Structure of transition-metal—oxygen-vacancy pair centers

Abstract

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 $\ensuremath{\Delta}=0.1$ $d=0.2$ \AA{} towards the vacancy ${V}_{\mathrm{O}}$. The four lateral oxygens move against the metal ion by 4% or 0.08 \AA{}. 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.