Theoretical studies of the spin Hamiltonian parameters and local structures for the tetragonal Cu2+ and Ni3+ centers in Mg2TiO4

Highlights: ► Spin Hamiltonian parameters and local structures are analyzed for the impurity Cu2+ and Ni3+ centers in Mg2TiO4. ► Ligand orbital and spin-orbit coupling contributions are included from the cluster approach due to moderate covalency. ► Ligand octahedra experience relative elongations along C4 axis due to Jahn-Teller effect. - Abstract: The local structures of the two impurity Cu2+ (and Ni3+) centers with low spin (S = 1/2) in Mg2TiO4 are theoretically studied by using the perturbation formulas of the spin Hamiltonian parameters for 3d9 (and 3d7) ions in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are quantitatively determined using the superposition model and the local structures of the impurity Cu2+ (and Ni3+) centers, and the ligand orbital and spin–orbit coupling contributions are included on the basis of the cluster approach in view of moderate covalency for the studied systems. The [CuO6]10− and [NiO6]9−clusters on the substitutional Mg2+ site are found to suffer relative elongations by about ΔZCu (≈3.2%) and ΔZNi (≈0.7%) for the impurity Cu2+ and Ni3+ centers, respectively, along the C4 axis due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above Jahn–Teller elongations show good agreement with the observed values. The results are discussed.