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The electron paramagnetic resonance of Gd3+ has been studied in a single crystal of thorium oxysulfide. The experimental results are interpreted using a spin Hamiltonian for tetragonal symmetry, and the crystal-field parameters and the g factor are determined. The very good agreement between the calculated and experimental data shows that the Gd + ion enters substitutionally, and that the charge compensation is far enough away to avoid any distortion of the local symmetry.

A method of calculation of bond parameters is proposed for magnetic molecular orbitals which describe the 3d9 configuration in the case of complexes with a rhombic surrounding (D2h symmetry), when the experimental spin Hamiltonian is known. This treatment contains as a particular case that of Kivelson et al. for square planar structures and allows us to evaluate both the anisotropic presence of unpaired spin at the various ligands, and the mixing of the d3z2−r2 orbital in the ground state which is essentially dx2−y2. Applications of this treatment in calculating the bond parameters in the CuCl2·2H2O and CuF2·2H2O compounds allow a comparison between the covalent characters of the Cu–Cl and Cu–F bonds in a similar surrounding. The results show that there is a relevant spin density at the halide ions in both compounds and that, in CuCl2·2H2O, the anisotropy of covalence in the principal plane of coordination is strong. The reliability of the proposed criteria is considered by a comparison between the covalence values obtained following our method and those obtained directly from the extrahyperfine structure in the case of a square planar copper complex.

The crystal structure of ThS2 has been refined starting from single crystal diffractometry data. The accurate determination of the positional parameters allows us to reexamine the SPM analysis of the second degree crystal field parameters for Gd3+ in ThS2 matrix. The fitting of the EPR data is still possible by means of a suitable two exponents power law for the intrinsic parameter only if the substitutional Gd3+ ion displaces in the mirror (bc) plane with respect to the undistorted Th4+ site. The resulting increase of the mean metal-ligand distances agrees with the expected expansion of the ligand cage around the trivalent Gd ion. The applicability of the proposed method to examine the impurity induced distortions in an host crystal is discussed too.

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