A semi-empirical analysis of ZFS parameters for transition metal (TM) ions (Cr3+ and Mn2+) centers in K2MgF4

The local structure around the transition metal (TM) ions (Cr³⁺ and Mn²⁺) centers in K₂MgF₄, namely tetragonal (TE) center I, monoclinic (MO) center II and orthorhombic (OR) centers III, have been explored by semi-empirical calculations in the frame of superposition model (SPM). We proposed the most appropriate structural model for each center by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. It is shown that the tetragonality of MgF₆ octahedra increases with the substitution of both Cr³⁺ and Mn²⁺ for Mg²⁺ sites as well as with a rather higher value for the latter one. A tilt angle of 12.51° is found for MO Cr³⁺ center, which is comparatively small, as compared to the other A₂BF₄ crystals such as K₂ZnF₄ and Tl₂ZnF₄. Almost three times larger relaxation of intervening F-ligand than that of other equatorial F-ligands is also indicated for OR Cr³⁺ center.