Optical thermometry has attracted many studies for non-contact high-resolution real-time temperature sensing. Most promising approaches are based on the ratio of up-converted luminescence intensities of two thermally coupled excited states. Here, we proposed a new strategy utilizing the temperature dependence of the anti-Stokes luminescence by exciting a thermally populated low-lying state to the excited state. Our scheme not only retains the advantage of previous approaches in reducing noise from the Stokes-type stray light, but also has the advantage of high quantum yield as a result of a one-photon excitation process. The temperature-dependent luminescence of Tb3+, Ho3+ codoped YPO4 is employed to demonstrate our scheme. The results show that, under a certain excitation, the emission of Tb3+ enhances dramatically while that of Ho3+ declines with increasing temperature. The sharp temperature-dependent intensity ratio was used to calibrate temperature. A maximum relative sensitivity of 2.51% K−1 at 310 K was obtained, substantially superior to values previously reported for acknowledged optical thermometry phosphors. These results indicate that the YPO4:Tb3+,Ho3+ can be a promising candidate to achieve accurate optical temperature sensing with a high sensitivity, and the mechanism proposed can be used to develop better optical thermometry.

•Novel transparent glass-ceramics Na5Yb9F32: Er3+ was successfully fabricated.•The glass-ceramics have already formed during melt-quenching process.•The structural and morphology properties of the samples were analyzed.•Both up-conversion and down-conversion are greatly enhanced after heat treatment.•Temperature dependent of GC680 indicates their good optical thermometry property.Novel self-crystallized transparent Na5Yb9F32: Er3+ glass-ceramics (GCs) were successfully elaborated through high temperature melt-quenching route for the first time. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and photoluminescence emission spectra confirmed that the Na5Yb9F32 nanocrystals (NCs) were well-formed without any additional phase. In addition, X-ray photoelectron spectroscopy (XPS) was studied for investigating valence states of elements in the Na5Yb9F32: Er3+ GCs and it was suggesting that Er3+ ions was doped into the Na5Yb9F32: Er3+ GCs. Interestingly, the XRD patterns and TEM images demonstrate that the Na5Yb9F32 NCs have already formed during melt-quenching process without any further heat treatment. Due to improved crystallinity, reduced surface-to-volume ratio and more Er3+ incorporation into NCs after further heat treatment, both up-conversion (UC) and down-conversion (DC) show enhanced luminescence intensity after heat treatment. The emission color of Na5Yb9F32 GCs changed under different ultraviolet excitation wavelength was addressed and interpreted by photoluminescence. Furthermore, via the fluorescence intensity ratio (FIR), the temperature-dependent green UC emissions of Na5Yb9F32: Er3+ GCs were studied under 980 nm laser excitation. Provided with broad operating temperature range (300–773 K), large energy gap of thermal coupled energy levels (835 cm−1) and high sensitivity (1.33% K−1 at 300 K), the Na5Yb9F32: Er3+ GCs may have potential application in temperature senor.

Eu3+/Nd3+-codoped Ba2LaF7 transparent bulk glass ceramics were successfully fabricated by glass self-crystallization. The structure and morphology of the sample were investigated by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and selected area electron diffraction. The fluorescence intensity ratios of Nd3+ emission at 800 nm to the Eu3+ emission at 699 nm (5D0 → 7F4) were measured under 578.3 nm laser excitation in a wide temperature range from 290 to 740 K. A relatively good temperature sensing performance was obtained with a maximum relative sensitivity of 1.02% K−1 at 420 K. Both the emission peaks for temperature sensing were located in the optical window of biological tissue, which is favorable for biomedical applications. The results indicate that Ba2LaF7:Nd3+/Eu3+ glass ceramics have a potential application as temperature probes.

