Energy structure of KTaO3 and KTaO3:Li

Ab initio energy structure calculations of pure ${\mathrm{KTaO}}_{3}$ (KTO) and ${\mathrm{KTaO}}_{3}$ doped with Li (KTL) were performed using the CASTEP code [M.C. Payne, M.P. Teter, D.C. Alan, T.A. Arias, and J.D. Joannopoulos, Rev. Mod. Phys. 64, 1045 (1992)] based on the density functional theory (DFT) and the intermediate neglect of the differential overlap INDO method. The optimized lattice parameters of perfect KTO calculated in this work are in a good agreement with the available experimental data. The population analysis shows that KTO has a mixed ionic-covalent type of bonds. DFT calculations for the KTL have been performed for a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}2$ supercell of KTO with one ${\mathrm{Li}}^{+}$ substituting ${\mathrm{K}}^{+}.$ The ${\mathrm{Li}}^{+}$ off-center ion and nearest oxygen displacements are defined. It was shown that oxygen displacements around ${\mathrm{Li}}^{+}$ off-center ions lead to the production of two local states in the bandgap at 60 meV and 90 meV from the top of the valence band, which in all probability play an important role in the photostimulated processes.