Theoretical studies of the spin Hamiltonian parameters and the local structures for the tetragonal Cu2+ centers in the LKB glasses

The spin Hamiltonian parameters (g factors g∥, g⊥ and the hyperfine structure constants A∥, A⊥) for the Cu2+ centers in the lithium potassium borate (LKB) glasses xLi2O·(30 − x)·K2O·70B2O3 (0 ≤ x ≤ 25) were theoretically studied using the high-order perturbation formulas of these parameters for a 3d9 ion in a tetragonally elongated octahedron. The [CuO6]10− clusters in the LKB glasses are found to suffer the relative elongations of about 3% along the tetragonal axis due to the Jahn–Teller effect. The concentration dependences of the g factors are illustrated by the approximately linear decrease of the cubic field parameter Dq as well as the increases of the covalency factor N and the relative elongation ratio ρ due to the slight expansion of the cell volume or bond lengths with increasing the Li2O concentration x. Meanwhile, the slow non-linear increases of the hyperfine structure constants are described as the rough exponential increase of the core polarization constant κ with x due to the increase of the tetragonality of the systems. The theoretical spin Hamiltonian parameters and their concentration dependences show good agreement with the experimental data. To evaluate validity and applicability of the present theoretical model and formulas, the EPR results of the Cu2+ centers in similar lithium sodium borate (LNB) xLi2O·(30 − x)·Na2O·70B2O3 (5 ≤ x ≤ 25 mol%) glasses are also analyzed and compared with those in the LKB systems using the uniform model and formulas.