Theoretical investigations of the local structures and spin Hamiltonian parameters for the trigonal Co2+ centers in LiNbO3 and LiTaO3

The local structures and spin Hamiltonian parameters (g factors g//, g⊥ and the hyperfine structure constants) for the trigonal Co2+ centers in LiNbO3 and LiTaO3 are theoretically studied from diagonalization of the 6 × 6 energy matrix within 4T1 ground state for a 3d7 ion under trigonal symmetry. The contributions from the admixtures of various J (=1/2, 3/2, 5/2) states and the ligand orbitals and spin–orbit coupling interactions, which were usually ignored in the previous studies, are taken into account in this work. From the calculations, the impurity Co2+ is found not to occupy exactly the host Li+ site but to undergo an off-center displacement 0.06 Å (or 0.03 Å) away from the oxygen octahedron center in LiNbO3 (or LiTaO3). The theoretical spin Hamiltonian parameters based on the above local structures show good agreement with the experimental data. The various contributions to these parameters are also discussed.