Compressibility of β-As4S4: an in situ high-pressure single-crystal X-ray study

Ambient temperature X-ray diffraction data were collected at different pressures from two crystals of beta-As4S4, which were made by heating realgar under vacuum at 295 degrees C for 24 h. These data were used to calculate the unit-cell parameters at pressures up to 6.86 GPa. Above 2.86 GPa, it was only possible to make an approximate measurement of the unit-cell parameters. As expected for a crystal structure that contains molecular units held together by weak van der Waals interactions, beta-As4S4 has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch-Murnaghan equation of state with a resulting volume V-0 = 808.2(2) angstrom(3), bulk modulus K-0 = 10.9(2) GPa and K' = 8.9(3). These values are extremely close to those reported for the low-temperature polymorph of As4S4, realgar, which contains the same As4S4 cage-molecule. Structural analysis showed that the unit-cell contraction is due mainly to the reduction in intermolecular distances, which causes a substantial reduction in the unit-cell volume (similar to 21% at 6.86 GPa). The cage-like As4S4 molecules are only slightly affected. No phase transitions occur in the pressure range investigated. Micro-Raman spectra, collected across the entire pressure range, show that the peaks associated with As-As stretching have the greatest pressure dependence; the S-As-S bending frequency and the As-S stretching have a much weaker dependence or no variation at all as the pressure increases; this is in excellent agreement with the structural data.