- m at . s of t ] 2 9 N ov 1 99 9 Test of a simple and flexible molecule model for α - , β - and γ - S 8 crystals

Test of a simple and flexible molecule model for α-, β-and γ-S 8 crystals S 8 is the most stable compound of elemental sulfur in solid and liquid phases, at ambient pressure and below 400K. Three crystalline phases of S 8 have been clearly identified in this range of thermodynamic parameters, although no calculation of its phase diagram has been performed yet. α-and γ-S 8 are orientationally ordered crystals while β-S 8 is measured as orientationally disordered. In this paper we analyze the phase diagram of S 8 crystals, as given by a simple and flexible molecule model, via a series of molecular dynamics (MD) simulations. The calculations are performed in the constant pressure-constant temperature ensemble, using an algorithm that is able to reproduce structural phase transitions. Introduction: Elemental sulfur shows a rich variety of molecular forms and therefore of chemical and physical properties in the crystalline and liquid phases 1,2. Recently, a high pressure super-conductor transition has been found, with T c =17K (at 162GPa), the highest known for any elemental solid 3. Natural sulfur is of relevance in geophysics, astrophysics, material sciences and also of massive industrial use 1. Nevertheless, the complex phase diagram and properties of its condensed phases are far from well known 1,2. The most stable sulfur allotrope at ambient temperature and pressure (STP) is S 8 , a crown-shaped cyclic S 8 molecule, stable in solid, liquid and gas phases. 1 Orthorhombic α-S 8 is the crystalline form stable at STP. Figs 1a shows its " crankshaft " structure with four molecules in the primitive unit cell. The space group is D 24 2h (Fddd), with 16 molecules in this non-primitive orthorhombic cell. If α-S 8 is slowly heated, it shows a phase transition to monoclinic β-S 8 at 369K (Fig. 1b), which melts at 393K 1. Nevertheless, most α-S 8 crystals do no easily convert to β-S 8 , they melt, instead, at 385.8K 1,2. β-S 8 is a monoclinic crystal, with six molecules in the primitive unit cell, two of them orientationally disordered 4,5. A third crystalline allotrope has been observed: γ-S 8 (Fig. 1c), that can be obtained from solutions of S 8 or from its melt 1,2. The space group is C 4 2h (P2/c), with four molecules in the pseudo-hexagonal closed-packed unit cell 6. The density of this allotrope, at STP, is 5.8 % higher than that of α-S …