Studies on the EPR parameters and local angular distortion for the tetragonal Cu²⁺ center in CaWO₄ crystal

Abstract

The electron paramagnetic resonance (EPR) parameters-g factors g i (i = jj and ⊥) and hyperfine structure constants A i (M) and A i (N), with M and N belonging to isotopes 63 Cu 2+ and 65 Cu 2+ -and local structure of Cu 2+ ion occupying W 6+ site in CaWO 4 crystal are theoretically studied based on the perturbation formulas of these parameters for a 3d 9 ion under tetragonally elongated tetrahedra. In these formulas, the ligand orbital (LO) and spin-orbit coupling (SOC) contributions are included due to the shorter impurity-ligand distance R (≈1.83 Å) and hence the strong covalency of the studied [CuO 4 ] 6À cluster, and the related molecular orbital coefficients are quantitatively determined from the cluster approach in a uniform way; meanwhile, the required crystal field (CF) parameters for the tetragonally distorted tetrahedron (TDT) are estimated from the superposition model and the local structure of the impurity Cu 2+ center. According to the calculation, the bond angle θ between the four equivalent Cu 2+ -O 2À bonds and the C 4 axis in the CaWO 4 :Cu 2+ is found to be about 2.1 smaller than that (θ 0 ≈ 54.74 ) for an ideal tetrahedron due to the Jahn-Teller (JT) effect and the size mismatch. The fitted results agree well with the observed values, and the validity of the present assignment for the local structure of the Cu 2+ center is also discussed.