Federico Aiace Gorelli

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, SiO ₂ , Si, etc. Here we present an example of such behavior in a simple molecular substance, SO ₂ . 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 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.

A review of experimental and theoretical studies performed over the past three decades on high pressure chemistry of solid CO₂, at 0–80 GPa and 40–3000 K, is presented. Emphasis is placed on the recently discovered non-molecular covalent crystalline phase V, and its glassy counterpart a-CO₂, along with other molecular phases, whose interpretation is crucial for determining the reaction path to non-molecular CO₂. The matter is still under debate, and many open issues are outlined, such as the true reaction mechanism for forming phase V. Finally, we propose arguments to stimulate possible future research in a more extended P–T range. This work is a tutorial review and should be of general interest both for solid state chemistry and condensed matter physics communities.

We report on the antiferromagnetic character of solid oxygen in the high‐pressure, low‐temperature region enclosed between the β and ε phases. The occurrence of a first‐order phase transition, positioned at 5.5–6.5 GPa, between the monoclinic α (C2/m) and the orthorhombic δ (Fmmm) phases was shown by means of an accurate x‐ray diffraction analysis. The two crystal structures were assessed to be antiferromagnetically ordered, owing to a narrow peak observed in the IR spectrum and assigned to a vibron component. The absorption of this mode is allowed owing to the coupling of the molecular spins, which produces a doubling of the primitive cell. The vibrational coupling parameter was quantitatively related to the exchange integralJinvolved in the Heisenberg interaction. Finally, absorption measurements at different pressures of the electronic transition$^{3}\Sigma_{g}^{-} \rightarrow{}^{1}\Delta_{g}$ , lying in the near‐IR spectral range, allowed us, by means of a simple model, to obtain directly the quantityJas a function of the nearest‐neighbour distance. Copyright © 2003 John Wiley & Sons, Ltd.