Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu2+-doped Sr(LiAl)1–xMg2xAl2N4 and Sr(LiAl3)1–y(Mg3Si)yN4 (x, y = 0–1), are successfully synthesized by a solid state reaction. They show narrow-band red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors.

•Afterglow duration is doubled for CaTiO3:Pr3+ by introducing Na+-Nb5+.•Charging wavelength is red-shifted from UV to visible-violet light.•Detailed electronic structure is explored for afterglow explanation.The red persistent luminescence of CaTiO3:Pr3+ needs further improvement in longer excitation wavelength to fulfill the application demand. By Na+-Nb5+ substituting for Ca2+-Ti4+, a series of Pr3+-doped perovskite-type (Ca1-xNax) [Ti1-xNbx]O3:0.001Pr3+ compounds, with a structural transition at about x = 0.7–0.8, have been successfully synthesized by a solid state reaction. The excitation band first red-shift and then blue-shift gradually with x. The afterglow duration has been prolonged from 30 s for CaTiO3:Pr3+ to about 60 s for the composition x = 0.1. Wavelength-dependent trap filling analysis show that the charging wavelengths can be red-shifted from UV irradiation (<358 nm) for CaTiO3:Pr3+ to visible violet light for compositions with x = 0.1. On the basis of spectral data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of trivalent lanthanide-doped (Ca1-xNax) [Ti1-xNbx] O3 compounds, and further explain these afterglow behaviors. This work is helpful for exploring novel persistent luminescent materials.

In this paper, narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor has been successfully prepared using a solid-state reaction method. The effects of sintering temperatures, times and fluxes on phase formation and luminescence properties are investigated, respectively. The addition of BaF2 flux not only enhances room-temperature emission intensity, but also improves the luminescence thermal stability, which is ascribed to the increase of crystallinity. Under blue light excitation, the as-prepared SrLiAl3N4:Eu2+ phosphor has a narrow emission band with a peak wavelength at ∼648 nm and a full-width at half-maximum of ∼1177 cm−1 (∼50 nm). The critical quenching concentration of Eu2+ is about 1 mol%. White light-emitting-diode (w-LED) devices have been fabricated which are obtained by combining a 455 nm chip with the commercial yellow phosphor and the present red phosphor. The results exhibit a potential application for phosphor-converted LEDs (pc-LEDs).

Red-emitting nitride phosphors excited with blue light have great potential for the fabrication of warm white light-emitting diodes (WLEDs). Chemical composition and structural modification are generally adopted to optimize the photoluminescence behaviors of the targeted phosphors. Herein, on the basis of the famous CaAlSiN3 phosphors, Eu2+-doped (Ca1–xLix)(Al1–xSi1+x)N3 solid solutions via the cations’ cosubstitution of (CaAl)5+ pair by (LiSi)5+ pair are successfully synthesized by a solid state reaction, and the lattice parameters show a linear decrease with chemical compositions suggesting the formation of the isostructural phase relationship. Four types of coordinated structure models, corresponding to different coordination environments of Eu2+, are proposed over the course of structural evolution, which induces different structural rigidity and stability, and then they are responsible for three-stage changes of emission spectra of Eu2+ in (Ca1–xLix)(Al1–xSi1+x)N3 solid solution.

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).

•Relationship between polarizability and electronegativity is demonstrated.•The magnitude of the centroid shift can be predicted in nitrides/oxynitrides.•Local structure features of Ce3+ can be predicted based on luminescence spectra.The host structure and composition of rare earth doped phosphors play important roles in tuning luminescence properties. The energy of the 5d levels of Ce3+ in nitrides has recently become available due to the development of nitride phosphors. In this work, we have collected information on 5d levels of Ce3+ in nitrides and obtained a linear relationship between the anion polarizability and the inverse square of the average electronegativity of the cations based on the Dorenbos model. Our result indicates that the magnitude of the centroid shift can be predicted not only in nitrides but also in oxynitrides. Furthermore, the local structural features of Ce3+ can be unveiled on the basis of excitation luminescence spectra; as an example, we show that Ce3+ ions occupy the Sr2+ site rather than the Y3+ site in SrYSi4N7 compound.

