The EPR 𝑔 factors $g_{i}$ $(i = x, y, z)$ for the interstitial Ti³⁺ in rutile are theoretically studied from the perturbation formulas of these parameters for a 3d¹ ion in rhombically compressed octahedra. The ligand octahedron in the impurity center is found to be less compressed than that on the host interstitial site due to the Jahn–Teller effect. The local compression parameter $(\approx 0.026)$ and the rhombic distortion angle $\delta \phi'$ $(\approx 0.7^{\circ})$ around the impurity Ti³⁺ are smaller than the host values ($\approx 0.091$ and 3.5°). The theoretical 𝑔 factors based on the above local structural parameters are in good agreement with the experimental data. In addition, the 𝑔 factors for a tetragonal interstitial Ti³⁺ center are also reasonably interpreted.

The anisotropic 𝑔 factors and local structure for the trigonal Pt³⁺ centre in 𝛼-Al₂O₃ are theoretically investigated from the perturbation formulas of the 𝑔 factors for a $5d^7$ ion in trigonal symmetry. The Pt³⁺ impurity is found to experience an outward displacement by about 0.18 Å away from the centre of the oxygen octahedron along the $C_3$-axis. The calculated g factors based on the above axial displacement show good agreement with the observed values. Importantly, the pending problem of $+3$ valence state of the doped Pt in 𝛼-Al₂O₃ is theoretically clarified, and the possibility of Pt⁺ ($5d^9$) is thus excluded in this work.