce3+/Eu2+ codoped Sr4Ca(PO4)2SiO4 phosphors have been discussed, which might act as potential candidates for blue-green components in UV-pumped white light emitting diodes (WLEDs).

A series of CaZnOS:Ce3+,Na+,Mn2+ phosphors were successfully synthesized by a conventional solid-state reaction and their luminescence properties were investigated in detail. The phosphors can be excited by blue light at 455 nm, matching well with the commercial blue light emitting diode (LED) chips. Tunable full-color emission can be realized by combining the blue chips (455 nm), yellow emission (about 530 nm) ascribed to Ce3+ and red emission (about 590 nm) originating from Mn2+ in the CaZnOS host. Energy transfer (ET) from Ce3+ to Mn2+ in the CaZnOS host was validated by the variation of emission intensities as well as the decay lifetimes of Ce3+ ions with increasing Mn2+ concentration, and the ET mechanism is analyzed and ascribed to be a quadrupole–quadrupole interaction. The composition-controlled CaZnOS:0.03Ce3+,0.03Na+,0.03Mn2+ phosphor shows improved chromaticity characteristics fabricated using a blue LED chip, with an Ra of 82.5, a CCT of 5582 K and color coordinates (0.33, 0.32) at the forward current of 30 mA. We demonstrate that CaZnOS:Ce3+,Na+,Mn2+ show potential applications as a single-component white-light emission phosphor for fabricating white LED devices.

Multifunctional materials based on rare earth ion doped ferro/piezoelectrics have attracted considerable attention in recent years. In this work, new lead-free multifunctional ceramics of Ca1−x(LiHo)x/2Bi4Ti4O15 were prepared by a conventional solid-state reaction method. The great multi-improvement in ferroelectricity/piezoelectricity, down/up-conversion luminescence and temperature stability of the multifunctional properties is induced by the partial substitution of (Li0.5Ho0.5)2+ for Ca2+ ions in CaBi4Ti4O15. All the ceramics possess a bismuth-layer structure, and the crystal structure of the ceramics is changed from a four layered bismuth-layer structure to a three-layered structure with the level of (Li0.5Ho0.5)2+ increasing. The ceramic with x = 0.1 exhibits simultaneously, high resistivity (R = 4.51 × 1011 Ω cm), good piezoelectricity (d33 = 10.2 pC N−1), high Curie temperature (TC = 814 °C), strong ferroelectricity (Pr = 9.03 μC cm−2) and enhanced luminescence. These behaviours are greatly associated with the contribution of (Li0.5Ho0.5)2+ in the ceramics. Under the excitation of 451 nm light, the ceramic with x = 0.1 exhibits a strong green emission peak centered at 545 nm, corresponding to the transition of the 5S2 → 5I8 level in Ho3+ ions, while a strong red up-conversion emission band located at 660 nm is observed under the near-infrared excitation of 980 nm at room temperature, arising from the transition of 5F5 → 5I8 levels in Ho3+ ions. Surprisingly, the excellent temperature stability of ferroelectricity/piezoelectricity/luminescence and superior water-resistance behaviors of piezoelectricity/luminescence are also obtained in the ceramic with x = 0.1. Our study suggests that the present ceramics may have potential applications in advanced multifunctional devices at high temperature.

Novel self-activated yellow-emitting BaLuAlxZn4−xO7−(1−x)/2 photoluminescent materials were investigated by a combined experimental and theoretical analysis. The effects of Al/Zn composition modulation, calcination atmosphere and temperature on the crystal structure and photoluminescence properties have been studied via engineering oxygen vacancies. Accordingly, BaLuAl0.91Zn3.09O7 prepared in an air atmosphere was found to be the stable crystalline phase with optimal oxygen content and gave a broad yellow emission band with a maximum at 528 nm. The self-activated luminescence mechanism is ascribed to the O-vacancies based on the density functional theory (DFT) calculation. A theoretical model originating from the designed oxygen vacancies has been proposed in order to determine the influence of O-vacancies on the band structure and self-activated luminescence. Therefore, the appearance of a new local energy level in the band gap will cause the wide-band optical transitions in the studied BaLuAlxZn4−xO7−(1−x)/2 materials.

A design principle for the chemical unit cosubstitution of [Al3+–F−] for [Si4+–O2−] has been adopted to establish the phase relations between isostructural (Sr,Ba)3SiO5 and (Sr,Ba)3AlO4F, and solid solutions of (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ with tunable luminescence properties have been prepared based on the comparative “one-step” and “two-step” methods. The structures and photoluminescence tuning of the (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ phosphors were investigated, and it was found that enhanced emission intensities could be obtained using the two-step method with high crystalline quality. The emission peaks of the (1 − x)Sr3SiO5–x(Sr,Ba)3AlO4F:Ce3+ phosphors were blue-shifted from 520 to 490 nm upon increasing the content from x = 0 to x = 1.0, and the corresponding mechanism has been discussed. The results of the temperature-dependent luminescence properties further verified that the formation of a solid solution can improve the thermal stability of the photoluminescence. White light-emitting diode (w-LED) lamps were fabricated based on the as-prepared bluish-yellow emitting phosphors, and commercial red phosphors, combined with 415 nm NUV-emitting InGaN chips. The best packaged w-LED lamp gave CIE chromaticity coordinates of (0.38, 0.42) with a warm color temperature of 4277 K and a color rendering index of 86.3.

Garnets have the general formula of A3B2C3O12 and form a wide range of inorganic compounds, occurring both naturally (gemstones) and synthetically. Their physical and chemical properties are closely related to the structure and composition. In particular, Ce3+-doped garnet phosphors have a long history and are widely applied, ranging from flying spot cameras, lasers and phosphors in fluorescent tubes to more recent applications in white light LEDs, as afterglow materials and scintillators for medical imaging. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sublattice. The flexibility in phosphor composition and the tunable luminescence properties rely on design and synthesis strategies for new garnet compositions with tailor-made luminescence properties. It is the aim of this review to discuss the variation in luminescence properties of Ce3+-doped garnet materials in relation to the applications. This review will provide insight into the relation between crystal chemistry and luminescence for the important class of Ce3+-doped garnet phosphors. It will summarize previous research on the structural design and optical properties of garnet phosphors and also discuss future research opportunities in this field.

Yb3+/Er3+ co-doped La2.4Mo1.6O8 up-conversion phosphors with thermometric properties were synthesized by a conventional solid-state reaction. The optimal synthesis conditions for the pure phase have been determined by varying the types and contents of fluxes. Structural studies verify that all samples retain the same face-centered cubic structure, and the morphology via SEM observation also verified the cubic-like crystal growth behaviour. Yb3+/Er3+ co-doped La2.4Mo1.6O8 phosphors mainly emit green emission peaks, ascribed to the emission from the excited states 2H11/2 and 4S3/2 to the ground state 4I15/2 of Er3+ ion, and the weak red emission from the excited state 4F9/2 to the ground state 4I15/2 of Er3+ ion when they are excited by 980 nm laser diodes. The up-conversion process has been discussed in detail by pump-power dependence of luminescence intensities and further explained via the energy level diagram. The temperature-dependence properties of La2.4Mo1.6O8 phosphors were investigated in detail, which show their potential prospect in temperature-sensing applications.