Symmetry-breaking Ta4+ centers in KTaO3

A study of photoinduced Ta4+ centers in nominally pure KTaO3 single crystals has been carried out by electron-spin resonance. Two of these centers (Ta4+-V-0 and Ta4+-V-0-Me4+) are connected with vacancies of oxygen (V-0); a third center is associated with an OH- molecular ion (Ta4+-OH-). This assignment is made on the basis of concentration measurements of the corresponding centers after annealing in argon, oxygen, hydrogen, and H2O vapor atmospheres. It has been shown that the Ta4+ centers are shallow donors at T greater than or equal to 30 K they are ionized and transformed into ordinary V-0 and 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 Ta4+-V-0 and Ta4+-V-0-Me4+ 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 Ta5+ ions next to an oxygen vacancy or OH-. The role of V-0 defects and OH- molecules in the nucleation of local polar clusters in nominally pure KTaO3 crystals at low temperatures is discussed. [S0163-1829(98)00225-2].