Crystal structure, powder X-Ray diffraction, Hirshfeld, thermal, Infrared-Raman studies, and symmetry analysis of the relationship between the novel tetragonal [(CH3)4N]2SiF6 and the cubic Fm3m antifluorite structure

Abstract

In the present paper a novel tetragonal form of the [(CH3)4N]2SiF6 compound is investigated and compared with its already studies cubic antifluorite form (Fm3m). The single crystal and powder X-ray diffractions showed that the novel compound form crystallizes at ambient temperature in the tetragonal space group I4/m (Z = 4). The comparable space groups and parametrs obtained from single crystal and powder diffractions checked the purity of the tetragonal form. The importance of H…F/F…H (55%) and H…H (45%) intercontacts in the crystal packing are determined by the 3D Hirshfeld surfaces and the 2D fringplots analyses. Based on the Landau theory analysis, the novel tetragonal form may be derived from the antifluorite cubic form by a transition made at the Γ-Point of the Brillouin zone of the high-symmetry phase Fm3m. The lowering symmetry distortion (Fm3m→ I4/m) could be due to the anions and cations rotations around the c axis [001]. The DSC analysis showed two endothermic anomalies (12 °C/-2 °C) and (155 °C/150 °C) observed in heating/cooling, indicating that [(CH3)4N]2SiF6 exhibits two continuous reversible phase transitions of second order character. The optimal molecular geometries and thermodynamic properties of [(CH3)4N]2SiF6 have been determined by the PM3 and DFT/B3LYP 6-31G** methods. The calculated vibrational frequencies were generally closed to the experimental frequencies; besides the IR and Raman spectra are interpreted on the basis of the theoretical group analysis, considering the local symmetries S4 and C4h for the cations and anions, respectively.