By calculating the optical spectrum band positions and EPR parameters (𝑔 factors, $g_{\parallel}$ , $g_{\perp}$ and zero-field splitting 𝐷) by diagonalizing the complete energy matrix of 3d⁸ ions in trigonal symmetry, the defect structure of Ni²⁺ centre in 𝛼-LiIO₃ crystal is studied. It is found that to reach the good fits of optical and EPR data between calculation and experiment, the Ni²⁺ ion should shift by $\Delta z \approx 0.298$ Å along C₃ -axis and the O²⁻ ions between the Ni²⁺ ion and Li⁺ vacancy (V$_{Li}$) should be displaced away from the V$_{Li}$ by $\Delta x \approx 0.097$ Å because of the electrostatic interaction. The results are discussed.

The EPR parameters (𝑔 factors $g\parallel$, $g\perp$ and zero-field splitting 𝐷) of Mn⁴⁺ ion in h-BaTiO₃ crystal are calculated from the complete high-order perturbation formulas based on a two-mechanism model for the EPR parameters of $3d^{3}$ ions in trigonal symmetry. In the model, not only the widely used crystal-field mechanism, but also the charge-transfer mechanism (which is not considered in crystal-field theory) are included. The calculated results are in reasonable agreement with the experimental values. The relative importance of charge-transfer mechanism to EPR parameters and the defect structure of Mn⁴⁺ centre in h-BaTiO₃ crystal obtained from the calculations are discussed.