Local structure distortion models for Cr3+ centers in Tl2MgF4 and Tl2ZnF4 fluorine compounds

Theoretical analysis of the EPR spectra of Cr3+ centers in Tl2MgF4 (Tl2MgF4 and Tl2ZnF4) fluorine compounds have been carried out for the first time. The correlation between the experimental data and theoretical values regarding zero-field splitting (ZFS) provides suitable structural models to understand the local structure around the Cr3+ centers in Tl2MgF4 and Tl2ZnF4. A clear compression of the MF6 octahedron around tetragonal (TE) Cr3+ center has been shown in both crystals. The calculations for the monoclinic (MO) Cr3+ center reveal that the length of the four equatorial F-ligands (R2) is about 1.5% longer than that of the octahedral ZnF6 in undoped Tl2ZnF4 and the length of the axial F-ligands (R1) is quite shorter (∼9.5%) than R2. Also, it yields a quite large b2−1 and declined angle (27.46°, ∼30%) for z-axis. Our results indicate that the presence of the different structural formations may be considered around the orthorhombic (OR) Cr3+ center III and IV. It was suggested that the latter one can be attributed to an angular distortion relevant to the equatorial F-ligands along z∥[110]-axis.