Local-structure model of rhombic-symmetry Fe3+ centre in KTaO3

An influence of high temperature annealing in oxygen, argon and H2O vapour on electron spin resonance (ESR) spectra of Fe3+ in different local environ ments has been studied in incipient ferroelectric KTaO3. An analysis of the experimental data allowed to suggest and consider the model of the rhombic Fe3+ centre: Fe3+ near two oxygen vacancies is substituted for Ta5+ (Fe3+- 2V(O)). The calculation of the crystal field parameters within the framewo rk of the Newman linear-superposition model showed that Fe3+ is shifted by 0.25 Angstrom from the Ta5+ site in the [011] direction inside the oxygen t etrahedron. The interpretation of recent second harmonic generation (SHG) r esults of Fe-doped KTaO3 is reported. It is shown that these rhombic Fe3+ c entres are the static dipoles in the temperature range 4.2-450 K and thus t hey cannot be the source of dielectric losses at 40 K in KTaO3, which was a ssumed earlier. (C) 1999 Elsevier Science Ltd. All rights reserved.