Non-contact optical thermometry using rare-earth materials has attracted a lot of attention due to its realization of non-invasive and real-time temperature determination. In the current work, a new mechanism, differing from the conventional approach utilizing the ratio of intensities emitted from two thermally coupled excited levels, was developed and demonstrated for temperature sensing using Eu3+-doped Ca3Sc2Si3O12 (CSSO). Under the excitation of 610.6 nm-wavelength light, Eu3+ ions at the 7F2 multiplet became excited to the 5D0 state, and then the luminescence intensity originating from the 5D0 state increased significantly as the temperature was increased from 123 K to 273 K. The thermoequilibrium of the 7F2 multiplet with the 7F0 ground state at a weak excitation ensured a steady increase of the luminescence intensity I with temperature T, which well fit the equation I = A exp (−B/T) for the transitions to both 7F1 and 7F4 multiplets. A relative sensitivity SR of 1008/T2 was obtained for the 7F1 case, with a value of 1.35% at 273 K. This scheme, as a result of detecting the blue-shifted emission, has the advantages of being less disturbed by stray light from the host and the object of the thermometry. In addition, the high quantum efficiency of a one-photon excited photoluminescence scheme has the advantage of improving the resolution of the thermometry. Furthermore, a near-infrared broadband emission observed in the sample can be adopted as a reference, so as to transform the scheme into one using a luminescence intensity ratio. These results indicated that CSSO:Eu3+ may be used in practical temperature sensing applications.

Rare-earth doped CaIn2O4 phosphors have been widely investigated due to their excellent luminescent property, but the site occupation of rare-earth ions in CaIn2O4 is not very clear and needs to be clarified. Using Eu3+ as a fluorescence probe, such a clarification has been made in this work. 1% and 2% Eu3+ doped CaIn2O4 powder samples have been prepared by the sol–gel method. The X-ray diffraction results indicate that the lanthanide doping does not influence the structure of CaIn2O4. Site selective excitation at low temperature disclosed five different luminescent centers marked as A, B, C1, C2 and C3. The spectral analysis revealed that the A and B sites belong to Eu3+ embedded in In3+ sites; the other three are attributed to Eu3+ substitution on Ca2+ sites, which show slight distortion. Energy transfers from the B site to the A and C1 sites were observed in the 2% Eu3+ doped CaIn2O4 sample. The transitions of Eu3+ ions in the Ca2+ sites make the main contribution to the emission spectra excited at room temperature. These results may provide a guide for optimizing rare-earth doped CaIn2O4 phosphors for their application in the solid state lighting field.

In general, adjusting the composition of a fluorescent material is an effective way to tune its luminescent properties such as peak energy and bandwidth. In most solid-solutions, the emission peak shifts linearly with the materials’ composition, which is referred to as Vegard’s Law. However, we found extraordinary variations in our samples Ba2xSr2–2xV2O7, that is, both the excitation and emission peaks show nonlinear dependence on the composition x, and the same is true for the spectral bandwidths. The nonlinearities are not due to structural anomaly, as all the samples are confirmed to be solid-solutions by X-ray diffraction measurements. To explain these phenomena, we proposed a model by considering the disorder of Ba2+ and Sr2+ distributions in solid-solutions and the changes of configurations between the ground and excited electronic states. This novel phenomenon could be applied to further exploit new fluorescent materials.

•Transparent GdPO4 glass-ceramics were successfully fabricated.•Their structural and luminescence properties were systematically investigated.•Energy transfer Gd3+→Tb3+ and Yb3+ pair →Tb3+ give a dual mode luminescence.•The hysteresis plot analysis result indicates their good paramagnetic property.Tb3+ and Yb3+ ions co-doped transparent phosphate glass-ceramics containing monoclinic GdPO4 nanocrystals were successfully synthesized using a conventional high temperature melting quenching method in air atmosphere. The structure and morphology properties were systematically analyzed by recording X-ray diffraction patterns and transmission electron microscopy images, which indicate the GdPO4 glass-ceramics was well formed. The luminescent properties of the GdPO4: Yb3+, Tb3+ were investigated based on excitation, emission spectra and decay curves. All samples exhibited the emission of Tb3+ ions from 5D4 → 7FJ (J = 6, 5, 4 and 3) and 5D3 → 7FJ (J = 6, 5 and 4) under irradiation with ultraviolet and near infrared light. Specially, strong green up-conversion emission of Tb3+ at 543 nm (5D4 → 7F5) is observed. And the decay curves illustrate more efficient UC process in GdPO4 glass-ceramics rather than precursor glass. Energy transfer from Gd3+ to Tb3+ as well as cooperative energy transfer from Yb3+ pair to Tb3+ gives a dual mode luminescence. Laser power dependence of up-conversion shows that two-photon process is responsible for the green emission as expected. Additionally, the paramagnetic property of this material was discussed. The hysteresis plot (M−H) analysis results at room temperature indicate their good paramagnetic property. Based on the above results, the block GdPO4 glass-ceramics have potentials to serve as multifunctional materials applied in laser field materials.

