Compressibility and polymorphism of α-As4S4 realgar under high pressure

The energy-dispersive x-ray diffraction technique has been employed to study the structure and equation of state of realgar As4S4 under pressures up to 8 GPa at room temperature. We have obtained pressure dependences of the unit cell parameters and volume for the monoclinic structure of realgar. An approximation of the equation of state through the Murnaghan equation gives the bulk modulus and its derivative, B0 = 8.1 ± 0.5 GPa and B0' = 9.0 ± 0.5, respectively. A comparison of the obtained values with the corresponding values for other molecular crystals is drawn and discussed. At a pressure of around 7 GPa, realgar showed a polymorph transition to a new molecular phase with a supposedly orthorhombic structure. Such identification is evidenced by the presence of geometrical correlations between the parameters of the parent monoclinic phase and those of the new phase, and the phase transition is likely to be associated with the removal of a monoclinic distortion in the unit cell.