Theoretical studies of the local structures and EPR parameters for various Rh2+ centers in AgCl

The local structures and electron paramagnetic resonance (EPR) parameters (the g factors, the hyperfine structure constants and the superhyperfine parameters) for various Rh2+ centers in AgCl are theoretically studied using the perturbation formulas of these parameters for a 4d7 ion in tetragonally and orthorhombically distorted octahedra. These defects include the tetragonally and orthorhombically elongated centers at low temperatures (labeled as TL and OL) and the tetragonally compressed center at high temperature (labeled as TH). The centers TL and OL are attributed to the substitutional Rh2+ on Ag+ site, associated with none and one next nearest neighbour cation vacancy VAg along [1 0 0] (or X) axis, respectively. The ligand octahedron in the orthorhombic center OL is found to suffer the relative elongation ΔZOL ≈ 0.079 Å along [0 0 1] (or Z) axis due to the Jahn-Teller effect, and the intervening ligand Cl− in the VAg and the Rh2+ may experience the inward displacement ΔX ≈ 0.008 Å towards the central Rh2+. In the center TL, the uncompensated [RhCl6]4− cluster is found to suffer the relative elongation ΔZTL ≈ 0.116 Å along [0 0 1] axis due to the Jahn-Teller effect. As for the center TH, one VAg occurs in the next nearest neighbour site along [0 0 1] axis, which may compel the intervening Cl− in the VAg and the Rh2+ to shift towards the central Rh2+ by an amount ΔZH ≈ 0.112 Å. In the calculations of the superhyperfine parameters, the related molecular orbital coefficients and the ligand unpaired spin densities are determined quantitatively from the cluster approach in a uniform way. The theoretical EPR parameters based on the above local structures show good agreement with the experimental data for all the centers.