A density-driven first-order phase transition in liquid sulfur

TL;DR

This paper provides experimental evidence for a rare first-order liquid-liquid phase transition in sulfur, characterized by a sharp density change and structural transformation under high pressure and temperature conditions.

Contribution

It reports the first combined in-situ measurements confirming a first-order liquid-liquid transition in sulfur, involving a sharp density jump and structural change.

Findings

Identification of a sharp density jump in sulfur under high pressure

Observation of a two-stage transition involving polymerization and densification

Experimental support for a first-order liquid-liquid phase transition in a pure element

Abstract

First-order phase transitions are characterized by a discontinuous first derivative of the Gibbs free energy, so that volumes and entropies are discontinuous. Such transitions are common in the crystalline state, but extremely rare in liquid substances and experimentally evidenced in only one pure element, phosphorus. Here we report combined in-situ Raman scattering, X-ray diffraction and density measurements that support the existence of a first-order liquid-liquid transition in sulfur at high pressures and temperatures. The transformation involves a sharp density jump between two structurally and dynamically distinct liquids. The first-order phase transition proceeds through an initial stage of temperature induced polymerization and a final stage where the low-density liquid abruptly converts to a denser polymeric state. This unique feature is explained by competing effects of pressure and temperature.