Symmetry-breakingTa4+centers inKTaO3

Abstract

A study of photoinduced ${\mathrm{Ta}}^{4+}$ centers in nominally pure ${\mathrm{KTaO}}_3$ single crystals has been carried out by electron-spin resonance. Two of these centers $({\mathrm{Ta}}^{4+}-V_{\mathrm{O}}$ and ${\mathrm{Ta}}^{4+}-V_{\mathrm{O}}-{\mathrm{Me}}^{4+})$ are connected with vacancies of oxygen ${(V}_{\mathrm{O}});$ a third center is associated with an ${\mathrm{OH}}^{-}$ molecular ion $({\mathrm{Ta}}^{4+}-{\mathrm{OH}}^{-}).\mathrm{}$ This assignment is made on the basis of concentration measurements of the corresponding centers after annealing in argon, oxygen, hydrogen, and ${\mathrm{H}}_2\mathrm{O}$ vapor atmospheres. It has been shown that the ${\mathrm{Ta}}^{4+}$ centers are shallow donors; at $T>~30$ K they are ionized and transformed into ordinary $V_{\mathrm{O}}$ and ${\mathrm{OH}}^{-}$ which are assumed to be the main lattice defects before illumination. Their energy levels are determined by the temperature dependence of the relaxation rate of the light-induced nonequilibrium localized electron population. The energy levels of ${\mathrm{Ta}}^{4+}{-V}_{\mathrm{O}}$ and ${\mathrm{Ta}}^{4+}{-V}_{\mathrm{O}}-{\mathrm{Me}}^{4+}$ centers are situated at 26 and 8 meV below the bottom of the conduction band, respectively. The symmetry of the centers is inverse broken in the sense that the photoelectron is localized near one of two equivalent ${\mathrm{Ta}}^{5+}$ ions next to an oxygen vacancy or ${\mathrm{OH}}^{-}.$ The role of $V_{\mathrm{O}}$ defects and ${\mathrm{OH}}^{-}$ molecules in the nucleation of local polar clusters in nominally pure ${\mathrm{KTaO}}_3$ crystals at low temperatures is discussed.