Study of sulphur dioxide at high pressure

Physicists from the Comenius University showed that sulphur dioxide SO2 represents a new example of existence of several disordered solid forms. Employing experiments and computer simulations they studied what happens in the system under high pressure and found two distinct disordered forms - molecular and polymeric. Study whose co-authors are Mgr. Ondrej Tóth a prof. Roman Martoňák was published at the beginning of April 2020 in the prestigious multidisciplinary journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) and is a result of collaboration of several international groups.

It is well known that there are three basic states, or phases, of matter: solid (typically crystal), liquid and gas. The basic difference between them is the amount of structural order. In crystal the atoms are ordered over long distances, in liquid over short distances and in gas there is no order at all. There are also forms which have at the same time properties of both crystals and liquids. For example, glass is actually a frozen liquid and even though it keeps shape, like a crystal, its structure on the atomic level is disordered, like in a liquid. Such states are called amorphous and window glass (amorphous SiO2) is the most common example.

Upon change of pressure or temperature the arrangement of atoms may change, resulting in change of properties of the system. A well-known example is the transformation of graphite to diamond at high pressure. Besides different forms of order (polymorphism), distinct forms of disorder may also exist in solids and this interesting and still not well understood phenomenon is called polyamorphism.

Scientists from the Comenius Unversity together with colleagues from China, Italy and UK studied sulphur dioxide. It is a simple molecule with multiple bonds and upon compression one can expect a creation of a polymeric form. Similar transformations were already observed in solid carbon dioxide (CO2) and nitrogen (N2) where double or triple bonds exist in molecules. “The behaviour of sulphur dioxide SO2 under pressure has not been theoretically studied at all and experimentally only to limited extent. One could expect that at not too high pressure the system would polymerize and chains would be created, similar to those found in analogous compound SeO2 (mineral Downeyite) at ambient pressure”, Roman Martoňák explained.

In the study scientists investigated what happens in a crystal of frozen sulphur dioxide at high pressure. It was found that the crystalline state first transforms to an amorphous one where molecules, initially regularly arranged in crystal, become disordered. At even higher pressure a transformation of chemical bonds in molecules takes place and precisely as expected, molecules join together and form polymeric chains. Upon decompression the system, however, reverts back to the original molecular amorphous form. This means that a new type of reversible transformation was discovered, occurring between two disordered states – molecular and polymeric. This provides further evidence that disordered matter may exist in several distinct forms. Slovak scientists performed in the study extensive computer simulations which allowed to interpret in detail the experimental observations.

 “It would be very interesting to study whether sulphur dioxide behaves in a similar manner also in the liquid state and whether a similar transition between molecular and polymeric liquid exists. This kind of transformation is rather rare and of interest for fundamental understanding of structure of condensed matter”, Roman Martoňák concluded.

Further information about the study can be found here.