Crystal structure and elastic and phonon properties of realgar versus pressure

Realgar α-As4S4 (space group P21/n) is one of the best-known arsenic sulphide minerals because of its extended use in the past as red pigment and its employment in modern times for advanced optical and electronic technological applications. From the point of view of geology, it is also a relevant phase found between the upper mantle and Earth’s crust, occurring in different types of deposits, and one of the main sources of arsenic. Despite this widespread use and interest, few experimental and theoretical studies were focused on the characterisation of the structural, elastic, and vibrational properties of realgar, especially their variation with pressure. Some quantities, such as the cohesive energy between the As4S4 units and the elastic moduli have never been reported in the scientific literature. The present work deals with a Density Functional Theory investigation of the cited properties of realgar, using the recently proposed PBEh-3c method, in particular to deal with crystalline solids characterized by weak van der Waals interactions. This approach was validated against the available crystal-chemical, mechanical, and spectroscopic data from previous studies, finding a generally good agreement. The equation of state parameters of the energy vs unit cell volume data were V0 = 767.13(9) Å3, B0 = 15.73(8) GPa and B’ = 9.1(2), with the bulk modulus value (B0) in good agreement with the value obtained from the elastic tensor analysis (B = 16.1 GPa). The cohesive energy was found to be about 146.1 kJ mol–1, a value that follows the typical ones of organic crystals. The present work provides new insights into this peculiar mineral that, from the mineralogical point of view, could be considered a prototype of a heterodesmic structure made by inorganic molecular clusters.