The high-order perturbation formulas for spin-Hamiltonian parameters (g-factors and zero-field splitting, D) for 3d3 ions in tetragonal symmetry were established by using a cluster approach. In these formulae, not only was the contribution to the spin-Hamiltonian parameters from the crystal-field mechanism included, but also that arising from the charge-transfer mechanism (which was neglected in the widely used crystal-field theory). From these formulae, the g-shifts and the zero-field splitting, D, for the tetragonal Fe5+ center in SrTiO3 crystals were calculated. The results (in particular, the positive g-shifts, which could not be explained on the basis of the crystal-field mechanism) were in good agreement with the observed values. The results showed that (i) the sign of ΔgiCT (i = || or ┴) due to the charge-transfer mechanism was opposite to that of ΔgiCF due to the crystal-field mechanism, but the sign of DCT was the same as that of DCF and (ii) the ratio |QCT/QCF| (which represents the relative importance of the charge-transfer mechanism) takes values of about 143, 143 and 114% for Q = ∆g||,∆g┴, and D, respectively. This suggested that the positive g shifts were due mainly to the contribution of the charge-transfer mechanism; therefore for the high valence state 3dn ions in crystals, the contribution to spin-Hamiltonian parameters from the charge-transfer mechanism should be taken into account. The defect structure of the Fe5+ center in SrTiO3 crystal was also obtained from the calculations. The result was consistent with the expectations based on charge compensation and electrostatic interaction.

Studies of the Spin-Hamiltonian Parameters and Defect Structure for the Tetragonal Fe5+ Center in SrTiO3 Crystals Using a Two-Mechanism Model. W.C.Zheng, X.X.Wu, W.Fang: Journal of Physics - Condensed Matter, 2007, 19[45], 456214 (6pp)