EPR Investigation of the Structure of a Rhombic Co2+ Center in an NaF Crystal

The local structure of the rhombic Co2+ center in NaF crystal is investigated by using the perturbation formulas of electron paramagnetic resonance (EPR) gfactors gi(i = x,y, z) and hyperfine constants Ai for a 3d7 (Co2+) ion in rhombic octahedral symmetry based on a cluster approach. In these formulas, the contributions from the admixture among different states, covalency effect as well as rhombic crystal field are included. By studying the EPR data of the rhombic Co2+ center, one can reasonably obtain the local structural (or rhombic distortion) parameters ΔRc(≈ 0.268 Å) for the central Co2+ and ΔRp(≈ 0.181 Å) for the two F− ions along [100] and [010] axes closest to the Na+ vacancy VNa. The reasonableness of the results is also discussed.