Bulk modulus and high-pressure crystal structures of tetrakis(trimethylsilyl)methane C[Si(CH₃)₃]₄ determined by X-ray powder diffraction

The pressure dependence of the crystal structure of cubic tetrakis(trimethylsilyl)methane C[Si(CH₃)₃]₄ (TC) (P <16.0 GPa, T = 298 K) is reported using high-resolution angle-dispersive X-ray powder diffraction. The compound has crystal structures with the molecules in a cubic-close-packed (c.c.p.) arrangement. It shows three phase transitions in the measured pressure range. At ambient conditions, TC has space group Fm3m (Z = 4) with a = 12.8902 (2) Å, V = 2141.8 (1) ų (phase I). Between 0 and 0.13 GPa TC exhibits a first-order phase transition into a structure with space group Pa3 (phase II). A second first-order phase transition occurs between 0.2 and 0.28 GPa into a structure with space group P2₁3 (phase III). Under non-hydrostatic pressure conditions (P > 10 GPa) a transformation is observed into a c.c.p. structure that is different from the face-centred-cubic structure at ambient conditions. A non-linear compression behaviour is observed, which could be described by a Vinet relation in the range 0.28-4.8 GPa. The extrapolated bulk modulus of the high-pressure phase III was determined to be K₀ = 7.1 (8) GPa. The crystal structures in phase III are refined against X-ray powder data measured at several pressures between 0.49 and 4.8 GPa, and the molecules are found to be fully ordered. This is interpreted to result from steric interactions between neighbouring molecules, as shown by analysing the pressure dependence of intramolecular angles, torsion angles and intermolecular distances. Except for their cell dimensions, phases I, II and III are found to be isostructural to the corresponding phases at low temperatures.