Theoretical investigations of the local structures and spin Hamiltonian parameters for the trigonal Co 2+ centers in LiNbO 3 and LiTaO 3

Abstract

The local structures and spin Hamiltonian parameters ( g factors g //, g ⊥ and the hyperfine structure constants) for the trigonal Co 2+ centers in LiNbO 3 and LiTaO 3 are theoretically studied from diagonalization of the 6 × 6 energy matrix within 4T 1 ground state for a 3d 7 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 Co 2+ 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 LiNbO 3 (or LiTaO 3). 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.