Investigations of electron-paramagnetic-resonance parameters of trigonal (CrO4)4−, (MnO4)3− and (FeO4)2− clusters in Y2SiO4:Cr4+, Ba3(VO4)2:Mn5+ and K2CrO4:Fe6+ crystals

The complete high-order perturbation formulas of EPR parameters (g-shifts Δg∥,Δg⊥ and zero-field splitting D), including not only the crystal-field (CF) mechanism, but also the charge-transfer (CT) mechanism (which is neglected in crystal-field theory), are established by a cluster approach for 3d2 ions in trigonal symmetry. From these formulas, the EPR parameters of Y2SiO4:Cr4+, Ba3(VO4)2:Mn5+ and K2CrO4:Fe6+ are calculated. The results are in reasonable agreement with the observed values. It is found that the relative importance of CT mechanism increases with increasing atomic number of 3d2 ion and so in the studies of EPR parameters for high valence state 3dn ions in crystals, both CF and CT mechanisms should be taken in account.