• Energy Research

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 ∼8.5 to ∼5.5 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 → 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 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.

A model of conductivity of nanocomposite ceramics consisting of solid-electrolyte and dielectric phases is proposed based on the assumption that the conductivity of grain boundaries between the solid-electrolyte and dielectric phases is higher than the conductivity of the volume of particles in the solid-electrolyte phase and its grain boundaries. Taking into account the size of particles, the thickness of grain boundaries, and the bulk and grain-boundary conductivities, the grain size of composite ceramics for which the conductivity may exceed the conductivity of single-phase solid-electrolyte ceramics is assessed. For testing this model, the composite samples are synthesized based on dielectric magnesium oxide and solid-electrolyte cerium oxide doped with samarium oxide. It is shown that introduction of 50 mol % magnesium oxide into composite ceramics has virtually no effect on its conductivity as compared with single-phase solid-electrolyte ceramics. This result can be explained by assuming the appearance of accelerated transport routes for oxygen ions in grain boundaries between dielectric and solid-electrolyte phases. Further dispersion, optimization of the ratio, and increase in distribution homogeneity of components can confirm the validity of the proposed conductivity model and open up the possibility of preparation of oxide solid-electrolyte materials with higher conductivity.