•Sr2Si5−xAlxN8−xOx:Eu2+ phosphors have been synthesized by alloy-nitridation method.•Preferential occupation of O atoms is confirmed.•Al and O have an adverse influence on the emission bands.•The energy transfer involves in phonon-assisted processes.•Thermal quenching behaviors were improved after substituting of AlO+ for SiN+.The influence of the replacement of SiN+ by AlO+ on crystal structure and luminescence properties is reported in a series of Sr2Si5−xAlxN8−xOx:0.02Eu2+ phosphors. Changes of refined lattice parameters of the powder X-ray diffraction data suggest that preferential occupation occurred in the evolution of crystal structure: only NII atoms which are connected with two Si atoms can be substituted by O atoms. It is evidenced that Al and O have adverse influence on the emission bands, which just keep the emission peak position around 620 nm. Time-resolved photoluminescence analysis has been employed to describe the energy transfer in Sr2Si5−xAlxN8−xOx:Eu2+ samples. The energy transfer effect depends partly on lattice vibration, i.e. phonon energy. Furthermore, the improved photoluminescence intensity and thermal quenching behavior after substituting of AlO+ for SiN+ allowed Sr2Si5−xAlxN8−xOx:0.02Eu2+ a promising candidate as a red phosphor in the white LED applications when x≤0.4.

The dependences of light efficiency of radiation (LER) and color-rendering index (CRI) of trichromatic white light-emitting diode (wLED), composed of blue LED die, green/yellow, and red phosphors, on the peak wavelength of each primary were investigated by theoretical calculations, at correlative color temperature (CCT) from 2,700 to 6,500 K. The peak wavelength of InGaN based blue LED chip ranges from 450 to 471 nm, while those of Ca3Sc2Si3O12:Ce3+, b-SiAlON:Eu2+, and Y3Al5O12:Ce3+ based green/yellow phosphors range from 511 to 572 nm, and those of Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+ red phosphors range from 620 to 650 nm, which cover almost all the practically used, commercially available wave bands until now. Then, based on the results, selection guides of peak wavelengths for blue LED chip and phosphors to obtain tradeoff LER >280 lm·W−1 as well as CRI >80 in all CCTs are proposed. The favorable wave bands of each primary are suggested.

Using ray-tracing simulation based on Monte Carlo method, the effects of phosphor concentration and thickness on light output and phosphor consumption of pc-wLEDs were investigated in this work. The simulation was improved to comprehensively imitate the whole optical process of pc-wLEDs, including total produce of chip and phosphor light, losses in the propagation, and output. Experiments were conducted to verify the simulation. Results show that, light output changes non-monotonously over phosphor concentration and thickness, having maximum value. Experimental maximum light efficiency of 158 lm·W−1 was obtained at concentration of 16 wt%, 6 % higher than that of 11 wt% and 17 % higher than that of 33 wt%. Phosphor consumption of pc-wLEDs increases linearly with the increase of phosphor concentration and the decrease of thickness. Experimental phosphor consumption of pc-wLEDs with concentration of 11 wt% is only 37 % of that of 33 wt%.

In this study, small- and large-particle-diameter phosphor powders were mixed together (hybrid phosphors) to balance light efficacy and angular color uniformity and pursue optimal results. Phosphor with small-particle-diameter of 4 μm was employed and it was mixed into each large-particle-diameter phosphor of 10, 16, 22, and 26 μm, at mass percentage from 0 % to 50 % with an interval of 10 %, respectively. Remote phosphor package was adopted and overall phosphor concentration was kept constant for better comparison. Moreover, absorption coefficient μabs, scattering coefficient μsca and extinction coefficient μext of each hybrid phosphors were calculated based on Mie theory to further discuss the experiment results. Results show that, the introduction of small-particle-diameter phosphor to large one can highly improve angular color uniformity while only slightly reduce light efficacy. The optimal performance with angular color uniformity of 91.6 % as well as normalized light efficacy of 95.7 % is achieved in the white light emitting diode with hybrid phosphors consisting of 60 wt% powder of 22 μm and 40 wt% powder of 4 μm.

