High-pressure phase transitions in tetrakis(trimethylsilyl)silane Si[Si(CH₃)₃]₄

Abstract

The compound tetrakis(trimethylsilyl)silane Si[Si(CH ₃)₃]₄ (TSi) has been studied at room temperature for pressures up to 21.5(1) GPa, using X-ray powder diffraction with synchrotron radiation. Three different phases are reported in the investigated pressure range. The c.c.p.-type structure is stable up to a pressure of 0.19(1) GPa (

, a = 13.5211(1) Å, V = 2471.95(1) ų, Z = 4 at 0.02(1) GPa). It shows a severe orientational disorder of the molecules. At pressures 0.19(1)–0.71(1) GPa a monoclinic phase (HP1) is found with space group P 2₁/n and Z = 8 (a = 17.746(1) Å, b = 16.098(1) Å, c = 16.871(1) Å, γ = 111.437(5)^ˆ and V = 4486.3(8) ų at 0.24(1) GPa). Another monoclinic phase (HP2) exists at pressures 0.52(1)–8.67(5) GPa. This phase has a smaller unit cell than the HP1 phase with Z = 2 (a = 8.3778(8) Å, b = 9.1050(7) Å, c = 13.3024(8) Å, γ = 111.63(1)^ˆ and V = 943.22(6) ų at 1.73(2) GPa). The pressure dependencies of the unit-cell volumes are successfully described by Murnaghan or Vinet-type equations of state. The structures at different pressures and temperatures of TSi and related compounds are described as distorted superstructures of the c.c.p. arrangement. The occurrence of different superstructures is rationalized as the result of an optimized packing of ordered molecules of various shapes.

Keywords:

• Tetrakis(trimethylsilyl)silane
• High-pressure
• X-ray powder diffraction
• Molecular crystal
• Plastic crystal
• Rotator phase

Measurements have been carried out at the European Synchrotron Radiation Facility (ESRF) at beamline ID30. Financial support by the Deutsche Forschungsgemeinschaft (DFG) (Project Di687-4), the FCI and the BMBF is gratefully acknowledged.