•The luminescence thermometry based on rise time is a reliable method.•The rise time of the 5D0 state is temperature dependent in range of 330–510 K.•The highest relative sensitivity could reach 2.2% K−1 at 490 K.•BaY2ZnO5:Eu3+ has advantage of being applicable in luminescence thermometry.Lanthanide-based luminescence thermometry, superior to conventional invasive thermometers, has evoked considerable interest in the field of temperature evaluation for practical applications. Herein, the emission rise time of the 5D0 level in Eu3+ doped BaY2ZnO5 phosphor was verified to be temperature dependent in a wide range from 330 K to 510 K with a high sensitivity. It exhibited a tremendous decrease with temperature elevating, varying from 207 μs at 330 K to 16 μs at 510 K, and the absolute sensitivity was about −1.1 μs K−1, while the highest relative sensitivity could reach 2.2% K−1 at 490 K. Furthermore, the detailed mechanism was carefully discussed with the experimental data, and this strong temperature dependence was explained in terms of thermal quenching. These results indicate that the accurate temperature evaluation can be realized by singly monitoring the emission rise time of the 5D0 state, and BaY2ZnO5:Eu3+ phosphor has the specific advantage of being applicable in luminescence thermometry.

Dy3+/Tb3+ codoped CaMoO4 phosphors were synthesized by a simple sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy. The energy transfer process of Dy3+→Tb3+ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results verified that the efficient emission of Tb3+ was sensitized by Dy3+ under the excitation of 354 nm, realizing tunable emission in CaMoO4 phosphors. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb3+: 5D4→7F5 (~546 nm) and Dy3+: 4F9/2→6H13/2 (~575 nm). It is expected that the investigated CaMoO4 nanograins doped with Dy3+/Tb3+ have prospective applications in display technology and optical thermometry.

Sm2+-doped SrB4O7 was synthesized for high-sensitivity thermometry. A high thermal-sensitive fluorescence intensity ratio and fluorescence lifetime were achieved in a wide temperature range. At 500 K, the relative sensitivity of the temperature sensing was 2.16% K–1 for the fluorescence intensity ratio and 3.36% K–1 for the fluorescence lifetime. Furthermore, the fluorescence color-shifted dramatically from deep red at room temperature to green at 700 K. On the basis of this color change, a visible temperature field was obtained on quartz glass covered with our sample, which made the thermal conduction and distribution visible to the human eye. The temperature of the temperature field was determined using two methods. These outstanding properties, combined with the high sensitivity, multimode for temperature sensing and thermal stability of the sample, make SrB4O7:Sm2+ a promising material for highly sensitive thermometry applications.Keywords: decay lifetime; fluorescence color; fluorescence intensity ratio; high sensitivity; temperature field; thermometry;

•A new optical temperature sensing based on thermal population of 7F2 state in Eu3+ ions was proposed.•Thermal coupling of 7F0 and 7F2 of Eu3+ ions was discussed.•High relative sensitivity of 3.93% K−1 is achieved at 150 K.The thermal coupling property of the ground state 7F0 and its adjacent thermally-excited state 7F2 of Eu3+ ion was firstly discussed and a new temperature sensing mechanism was proposed based on the thermal population of 7F2 state in Y2O3: 5% Eu3+ powder sample. Under a constant 611.2 nm excitation, the fluorescence originating from 5D0 state increases monotonously as the temperature rises in the region from 150 K to 300 K. The reason leading to this temperature-dependent fluorescence behavior was analyzed in detail. It is demonstrated that this present work provides a new mechanism for non-contact optical thermometry.