A series of phosphors, Eu-doped Ca(Al/Si)2N2(N1−xOx), derivatives of CaAlSiN3, were synthesized by alloy-nitridation method. We demonstrated that their emission peaks can be tuned from 650 nm to 610 nm by oxygen preferential substitutions of nitrogen located at one of two crystallographic sites. Two luminescent centers corresponding to two types of Eu2+-coordination modes, i.e. EuNI2NII3 and EuNI2NII2O, were identified and accounted for the emission band structures, emission band shifts with oxygen/nitrogen substitutions, and the dependence of peak position and integrated emission intensity on temperature. As a typical example, Ca(Al/Si)2N2(N0.80O0.20):0.02Eu showed intense orange–red emission peaking at 622 nm and kept the feature of excellent chemical stabilities, which would have potential applications in fabricating the white light-emitting diode. The excellent luminescent properties of these materials, such as wavelength-tunable red emission and excellent chemical stabilities, make them practical for use in typical LED package.Highlights► Eu-doped Ca(Al/Si)2N2(N1−xOx) were synthesized by alloy-nitridation method. ► Two types of Eu2+-coordination modes are formed due to preferential occupancy of oxygen. ► Two types of Eu2+-coordination modes account for luminescence properties.

Ce3+-doped Ba9Sc2Si6O24 phosphors with a charge-compensating ion Na+ were prepared by solid-state reaction. The crystal structures of the samples with different Ce3+ concentration were investigated, as well as photoluminescence properties in the near ultraviolet–visible range. From the low temperature excitation and emission spectra of samples with low and high Ce3+ concentrations, we found that Ba2+ ions in three different crystallographic sites were replaced by Ce3+ ions in the host, while two of them were very similar. Accordingly, there are two types of luminescent centers in BSSO:Ce3+ i.e. Ce3+(1) in the Ba(1) site and Ce3+(2) in the Ba(2) and Ba(3) sites. With the increase of Ce3+ concentration, the emission intensity from Ce3+(1) becomes lower compared with that of Ce3+(2) due to the energy transfer between these two centers, which were confirmed by the life-time and low temperature test results.Highlights► Ce3+-doped Ba9Sc2Si6O24 phosphors with a charge-compensating Na+ were prepared. ► Ba9Sc2Si6O24:Ce3+,Na+ has two luminescent centers. ► Energy transfer from Ce3+(1) to Ce3+(2) were confirmed.

A novel synthesis route of Eu2+-doped pure-nitride α-sialons has been reported. It is through an alloy-nitridation method at ∼2173 K in nitrogen atmosphere, with stable alloys (CaAl, SiEu), AlN, and α-Si3N4 powders as starting materials. A linear relationship between the lattice parameters and m values of (Ca0.995Eu0.005)m/2Si12−mAlmN16 compositions is obtained, indicating that our samples contain very little oxygen, i.e. herein so-called Eu-doped pure-nitride α-sialons. The (Ca0.995Eu0.005)m/2Si12−mAlmN16 compounds with 2.4≤m≤4.0 give the strongest emission. The emission shifts to longer wavelength with m values increasing as well as Eu contents increasing. (Ca0.995Eu0.005)m/2Si12−mAlmN16 compositions with smaller m values exhibit better thermal quenching properties.Highlights► Eu-doped pure-nitride α-sialons were fabricated by the alloy-nitridation method. ► Relationship between lattice parameters and compositions confirms that samples contain very little oxygen. ► Luminescence properties were investigated.

