Polymorphism of As4S3 (tris-(μ2-sulfido)-tetra-arsenic) : accurate structure refinement on natural α- and β-dimorphites and inferred room temperature thermodynamic properties

An accurate structure refinement, using singlecrystal X-ray diffraction data, on natural a- and b-dimorphites collected at Solfatara di Pozzuoli, in the Phlegraean Fields, near Naples and at Vesuvius (Italy) is reported. Theoretical calculations at various levels of sophistication have been used to calculate molecular vibrational frequencies and gas-phase specific heats, to analyze the crystal packing, and to estimate lattice energies of both phases. All computational methods contribute to demonstrate that the b-phase is the thermodynamically stable one at room temperature.