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

Abstract

We present results of \textit{ab initio} molecular dynamics study of a structural transformation occurring in hot liquid sulfur under high pressure, which corresponds to the chain-breakage phenomenon recently observed experimentally by Liu \textit{et al.} [1] and to the electronic transition reported by Brazhkin \textit{et al.} [2,3]. We performed an extensive \textit{ab initio} study and confirmed the existence of one transformation separating two distinct liquid polymeric phases: one composed of short chain-like fragments and another one with very long chains. We have not observed additional transformations reported in Refs. [2,3] and in the recent theoretical study by Zhao and Mu [4] and our findings are in agreement with the most recent experiment [1]. We offer a structural description of this liquid-liquid transformation in terms of chain lengths, cross-linking and geometry and investigate several physical properties. We conclude that the transformation is accompanied by changes in configurational energy and entropy as well as in diffusion coefficient and electronic properties (semiconductor-metal transition), but no density discontinuity was found. We also describe analogy of the investigated phenomenon to similar processes present in two other chalcogenes selenium and tellurium. Finally, we discuss possibility of a critical point terminating the studied transition in relation to the long-known behavior of ambient-pressure heated sulfur melt.