Divalent europium-doped alkaline earth metal silicate phosphors, (Ba1−x−ySryEux)9Sc2Si6O24 (x=0.005–0.1, y=0–0.95), have been successfully prepared by solid-state reaction at 1350 °C. The analysis of X-ray diffraction shows that the compounds are in a single phase at the proper concentration of Sr2+. At room temperature, the Eu2+-activated Ba9Sc2Si6O24 phosphor exhibits a single emission band peaking at about 506 nm. With the increasing content of Sr2+, the luminescent intensity of (Ba1−x−ySryEux)9Sc2Si6O24 weakens, and the emission peak shifts towards red. Luminescence concentration quenching occurs when Eu2+ content x is more than 1 mol% in (Ba1−x−ySryEux)9Sc2Si6O24 (y=0/0.2). At low temperatures (Ba0.9−ySryEu0.1)9Sc2Si6O24 (y=0/0.2) phosphors have two emission bands corresponding to different Eu2+ crystallographic sites. The high energy peak (P1) is quenched at room temperature, while the low energy peak (P2) weakens much more slowly owing to the energy transfer from P1 to P2.Highlights► Green (Ba1−x−ySryEux)9Sc2Si6O24 phosphors were synthesized by a solid-state reaction. ► Emission wavelength of (Ba1−x−ySryEux)9Sc2Si6O24 is successfully tuned from 506 nm to 526 nm by substituting Ba2+ with Sr2+ ions. ► Two emission bands correspond to different Eu2+ crystallographic sites.

This work investigates the stability of Eu2+ and Eu3+ in some Sr-based inorganic compounds. Generally reducing condition is adopted in order to obtain Eu2+, however, the Eu doped SrAl2O4/SrLaAlO4 case indicates that for some compounds Eu3+ is stabilized even in reducing atmosphere. Bond valence method is applied to explain this phenomenon and it reveals that crystal structure also determines the valence state of europium cations along with reducing/oxidizing condition. An analysis of other Eu doped Sr-based materials is performed which shows the relationship between Eu2+/Eu3+ stability and the Global Instability Index (GII). This research provides a guideline for synthesizing specific novel Eu2+/Eu3+ phosphors.Highlights► A general rule that controls the stability of Eu2+/Eu3+ is illustrated. ► Bond valence theory is applied to reveal the relation between structure and valence. ► Crystal structure constraints are adopted to explore the valence stability.

We have synthesized CaAlSiN3:Eu phosphors through an alloy-nitridation method with stable alloys as main starting materials, and investigated their crystal structures and photoluminescence properties in three aspects as below: (1) different Eu concentrations; (2) different Eu dopants; (3) different Al/Si molar ratios. The lattice volume calculated by using Rietveld refinement on the base of XRD analysis, decreases with oxygen incorporating, increases with the Al/Si ratios, and shows a limited expansion with the raise of Eu concentrations in CaAlSiN3:Eu crystal lattice. The concentration quenching and the shifts of emission bands are investigated and discussed in details. The different lattice volumes deriving from corresponding compositions have an important effect on photoluminescence properties. This work provides some methods to tune the emission wavelengths of CaAlSiN3:Eu phosphors.Highlights► The CaAlSiN3:Eu phosphors have been synthesized through alloy-nitridation. ► Different Al/Si molar ratios have an important effect on lattice volume and luminescence. ► The incorporation of oxygen results in blue shift of emission.

Oxonitridosilicate phosphors with compositions of (Y1−xCex)2Si3O3N4 (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce3+ ions have substituted for Y3+ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce3+ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce3+ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.

In this paper, narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor has been successfully prepared using a solid-state reaction method. The effects of sintering temperatures, times and fluxes on phase formation and luminescence properties are investigated, respectively. The addition of BaF2 flux not only enhances room-temperature emission intensity, but also improves the luminescence thermal stability, which is ascribed to the increase of crystallinity. Under blue light excitation, the as-prepared SrLiAl3N4:Eu2+ phosphor has a narrow emission band with a peak wavelength at ∼648 nm and a full-width at half-maximum of ∼1177 cm−1 (∼50 nm). The critical quenching concentration of Eu2+ is about 1 mol%. White light-emitting-diode (w-LED) devices have been fabricated which are obtained by combining a 455 nm chip with the commercial yellow phosphor and the present red phosphor. The results exhibit a potential application for phosphor-converted LEDs (pc-LEDs).

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).