Chain-ring transition during exothermic "melting" in amorphous sulfur

Abstract

A normal melting process is endothermic and causes volume expansion. Exothermic melting or volume contraction is therefore abnormal, and has important effects in condensed-matter physics. In the present study, we investigated the mechanism of abnormal exothermic "melting" of amorphous sulfur. Analysis of Raman spectra obtained before and after exothermic melting showed that before melting the amorphous sulfur contains clusters in the form of polymeric chains. After melting, sulfur clusters in the form of S 8 predominate, i.e., the polymeric chains are converted to S 8 rings during melting, which affects the overall internal energy of the system. Density functional method calculations confirmed that the net effect in the chain-ring transition is exothermic. This abnormal exothermic melting in amorphous sulfur can be described as chain-ring transitions coupled with melting. The difference between the chain-ring transition reported in this work and that reported in crystallization relaxation of amorphous sulfur is also discussed.