Local structure of the rhombic Fe3+ center in KTaO3

A study is reported of the effect of high-temperature annealing in oxygen, an inert gas, and water vapor on EPR spectra of the Fe3+ center of differen t local symmetries in the incipient ferroelectric KTaO3. An analysis of the relations obtained permits one to propose and substantiate a model of the rhombic Fe3+ center, by which Fe3+ substitutes for Ta5+ near two oxygen vac ancies (Fe3+ -2V(O)). Calculations of the crystal-field parameters performe d within the Newman superposition model showed in the rhombic center the Fe 3+ ion is displaced along [011] from the Ta5+ position it occupies within t he tetrahedron formed by four oxygens, to a distance of about 0.25 Angstrom . Some of the recent results obtained in second-harmonic light scattering i n iron-doped KTaO3 samples are interpreted. It is shown that, within the te mperature range of 4.2-300 K, rhombic Fe3+ centers are static electric dipo les, and that they cannot therefore be a source of dielectric losses in KTa O3 at T approximate to 40 K, as suggested earlier in some publications. (C) 1998 American Institute of Physics. [S1063-7834(98)01212-X].