Abstract: The quantitative relationship between the EPR zero-field splitting parameter D and the structure parameters of the tetragonal Cr³⁺ centre in KCdF₃:Cr³⁺ crystals has been established according to the superposition model and third-order perturbation theory. The existence of Cd²⁺ vacancy and the lattice distortion have been verified. Meanwhile, we observed that the F^- ion moves toward the central Cr³⁺ ion by Δ₁=0.00294nm, Δ₂==0.0010nm, Δ₃==0.0028nm (see Fig.2). Good agreement between the theoretical values and the experimental results shows that the assumption of the Cd²⁺ vacancy and the lattice distortion is reasonable. Although the main source of the tetragonal crystal field comes from the Cd²⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion can not be neglected.

Key words: KCdF₃:Cr³⁺ crystals, EPR parameter D, The charge compensation, Cd²⁺ vacancy, Lattice distortion