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

The crystal structures of tert-butyl-tris(trimethylsilyl)silane, Si[C(CH[sub 3])[sub 3]][sub l][Si(CH[sub 3])[sub 3]][sub 3] (Bu1), and di-tert-butyl-bis(trimethylsilyl)silane, Si[C(CH[sub 3])[sub 3]][sub 2][Si(CH[sub 3])[sub 3]][sub 2] (Bu2), at room temperature and at 105 K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5 GPa is reported. The room-temperature structures have cubic Fm&3macr;m symmetry (Z=4) with a = 13.2645 (2) Å, V = 2333.87 (4) ų for Bul and a = 12.9673 (1) Å, V = 2180.46 (3) ų for Bu2. The molecules are arranged in a cubic close packing (c.c.p.) and exhibit at least 48-fold orientational disorder. Upon cooling both compounds undergo a first-order phase transition at tempera- tures T[sub c] = 230 (5) K (Bu1) and T[sub c] = 250 (5) K (Bu2) into monoclinic structures with space group P2[sub 1]/n. The structures at 105 K have a = 17.317 (1), b = 15.598 (1), c = 16.385 (1) Å, γ = 109.477(4)°, V= 4172.7 (8) ų and Z=8 for Bu1 and a = 17.0089 (9), b = 15.3159 (8), c = 15.9325 (8) Å, γ = 110.343 (3)°, V= 3891.7 (5) ų and Z = 8 for Bu2. The severe disorder of the room-temperature phase is significantly decreased and only a two- or threefold rotational disorder of the molecules remains at 105 K. First-order phase transitions have been observed at pressures of 0.13-0.28 GPa for Bul and 0.20-0.24 GPa for Bu2. The high-pressure structures are isostructural to the low-temperature structures. The pressure dependencies of the unit-cell volumes were fitted with Vinet equations of state and the bulk moduli were obtained. At still higher pressures further anomalies in the pressure dependen- cies of the lattice parameters were observed. These anomalies are explained as additional disorder-order phase transitions. [ABSTRACT FROM AUTHOR]