Theoretical studies of EPR parameters and defect structures for Ni2+ ions in corundum

In this paper, fourth-order perturbation formulae of zero-field splitting (ZFS) parameter D and g-factors g//, g⊥ for ground state 3A2g of 3d8 ions in the trigonal crystal field (CF) have been derived by perturbation theory method (PTM), and the complete energy matrices (45×45) including low symmetry ligand field (C3v), Coulomb interactions and spin-orbital (SO) coupling interaction for 3d8 ions have been constructed, in the strong-field scheme. In these formulae, both the contributions from the SO of the central 3d ion and the ligands taken into account by using the two SO coupling parameter model, and the contributions to the ZFS from all excited states are included. These PTM formulae and complete energy matrices are applied to investigate the EPR parameters D and g-factors for Ni2+ ions in Al2O3 crystals unifiedly, and the calculated results of PTM and complete diagonalization method (CDM) are not only close to each other but also in good agreement with the experimental data, and the local structure of [NiO6]10− cluster is determined quantitatively. Furthermore, the validities of the present formulae are tested further by the comparison of calculated results of the formula and these of CDM based on intermediate –CF scheme, and the comparison shows that the calculated results with these two different methods are in mutually good agreement with each other in all different trigonal crystal environments, which also suggest that these fourth-order perturbation formulae are suitable for explaining both D and g-factors for 3d8 ions in trigonally distorted octahedral field.

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