An abnormal fluorescence intensity ratio (FIR) between two green emissions of Er3+ at room temperature, which is larger than a normal value, emerged in many reported articles. However, up to now detailed work has seldom been done to clarify this abnormal phenomenon. In this paper, green upconversion luminescence of the β-NaLuF4:20%Yb3+,2%Er3+ powder sample was investigated under 980 nm excitation at different circumstances, different pump power densities and different temperatures as well as different air pressures. The corresponding local temperature calculated using FIR technique increased gradually with the enhancement of the pump power density. It was demonstrated that high pump power density of 980 nm laser led to the increase of local temperature of the luminescent material, which further gave the abnormal FIR.Normalized UC spectra of the as-prepared sample under 980 nm excitation with power density of 2 mW/mm2 at 79.1 °C and 62.5 mW/mm2 at room temperature

Nano-sized CaMoO4 phosphors tri-doped with Er3+, Yb3+ and Tm3+ ions were successfully synthesized by sol-gel method. Intense blue emission from Tm3+ ions was observed upon excitation of 1532 nm infrared light in Er-Yb-Tm system, while this blue upconversion could not be achieved with the absence of Yb3+ ions in Er-Tm co-doped sample. In order to understand this upconversion process, the upconversion spectra in these samples were investigated, and the possible mechanism was proposed based on experimental results. It showed that two different energy transfer from Er3+ to Tm3+ existed simultaneously in Er-Yb-Tm system, the one-step direct energy transfer from Er3+ to Tm3+, and the two-step Er3+→Yb3+→Tm3+ energy transfer. In particular, the 1G4 state of Tm3+ could only be populated from the 3H4 state by cross-relaxation with an excited Yb3+ ion, producing blue emission of Tm3+. In this upconversion process, Yb3+ ions acted as an energy transfer bridge between Er3+ and Tm3+, which also meant that the upconversion of other rare-earth ions under the excitation of 1532 nm was possible with the presence of Er3+ and Yb3+.Schematic illustration of the upconversion mechanism for the Er3+, Yb3+, Tm3+ co-doped CaMoO4 sample under 1532 nm excitation (The dashed, dashed-dotted, curved, and full arrows indicate one-step energy transfer, two-step energy transfer, multiphonon relaxation, and emission processes, respectively)

Yb3+/Er3+ co-doped transparent oxyfluoride glass ceramics (GC) have been successfully fabricated and characterized for the purpose of optical thermometry by measuring the green upconverted fluorescence intensity ratio (FIR) between the two thermally coupled levels (2H11/2 and 4S3/2). The green upconverted emissions in the GC are enhanced by 15 times compared to that of the precursor glass. The dependence of the FIR on temperature ranging from 298 K to 693 K is well fitted with an exponential function by using an effective energy difference of 775 cm−1. Owing to its low phonon energy and high thermal stability, an improved measurement accuracy and widened operating temperature range can be achieved with the GC sample, suggesting that transparent glass ceramics containing NaYF4:Yb3+/Er3+ nanocrystals are promising for application as optical-fiber temperature sensors.

•Yb2(MoO4)3 which was introduced by molybdenum doping was discovered in YbAG: Er, Mo.•Yb2(MoO4)3 was found to be the origin of the anomalous upconversion luminescence.•The previously reported upconversion mechanism in YbAG: Er, Mo has been analyzed.In molybdenum (Mo6+) doped upconversion (UC) materials, such as Yb3Al5O12 and Yb2Ti2O7, anomalous UC luminescence has been discovered recently. In this paper, Yb2.75−xEr0.25MoxAl5O12 phosphors with different Mo-content were synthesized via the sol-gel method. According to the analysis of the structural components and the luminescence spectra of the samples, we found that the Yb2(MoO4)3 impurity which was introduced into YbAG sample by Mo6+ doping is responsible for the anomalous intense UC luminescence. Quantitive analyses on the dependence of UC luminescence intensity upon pump power indicate UC green luminescence is a two-photon process for Yb2(MoO4)3: Er3+ 1% and Yb2.25Er0.25Mo0.5Al5O12 systems, which is far less than the lowest excited state level of MoO42−.

