Compressibility of beta -As (sub 4) S (sub 4) ; 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 -As (sub 4) S (sub 4) , 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 -As (sub 4) S (sub 4) has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch-Murnaghan equation of state with a resulting volume V (sub 0) = 808.2(2) Aa (super 3) , bulk modulus K (sub 0) = 10.9(2) GPa and K' = 8.9(3). These values are extremely close to those reported for the low-temperature polymorph of As (sub 4) S (sub 4) , realgar, which contains the same As (sub 4) S (sub 4) 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 (approximately 21% at 6.86 GPa). The cage-like As (sub 4) S (sub 4) 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.