Modeling local structure using crystal field and spin Hamiltonian parameters: the tetragonal FeK3+-OI2- defect center in KTaO3 crystal

The local structure and the spin Hamiltonian (SH) parameters, including the zero-field-splitting (ZFS) parameters D and (a + 2F/3), and the Zeeman g factors g(parallel to) and g(perpendicular to), are theoretically investigated for the Fe-K(3+)-O-I(2-) center in KTaO3 crystal. The microscopic SH (MSH) parameters are modeled within the framework of the crystal field (CF) theory employing the CF analysis (CFA) package, which also incorporates the MSH modules. Our approach takes into account the spin-orbit interaction as well as the spin-spin and spin-other-orbit interactions omitted in previous studies. The superposition model (SPM) calculations are carried out to provide input CF parameters for the CFA/MSH package. The combined SPM-CFA/MSH approach is used to consider various structural models for the Fe-K(3+)-O-I(2-) defect center in KTaO3. This modeling reveals that the off-center displacement of the Fe3+ ions, Delta(1)(Fe3+), combined with an inward relaxation of the nearest oxygen ligands, Delta(2)(O2-), and the existence of the interstitial oxygen O-I(2-) give rise to a strong tetragonal crystal field. This finding may explain the large ZFS experimentally observed for the Fe-K(3+)-O-I(2-) center in KTaO3. Matching the theoretical MSH predictions with the available structural data as well as electron magnetic resonance (EMR) and optical spectroscopy data enables predicting reasonable ranges of values of Delta(1)(Fe3+) and Delta(2)(O2-) as well as the possible location of O-I(2-) ligands around Fe3+ ions in KTaO3. The defect structure model obtained using the SPM-CFA/MSH approach reproduces very well the ranges of the experimental SH parameters D, g(parallel to) and g(perpendicular to) and importantly yields not only the correct magnitude of D but also the sign, unlike previous studies. More reliable predictions may be achieved when experimental data on (a + 2F/3) and/or crystal field energy levels become available. Comparison of our results with those arising from alternative models existing in the literature indicates considerable advantages of our method and presumably higher reliability of our predictions.