Band Gap Engineering and Trap Depths of Intrinsic Point Defects in RAlO3 (R = Y, La, Gd, Yb, Lu) Perovskites

The work was supported by the Polish National Science Centre (Project No. 2018/31/B/ST8/00774), by the NATO SPS Project G5647, and by the Ministry of Education and Science of Ukraine (Project DB/Kinetyka no. 0119U002249). L.V. acknowledges support of the National Research Foundation of Ukraine under Grant No. 2020.02/0373 “Crystalline phosphors’ engineering for biomedical applications, energy saving lighting and contactless thermometry”. Researchers from Tartu were supported by the ERDF fundings in Estonia granted to the Centre of Excellence TK141 “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics (HiTechDevices)” (Grant No. 2014-2020.4.01.15-0011) and Estonian Research Council Grant PRG-629. The Institute of Solid State Physics, University of Latvia as the Center of Excellence acknowledges funding from the H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under Grant Agreement No. 739508, Project CAMART2. N.K. was supported by the National long-term project No. WQ20142200205 (Recruitment Program of Global Experts, PRC). Authors are thankful to George Loutts from Norfolk State University, United States, and Dorota Pawlak from Institute of Electronic Materials Technology, Poland for providing some single crystals studied in the work, as well as to Kirill Chernenko from FinEstBeAMS of MAX IV for his assistance with synchrotron experiments.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,Eg, 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 ofEgreduction from ∼8.5 to ∼5.5 eV in RAlO3perovskites with increasing R3+ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3crystals (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 → Gd → 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 YAland VY, while shallower electron traps of 0.9-1.0 eV were related to the energy level of YAlantisite complexes with neighboring VOor (VO+ VY). The effect of the lowering of electron trap depths in RAlO3was demonstrated for the VO-related level of the (YAl+ VO+ VY) complex defect for the particular case of La substituting Y. © 2021 The Authors. Published by American Chemical SocietyNATO SPS G5647; National Research Foundation of Ukraine 2020.02/0373; Polish National Science Centre 2018/31/B/ST8/00774; Eesti Teadusagentuur PRG-629; Latvijas Universitate 739508, WQ20142200205; Institute of Solid State Physics, Chinese Academy of Sciences; Ministry of Education and Science of Ukraine 0119U002249; European Regional Development Fund 2014-2020.4.01.15-0011, TK14

Investigations of LiNb1−xTaxO3 nanopowders obtained with mechanochemical method

Nanocrystalline compounds LiNb1−xTaxO3 of various compositions (x = 0, 0.25, 0.5, 0.75, 1) were synthesized by high-energy ball milling of the initial materials (Li2CO3, Nb2O5, Ta2O5) and subsequent high-temperature annealing of the resulting powders. Data on the phase composition of the nanopowders were obtained by X-ray diffraction methods, and the dependence of the structural parameters of LiNb1−xTaxO3 compounds on the value of x was established. As a result of the experiments, the optimal parameters of the milling and annealing runs were determined, which made it possible to obtain single-phase compounds. The Raman scattering spectra of LiNb1−xTaxO3 compounds (x = 0, 0.25, 0.5, 0.75, 1) have been investigated. Preliminary experiments have been carried out to study the temperature dependences of their electrical conductivit

Mechanosynthesis, Structure and Photoluminescent Properties of the Pr³⁺ Doped LiNbO₃, LiNbO₃:Mg, LiTaO₃ Nanopowders

In the current work, nanocrystalline powders with different compositions, namely Li0.98Pr0.02NbO3, Li0.93Pr0.02Mg0.05NbO3 and Li0.98Pr0.02TaO3 were synthesized for the first time using the method of high-energy ball milling of the starting materials (Li2CO3, Nb2O5, Ta2O5, MgO, Pr6O11), followed by high-temperature annealing. XRD data analysis confirmed the absence of parasitic phases in the obtained nanocrystalline compounds. The estimated particle sizes ranged from 20 to 80 nm. From the obtained nanopowders, ceramic samples were prepared using specially developed equipment, which allowed for pressing at elevated temperatures with a simultaneous application of a constant electric field. The obtained photoluminescence spectra exhibit characteristic features of Pr3+ ions in the crystal structure of LiNbO3 and LiTaO3 and are most efficiently excited by UV light. Samples pressed with an electric field application show higher intensity of photoluminescence. Investigations of the temperature dependence of electrical conductivity of the Li0.98Pr0.02NbO3 sample, pressed with the application of an electric field, indicate that the conductivity mechanism is similar to that of LiNbO3 single crystals and, at high temperatures, is attributed to the lithium conduction mechanism