Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals

Acta Cryst. (2003). B59, 60–71

The unit cell and the molecular packing of the low-temperature structure of Si[C(CH₃)₃][Si(CH₃)₃]₃ viewed along the monoclinic axis. Only the most probable orientations of the molecules are shown.

The compounds tert-butyl-tris(trimethylsilyl)silane, Si[C(CH₃)₃][Si(CH₃)₃]₃ (Bu1), and Si[C(CH₃)₃]₂[Si(CH₃)₃]2 (Bu2) belong to a class of crystalline molecular compounds with orientationally disordered, quasi-spherical molecules in a cubic closest packed (ccp) arrangement. Solid-state NMR studies have shown that the molecules undergo rapid (kHz scale) reorientational jumps about fixed centers of mass forming so-called 'rotator-phases' or 'plastic' crystals. X-ray powder diffraction with synchrotron radiation is used to characterize the order-disorder phase transitions at low temperatures and high pressures. The structure at 105 K corresponds to a monoclinically distorted ccp arrangement with partial orientational order of the molecules. At room temperature the same structures are found at 0.3 GPa, but additional phase transitions are found at still higher pressures, up to 5 GPa, that are tentatively interpreted as due to further orderings of the molecules.