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Pressure-induced amorphization and existence of molecular and polymeric amorphous forms in dense SO2

Significance Some substances are known to exist in several different structurally disordered solid states and transform between them similarly to structural phase transitions between crystalline polymorphs. This interesting and yet not fully understood phenomenon is called polyamorphism, and notable examples include water, SiO2, Si, etc. Here we present an example of such behavior in a simple molecular substance, SO2. By using experimental high-pressure techniques, we observe a reversible transition between different amorphous states around 26 GPa. Experimental results are well supported by ab initio simulations, which identify the high-pressure amorphous form as a nonmolecular, polymeric one consisting of intertwined chains. Our findings contribute to the fundamental understanding of structure of disordered matter as well as high-pressure behavior of simple archetypal molecules. We report here the pressure-induced amorphization and reversible structural transformation between two amorphous forms of SO2: molecular amorphous and polymeric amorphous, with the transition found at 26 GPa over a broad temperature regime, 77 K to 300 K. The transformation was observed by both Raman spectroscopy and X-ray diffraction in a diamond anvil cell. The results were corroborated by ab initio molecular dynamics simulations, where both forward and reverse transitions were detected, opening a window to detailed analysis of the respective local structures. The high-pressure polymeric amorphous form was found to consist mainly of disordered polymeric chains made of three-coordinated sulfur atoms connected via oxygen atoms, with few residual intact molecules. This study provides an example of polyamorphism in a system consisting of simple molecules with multiple bonds.