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(trimethy lsilyl)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 compou nd has crystal structures with the molecules in a cubic-close-packed (c.c.p .) arrangement. It shows three phase transitions in the measured pressure r ange. At ambient conditions, TC has space group Fm (3) over bar m (Z = 4) w ith a = 12.8902 (2) Angstrom, V = 2141.8 (1) Angstrom(3) (phase I). Between 0 and 0.13 GPa TC exhibits a first-order phase transition into a structure with space group Pa (3) over bar (phase II). A second first-order phase tr ansition occurs between 0.2 and 0.28 GPa into a structure with space group P2(1)3 (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 compr ession 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-pressu re phase III was determined to be K-o = 7.1(8)GPa. The crystal structures i n phase III are refined against X-ray powder data measured at several press ures between 0.49 and 4.8 GPa, and the molecules are found to be fully orde red. This is interpreted to result from steric interactions between neighbo uring molecules, as shown by analysing the pressure dependence of intramole cular 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.