Molecular and polymeric amorphous forms in dense SO$_2$

TL;DR

This study investigates reversible pressure-induced structural changes between molecular and polymeric amorphous SO$_2$ using experiments and simulations, revealing a potential liquid-liquid transition and polyamorphism in simple molecules.

Contribution

It provides the first combined experimental and computational analysis of amorphous SO$_2$ phases and details the atomic structure of the high-pressure polymeric form.

Findings

Reversible amorphous phase transition observed at 26 GPa.

Polymeric amorphous SO$_2$ consists mainly of disordered chains.

Simulation results agree with experimental data.

Abstract

We report here a study of reversible pressure-induced structural transformation between two amorphous forms of SO$_2$: molecular at pressures below 26 GPa and polymeric above this pressure, at temperatures of 77 - 300 K. The transformation was observed by Raman spectroscopy and x-ray diffraction in a diamond anvil cell. The same phenomenon was also observed in ab initio molecular dynamics simulations where both forward and reverse transitions were detected, allowing to analyze in detail the atomic structure of both phases. The high-pressure polymeric amorphous form was found to consist mainly of disordered polymeric chains made of 3-coordinated sulfur atoms connected via oxygen atoms, and few residual intact molecules. The simulation results are in good agreement with experimental data. Our observations suggest a possible existence of molecular liquid - polymeric liquid transition in SO$_2$ and show a case example of polyamorphism in system consisting of simple molecules with multiple bonds.