Structure of transition-metal—oxygen-vacancy pair centers

The microscopic structure of the Fe³⁺-VO and Mn²⁺-VO pair centers has been considered using the Newman superposition model. This model yields the EPR parameter b₂⁰ 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 Δ=0.1 $d=0.2$ {} towards the vacancy VO. The four lateral oxygens move against the metal ion by 4% or 0.08 {}. This is in agreement with the reduction of the metal ionic radius due to the five-fold coordination. The intrinsic two-center parameter b̄₂ needed in the Newman model is based on the uniaxial strain data of MgO and SrTiO₃, as well as on the measured spin-Hamiltonian parameter b₂⁰ in the tetragonal phase of SrTiO₃. The b̄₂ parameters obtained also provide information on other centers reported in oxide and fluoride compounds.