Theoretical investigations on the defect structures and spin Hamiltonian parameters for various orthorhombic Rh2+ centres in KTiOPO4 and KTiOAsO4

The defect structures and spin Hamiltonian parameters (SHPs) for three Rh2+ centres (denoted C1 in KTiOAsO4 and C2 and C3 in KTiOPO4) are theoretically investigated by utilising the perturbation formulae for a 4d7 ion under orthorhombically (D2h) elongated octahedra. The defect structures are characterized by the axial elongation ratios of 4.91%, 4.93% and 4.90% along the Z axis and the planar bond length variation ratios of 0.05%, 0.01% and 0.04% for centres C1, C2 and C3, respectively, owing to the Jahn-Teller effect. The nearly identical moderate axial elongation ratios and the slightly different tiny planar bond length variation ratios may suitably account for the comparable moderate axial g anisotropies ∆g (≈0.6087, 0.6124 and 0.6067) and the slightly dissimilar tiny perpendicular g anisotropies δg (≈0.0649, 0.0097 and 0.0509) of the three centres, respectively. All centres demonstrate similar strong crystal-field interactions and moderate covalence arising from the comparable short impurity-ligand distances.