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

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Wunschel, Markus ; Dinnebier, Robert E. ; Carlson, Stefan ; Bernatowicz, Piotr ; van Smaalen, Sander:
Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals.
In: Acta Crystallographica Section B. Bd. 59 (2003) Heft 1 . - S. 60-71.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768102021791

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Abstract

The crystal structures of \it tert-butyl-tris(trimethylsilyl)silane, SiC(CH_3)_3_1Si(CH_3)_3_3 (Bu1), and di-{\it tert}-butyl-bis(trimethylsilyl)silane, SiC(CH_3)_3_2Si(CH_3)_3_2 (Bu2), at room temperature and at 105K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5GPa is reported. The room-temperature structures have cubic Fm\bar{3}m symmetry ({\it Z}=4) with {\it a}=13.2645(2){\AA}, {\it V}=2333.87(4){\AA}${\sp 3}$ for Bu1 and {\it a}=12.9673(1){\AA}, {\it V}=2180.46(3){\AA}${\sp 3}$ 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 temperatures T_c=230(5)K (Bu1) and T_c=250(5)K (Bu2) into monoclinic structures with space group P2_1/n. The structures at 105K have {\it a}=17.317(1), {\it b}=15.598(1), {\it c}=16.385(1){\AA}, \gamma=109.477(4){$^\circ$}, {\it V}=4172.7(8){\AA}${\sp 3}$ and {\it Z}=8 for Bu1and {\it a}=17.0089(9), {\it b}=15.3159(8), {\it c}=15.9325(8){\AA}, \gamma=110.343(3){$^\circ$}, {\it V}=3891.7(5){\AA}${\sp 3}$ and {\it 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 105K. First-order phase transitions have been observed at pressures of 0.13{--}0.28 GPa for Bu1 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 dependencies of the lattice parameters were observed. These anomalies are explained as additional disorder{--}order phase transitions.