In order to obtain a single-host-white-light phosphor, a series of Ba1.8–w–x–y–zSrwLi0.4–xCexEuyMnzSiO4 (BSLS:Ce3+,Eu2+, Mn2+) powder samples were synthesized via high temperature solid-state reaction. The structure and photoluminescence properties were investigated. Under ultraviolet excitation, the emission spectra contained three bands: the 370–470 nm blue band, the 470–570 nm green band and the 570–700 nm red band, which arose from the 5d→4f transitions of Ce3+ and Eu2+, and the 4T1→6A1 transition of Mn2+, respectively. The excitation spectra of the emissions of Ce3+ and Mn2+ ions showed the energy transfer from Ce3+ to Mn2+. White light emission was obtained from the tri-doped samples of appropriate doping concentration under 310–360 nm excitation.CIE chromaticity coordinate points of the samples Ba1.245 Sr0.4Li0.3Ce0.10Eu0.005Mn0.05SiO4 (sample 1) and Ba1.243Sr0.4Li0.3Ce0.10Eu0.007Mn0. 05SiO4 (sample 2) under various excitation wavelengths increasing from 310 to 370 nm along the direction of the arrow

Rare earth doped luminescent materials are promising candidates for temperature sensing. In this paper, pure β-NaYF4:20%Yb3+, 2%Er3+ microprisms were synthesized by hydrothermal method. The temperature dependence of the fluorescence intensity ratio (FIR) of emission bands centered at 525 nm and 545 nm was measured in the temperature range of 160–300  K under excitation of a 980 nm diode laser. The monotonous increase of FIR with temperature shows that this material can be used for temperature sensing. The dependence of the FIR on temperature is fitted with an exponential function and the effective energy difference obtained is 752 cm−1, which gives further a temperature sensitivity of 1.20% K−1 at 300 K. The FIR technique was then used to study the effect of heating caused by laser excitation under various powers.

Y2O3:Eu3+ nano-powders were synthesized by homogeneous precipitation and the influence of solution acidity was investigated. IR and TG–DTA analysis showed that in different acidic solutions different kinds of sediments were generated. TEM images revealed distinct morphologies of obtained powders. The resultant powders were sintered into ceramics in vacuum and then in N2 atmosphere without any additives. SEM images indicated that the morphologies of powders have significant impact on the microstructures of sintering-derived ceramics. Low-agglomerated and uniform powders were in favor of production of ceramics with homogeneous microstructures. Excitation and emission spectra of both powders and ceramics were measured and some changes were observed.

Y2O3:Eu3+ powders were prepared by combustion method with and without gel process. The crystal structures and morphologies of the powders were studied by XRD, SEM and HRTEM. The combustion-derived powders were sintered into ceramics in vacuum and then in N2 atmosphere without any additives. SEM images indicate that the ceramics derived from different powders have different microstructures. Excitation and emission spectra were recorded for both powders and ceramics. Pronounced changes in excitation spectra of them were observed.

The red long-lasting phosphor Mg2SiO4:Dy3+, Mn2+ was prepared by solid state reaction, the luminescence and afterglow properties of which were investigated. The emission spectrum of Mg2SiO4:Mn2+ shows a broad band centered at about 650 nm, which is attributed to the 4T1(4G) → 6A1(6S) transitions of Mn2+ ions occupying two nonequivalent Mg2+ sites. The excitation bands of 650 nm emission in near ultraviolet and visible region were assigned. Mg2SiO4:Mn2+ samples co-doped with Dy3+ show higher initial phosphorescence intensity and longer afterglow time than Mn2+ doped ones. Both of them represent red afterglow. In addition, investigation of the effects of Dy3+ ions in co-doped samples was conducted, indicating that Dy3+ ions serve as trap centers.

