Toward an anisotropic atom–atom model for the crystalline phases of the molecular S8 compound

Abstract

We analize two anisotropic atom-atom models used to describe the crystalline α, β and γ phases of S 8 crystals, the most stable compound of elemental sulfur in solid phases, at ambient pressure and T < ∼ 400 K. The calculations are performed via a series of classical molecular dynamics (MD) simulations, with flexible molecular models and using a constant pressure-constant temperature algorithm for the numerical simulations. All intramolecular modes that mix with lattice modes, and are therefore relevant on the onset of structural phase transitions, are taken into account.Comparisons with experimental data and previous results obtained with an isotropic atom-atom molecular model are also performed.

Structural transformation between long and short-chain form of liquid sulfur from ab initio molecular dynamics

“…Further compression leads to extended-network phases -incommensurately modulated bcm phase S-IV [59][60][61] and S-V with β-Po structure [57,[60][61][62], which are both metallic [61,63,64] and superconducting [65]. Other structures of S were predicted from computer simulations -α -S [66], monoclinic-S with two different conformations of S 8 molecules [67], chain forms similar to S-II and S-III [68] or higher-pressure superconducting sc and bcc structures [69,70]. Besides the crystalline regime, sulfur also undergoes pressureinduced amorphization [62,67,[71][72][73] and the possibility of amorphous-amorphous transition (polymorphism in glassy state) [73] has also been proposed.…”

Free-energy calculations of elemental sulphur crystals via molecular dynamics simulations

“…The MD simulations performed with this anisotropic potential did not improve the previous result (shown in Fig. 1), unless a quite unrealistic atomic anisotropy is used [11,14].…”

“…1͒, unless a quite unrealistic atomic anisotropy was used. 9,10 It has to be pointed out that the search for a reliable molecular model for elemental sulphur molecules is extremely useful due to the practical impossibility of performing quantum-mechanical simulations of the complex phase diagram of these molecular crystals, with a large number of atoms in the primitive cells.…”

Order–disorder transition in monoclinic sulfur: a precise structural study by high-resolution neutron powder diffraction

“…The rich structural variety of cyclo-octasulphur provides case studies for the analysis of a number of general phenomena that include allotrope comparison and reaction kinetics, pre-melting behaviour, premonitory reconstructive phase-transition effects, the modelling of static and dynamic disorder, critical behaviour and anomalous thermal expansion. The complex phase diagram of elemental sulfur also provides an attractive testing ground for theoretical studies and has been the subject of recent molecular dynamics simulations (Pastorino & Gamba, 2001, 2003.…”