Investigations of the EPR parameters and local structures for the substitutional Cu2+centers in the tungstates

Abstract

The high order perturbation formulas of the electron paramagnetic resonance (EPR) parameters (the g factors and the hyperfine structure constants) are established for a 3d(9) ion in orthorhombically compressed octahedra. These formulas are applied to the investigations of the EPR spectra and local structures for the substitutional Cu(2+) centers in the wolframite-type tungstates AWO(4) (where A = Cd, Zn, Mg). Based on the studies, the [CuO(6)](10-) clusters are found to experience the local planar bond length variations delta R' (approximate to 0.096, 0.021 and 0.028 angstrom for CdWO(4), ZnWO(4) and MgWO(4), respectively) along X and Y-axes. These values are quite different from the host planar bond length variations delta R (approximate to 0.013, 0.069 and 0.005 angstrom) for the A(2+) sites in the pure tungstates. The above local bond length variations in the impurity centers can be attributed to the Jahn-Teller effect and size mismatching substitution. The theoretical EPR parameters based on the above local structures agree well with the experimental data. By adopting the uniform theoretical formulas and fewer adjustable parameters in this work, the improvements are achieved for the EPR parameters as compared with the previous calculation results.