In this work, a red long-lasting phosphor: MgSiO3:Eu2+, Dy3+, Mn2+ was prepared by sol–gel method. The XRD pattern of this phosphor is indexed on a proto-enstatite structure (JCPDS No. 11-0273). Its red long-lasting phosphorescence was confirmed and attributed to the Mn2+ emission. In MgSiO3:Eu2+, Dy3+, Mn2+, the functions of co-doped Eu2+ and Dy3+ ions are affirmed as sensitizers and trap centers, respectively. Further TL study was taken on the trap centers introduced by Dy3+, including fitting of trap depths and frequency factors. Moreover, a long-lasting phosphorescence model of MgSiO3:Eu2+, Dy3+, Mn2+ was suggested.

Nanocrystalline Lu2O3 doping with different Eu3+ concentrations was prepared by co-precipitation method. The crystal structure and morphology were analyzed by means of XRD and HRTEM. The resultant powders were sintered into translucent ceramics in vacuum and then in nitrogen without any additive. The surface morphology of the unpolished sintered specimens was characterized using SEM. The excitation and emission spectra of Lu2O3:Eu3+ powders and ceramics were measured at room temperature by using synchrotron radiation as the light source. Pronounced changes of the spectroscopic properties with increasing Eu3+ content were observed and the reasons were studied. The fluorescent lifetimes of nanopowders shortened obviously with increasing Eu content, but no such effect was observed for the sintered ceramics. Luminescence of ceramics samples decays faster than the corresponding nanopowders.

Nanocrystalline Lu2O3:Eu3+ was prepared by co-precipitation method using ammonium hydrogen carbonate and ammonium oxalic acid as precipitants, respectively. The crystal structure and morphology were analyzed by means of XRD and TEM. The resultant powders were sintered into transparent ceramics in vacuum and then in nitrogen without any additive. The surface morphology of the unpolished sintered specimens was characterized using SEM. The effect of different precipitants on microstructure of the nanopowders and transparency of the ceramics are compared. The excitation and emission spectra of Lu2O3:Eu3+ powders and ceramics were measured at room temperature by using synchrotron radiation as the light source. The fluorescence decay times of all specimens were analyzed. Luminescence of the ceramics decays faster than the corresponding nanopowders.

Low temperature photoluminescence spectra were measured under the selective excitation to the energy levels of Gd3+, Er3+ and Sm3+, respectively. Corresponding transitions were assigned with the help of fluorescence decay measurements. By assuming that the rare earth ion is in C2v symmetry instead of the actual C2, crystal field calculations were carried out based on the deduced experimental energy level data of Er3+ and Sm3+. The theoretical result is in good agreement with the experimental data.

Nanoscale Lu2O3:Eu powders were prepared by solution combustion synthesis. X-ray diffraction (XRD), high-resolution electronic microscope (HREM), Fourier transform infrared spectroscopy (FT-IR), excitation and emission spectra, as well as fluorescent decay curves were measured to characterize the structure and luminescent properties of the samples. The results show that the compound of composition Lu2O3 crystallizes in pure cubic structure. By changing the ratio of glycine to nitrate in the combustion process, the particle size varies from 40 nm to less than 5 nm. The emission and excitation spectra strongly depend on the particle size of the samples. Novel emission band, red-shift of charge transfer band (CTB) and shortening of lifetime were observed in nanoscale samples.Excitation spectra of CS-made nanocrystalline Lu2O3:Eu 2% with different size.

The luminescent properties of (NH4)3[Cr(OH)6Mo6O18]·nH2O were investigated for the first time. It was determined that two centers, single Cr3+ ion and Cr3+ ion pair, are responsible for the luminescence properties of this compound. By conducting time-resolved and temperature-dependent emission spectra, energy transfer between the two centers was observed and analyzed. A transfer rate kC equal to 4.6×104 s−1 was determined.

