Structure of transition-metal—oxygen-vacancy pair centers

The microscopic structure of the ${\mathrm{Fe}}^{3+}$-$V_{\mathrm{O}}$ and ${\mathrm{Mn}}^{2+}$-$V_{\mathrm{O}}$ pair centers has been considered using the Newman superposition model. This model yields the EPR parameter $b_2^0$ as a function of transition-metal-ion-oxygen distance $d$ and position. It is found, for the oxide perovskites, that the transition-metal ion moves by a distance $\Delta =0.1\mathrm{}$ $d=0.2\mathrm{}$ Å towards the vacancy $V_{\mathrm{O}}$. The four lateral oxygens move against the metal ion by 4% or 0.08 Å. This is in agreement with the reduction of the metal ionic radius due to the five-fold coordination. The intrinsic two-center parameter ${\overline{b}}_2$ needed in the Newman model is based on the uniaxial strain data of MgO and SrTi${\mathrm{O}}_3$, as well as on the measured spin-Hamiltonian parameter $b_2^0$ in the tetragonal phase of SrTi${\mathrm{O}}_3$. The ${\overline{b}}_2$ parameters obtained also provide information on other centers reported in oxide and fluoride compounds.