SYMMETRY-BREAKING TA4+ CENTERS IN KTAO3

A study of photoinduced Ta4+ centers in nominally pure KTaO3 single cr ystals has been carried out by electron-spin resonance. Two of these c enters (Ta4+-V-0 and Ta4+-V-0-Me4+) are connected with vacancies of ox ygen (V-0); a third center is associated with an OH- molecular ion (Ta 4+-OH-). This assignment is made on the basis of concentration measure ments of the corresponding centers after annealing in argon, oxygen, h ydrogen, and H2O vapor atmospheres. It has been shown that the Ta4+ ce nters are shallow donors at T greater than or equal to 30 K they are i onized and transformed into ordinary V-0 and OH- which are assumed to be the main lattice defects before illumination. Their energy levels a re determined by the temperature dependence of the relaxation rate of the light-induced nonequilibrium localized electron population. The en ergy levels of Ta4+-V-0 and Ta4+-V-0-Me4+ centers are situated at 26 a nd 8 meV below the bottom of the conduction band, respectively. The sy mmetry of the centers is inverse broken in the sense that the photoele ctron is localized near one of two equivalent Ta5+ ions next to an oxy gen vacancy or OH-. The role of V-0 defects and OH- molecules in the n ucleation of local polar clusters in nominally pure KTaO3 crystals at low temperatures is discussed. [S0163-1829(98)00225-2].