Investigation of EPR parameters and the local structure of the rhombic Cu2+ center in zinc fluoride

Based on a cluster approach, the local structure of the rhombic Cu²⁺ center in a ZnF₂ crystal is investigated by using the high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters g factors g _i (i = x, y, z) and hyperfine structure constants A _i in rhombically compressed octahedral symmetry. According to the studies, the local axial distortion angle Δα′(≈ 2.9°) in the impurity center is found to be by ～2.5° larger than the host value, which is characterized as a contraction and stretching of the parallel and perpendicular bond lengths by about 0.116 Å and 0.058 Å respectively. This results in a more compressed ligand octahedron because of the Jahn-Teller effect via the interaction of the vibrations of impurity-ligand bonds in the Cu²⁺ center. The reasonableness of the results is also discussed.