Crystal structure and elastic and phonon properties of realgar versus pressure

Realgar, α-As 4 S 4 (space group P 2 1 / n ), is one of the best-known arsenic sulfide minerals because of its extended use in the past as a red pigment and its employment in modern times for advanced optical and electronic technological applications. From a geological perspective, the main realgar deposits are hydrothermal and epithermal, but it is also a relevant phase found between the upper mantle and Earth's crust and therefore one of the main sources of arsenic. Despite this widespread use and interest, few experimental and theoretical studies have been focused on the characterization of the structural, elastic and vibrational properties of realgar, especially their variation with pressure. Some quantities, such as the cohesive energy between the As 4 S 4 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, which was devised in particular to deal with crystalline solids characterized by weak van der Waals interactions. This approach is 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 versus unit-cell volume data were V 0 = 767.13 (9) Å 3 , B 0 = 15.73 (8) GPa and B ′ = 9.1 (2), with the bulk modulus value ( B 0 ) 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 a mineralogical point of view, could be considered a prototype of a heterodesmic structure made by inorganic molecular clusters.