Emission and absorption spectra of Tm3+ in single crystal K2YF5 with different concentrations are reported and analyzed. The non-exponential feature of the fluorescence decay of 1D2 level for the high concentration sample indicates the existence of a cross-relaxation process. Energy level simulation has been carried out by using a phenomenological model with 16 parameters accounting simultaneously for the free ion and crystal field (CF) effects. A good fit was achieved with a root mean square deviation σ=18 cm−1. The free ion and CF effects were then discussed by comparing the available data of K2YF5:Nd3+, LiYF4:Tm3+ and LiYF4:Nd3+.

Optical properties of Nd3+ doped into K2YF5 single crystal have been analysed. A detailed electronic energy level scheme of Nd3+ ion in the crystalline structure has been deduced from the absorption, emission and time resolved spectra recorded at 12 K. Energy level simulation of Nd3+ ions in K2YF5 has been carried out and the phenomenological crystal field parameters were determined. The root mean square (r.m.s.) standard deviation is 19.0 cm−1, indicating a satisfying agreement between the calculated and experimental levels. The results are also compared to those reported for BaY2F8:Nd3+ and LiYF4:Nd3+ crystals, and the tendency of a covalence and crystal field decreasing effect was observed from K2YF5, BaY2F8 to LiYF4.

Crystals of K2Y0.999Pr0.001F5 and K2Y0.98Pr0.02F5 were grown by the hydrothermal synthesis technique. The luminescent properties are investigated by performing low-temperature selective excitation. With laser excitation set at 583.7 nm, blue emission from 3P0 level of Pr3+ ions in K2YF5:Pr is observed and analyzed via an up-conversion model. Crystal field calculation of Pr3+ ions in K2YF5 has been performed and the phenomenological crystal field parameters determined. The root mean square standard deviation is 17.6 cm−1, indicating a satisfying agreement between the calculated and experimental levels.

Analysis of the excitation of Eu3+ ions in site A and site B in L–EuBO3 and L–GdBO3:Eu3+ at several temperatures have shown very efficient energy transfer and reciprocal processes between both sites. At 12 K, the transfer from site A to site B dominates the process and results in the fluorescence quenching of site A in L–EuBO3. At higher temperature, 77 and 300 K, the transfer and back transfer have nearly an equal rate and accordingly the same spectra from both sites are obtained despite of exciting site A or site B.

BaMgAl10O17:Eu2+,Yb3+ was investigated as a possible quantum cutting system to enhance solar cells efficiency. Phosphors were synthesized by combustion method and composed of nanorods. Photoluminescence spectra showed that Eu in the sample was reduced to bivalence while Yb remained trivalence. Through a cooperative energy transfer process, the obtained powders exhibited both blue emission of Eu2+ (around 450 nm) and near infrared emission of Yb3+ (around 1020 nm) under broad band excitation (250–410 nm) originating from 4f→5d transition of Eu2+. Energy transfer phenomenon between the sensitizer Eu2+ and the activator Yb3+ was investigated via the luminescent spectra and the decay curves of Eu2+ with different Yb3+ concentrations. Results indicated that energy transfer efficiency from Eu2+ to Yb3+ was not high. The poor efficiency can be explained by the long distance between rare earth ions.

Rare-earth doped CaIn2O4 phosphors have been widely investigated due to their excellent luminescent property, but the site occupation of rare-earth ions in CaIn2O4 is not very clear and needs to be clarified. Using Eu3+ as a fluorescence probe, such a clarification has been made in this work. 1% and 2% Eu3+ doped CaIn2O4 powder samples have been prepared by the sol–gel method. The X-ray diffraction results indicate that the lanthanide doping does not influence the structure of CaIn2O4. Site selective excitation at low temperature disclosed five different luminescent centers marked as A, B, C1, C2 and C3. The spectral analysis revealed that the A and B sites belong to Eu3+ embedded in In3+ sites; the other three are attributed to Eu3+ substitution on Ca2+ sites, which show slight distortion. Energy transfers from the B site to the A and C1 sites were observed in the 2% Eu3+ doped CaIn2O4 sample. The transitions of Eu3+ ions in the Ca2+ sites make the main contribution to the emission spectra excited at room temperature. These results may provide a guide for optimizing rare-earth doped CaIn2O4 phosphors for their application in the solid state lighting field.