RESUMEN
Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy; 0 ≤ x ≤ 0.2) phosphors were synthesized by a high-temperature solid-state reaction. Sc : R ratios for the NSP:xR samples were determined by ICP-MS, EDX-SEM and TEM-EDX measurements. An X-ray diffraction study revealed that solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.1. The luminescence properties of Na3Sc2(PO4)3 and Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) were studied in the range of 80-500 K. The highest R3+ luminescence intensity in Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) depending on R was found for x = 0.05 in the case of Dy and x = 0.1 in the case of Eu and Tb. The temperature behaviour of the R3+ emission intensity of Na3Sc2-xRx(PO4)3 (R = Eu, Tb, Dy) depends on R that replaces Sc. The decrease of the Eu3+ emission intensity depending on the transition energy by â¼26% and 18% at â¼420 K compared to TR allowed us to consider NSP:0.1Eu3+ as a suitable phosphor for pc-LEDs. The temperature dependence of the Dy3+ emission for NSP:0.05Dy3+ demonstrates a strong thermal quenching. Different temperature dependences of the Tb3+ emission intensity of NSP:0.1Tb3+ were found for two excitation bands at λex = 220 and 378 nm representing f-d and f-f intracentre transitions. No thermal quenching for f-f transitions takes place while the emission intensity for f-d transitions increases with a temperature rise from 80 to 500 K. The dielectric measurements for Na3Sc2(PO4)3 and Na3Sc1.9Eu0.1(PO4)3 were provided on ceramic pellets sintered under vacuum using a spark plasma sintering technique. Different dependences of conductivity were found for two samples. The calculated conductivity for Na3Sc1.9Eu0.1(PO4)3 with an R3Ìc structure (σbulk = 6.4 × 10-5 S cm-1 at 300 K, 1.14 × 10-3 S cm-1 at 360 K and 5.0 × 10-2 S cm-1 at 500 K) is higher than that for pure α-Na3Sc2(PO4)3 but lower than that for ß- and γ-Na3Sc2(PO4)3.
RESUMEN
The structural and luminescence properties of undoped Y1-xScxPO4 solid solutions have been studied. An intense thermally stable emission with fast decay (τ1/e ~ 10-7 s) and a band position varying from 5.21 to 5.94 eV depending on the Sc/Y ratio is detected and ascribed to the 2p O-3d Sc self-trapped excitons. The quantum yield of the UV-C emission, also depending on the Sc/Y ratio, reaches 34% for the solid solution with x = 0.5 at 300 K. It is shown by a combined analysis of theoretical and experimental data that the formation of Sc clusters occurs in the solid solutions studied. The clusters facilitate the creation of energy wells at the conduction band bottom, which enables deep localization of electronic excitations and the creation of luminescence centers characterized by high quantum yield and thermal stability of the UV-C emission.
RESUMEN
The possibility of band gap engineering (BGE) in RAlO3 (R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials, E g, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect of E g reduction from â¼8.5 to â¼5.5 eV in RAlO3 perovskites with increasing R3+ ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3 crystals (R = Lu, Y, La; MIII = Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO3:Mn4+ phosphors, which confirmed decreasing of the trap depths in the cation sequence Y â Gd â La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO3) were considered to be formed by the energy level of oxygen vacancy (VO) with different arrangement of neighboring YAl and VY, while shallower electron traps of 0.9-1.0 eV were related to the energy level of YAl antisite complexes with neighboring VO or (VO + VY). The effect of the lowering of electron trap depths in RAlO3 was demonstrated for the VO-related level of the (YAl + VO + VY) complex defect for the particular case of La substituting Y.
