Study of EPR parameters and defect structure for two tetragonal impurity centers in MgO:Cr3+ and MgO:Mn4+ crystals

The electron paramagnetic resonance (EPR) parameters (g-factors g (aEuro-), g (aSyen) and zero-field splitting D) of two tetragonal 3d(3) impurity centers M-3d-V-Mg and M-3d-Li+ (where M-3d = Cr3+ or Mn4+, V-Mg is the Mg2+ vacancy) in M-3d-doped MgO crystals are calculated from the high-order perturbation formulas including both the crystal-field (CF) and the charge-transfer (CT) mechanisms for 3d(3) ions in the tetragonal symmetry. The calculated results are in reasonable agreement with the experimental values. From the calculations, it can be found that the relative importance of the CT mechanism for EPR parameters increases with increasing valence state of the 3d(3) ion. So, for the high-valence 3d (n) ions in crystals, a reasonable explanation of EPR parameters should take into account both CF and CT mechanisms. The defect structures (characterized by the displacement Delta R of O2- in the intervening M-3d and V-Mg or Li+ at the Mg2+ site) for these tetragonal impurity centers are obtained from the calculations. The results are consistent with the expectations based on the electrostatic interactions.