From disordered molecules to distorted chains and back
Experimental and theoretical study of polyamorphism in SO2 was on April 3, 2020 published in the prestigious multidisciplinary journal PNAS (Proceedings of the National Academy of Sciences of the United States of America). Co-authors are Mgr. Ondrej Tóth and Prof. Roman Martoňák from the Department of Experimental Physics of the Faculty of Mathematics, Physics and Informatics at the Comenius University in Bratislava.
By: Roman Martoňák
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. From macroscopic point of view, crystals keep their shape while liquids and gases flow. It might appear that the above states are sharply defined, but it is not quite true. Similarly to biology, where e.g. carnivorous plants exist, here we also find forms which have at the same time properties of both crystals and liquids. Some of them are in fact close to us. The window glass which allows us to look outside, 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.
In case of crystals there are many possibilities how atoms may order. A transformation of one order to another, typically induced by change of pressure or temperature, is called structural phase transition (or structural transformation). Well-known example is the transformation of graphite to diamond at high pressure - it is still carbon, but all properties are dramatically different. The existence of a number of possible structural orders in crystals appears natural and does not surprise. But could different forms of disorder exist? Naively, it might appear that all disorders are alike. However, disorder in amorphous solids is far from random. Also in liquids and amorphous solids atoms are to some extent ordered over short distance, similarly to crystals at the same conditions. Could there be transformations between different amorphous forms of the same system?
The answer is positive and the phenomenon is called polyamorphism, similarly to polymorphism which refers to existence of several different crystalline structures. Again, an example is close - polyamorphism was first observed in water ice cooled to the liquid nitrogen temperature of 77 K (Mishima 1984). At such low temperature at least two different forms of amorphous ice exist, with quite different structure, depending on pressure. Afterwards the phenomenon was observed in several systems, such as Si, SiO2, but the list is still rather short.
In our work we found a new example of a system which exhibits this phenomenon, in an interesting form. In collaboration with colleagues from Italy, UK and China we investigated sulphur dioxide, SO2, well known e.g. for its use in winemaking. The molecule SO2, similarly to that of water H2O, has a shape of an isosceles triangle (Fig. 2). At ambient conditions SO2 is gas, upon cooling to 263 K (-10 C) becomes liquid and at 201 K (-72 C) freezes to a molecular crystal. In our study we investigated what happens to the crystal at high pressure. Experimental study by Raman spectroscopy showed that at pressure 10-15 GPa the system first transforms from crystalline to amorphous state. This is called pressure-induced amorphization - molecules which were regularly arranged in the crystal, now become disordered. Upon further compression to 26 GPa (260000 times higher than atmospheric pressure) a dramatic structural transformation takes place, where the changes of the Raman spectrum suggest a change of chemical bonding in molecules. Interestingly, this transition is reversible (this is non-trivial) and upon decrease of pressure the system reverts back to the molecular amorphous form. Upon decompression, X-ray diffraction was measured as well, providing clue to microscopic structure. The back-transformation takes place at pressure of 25 GPa and is visible both on Raman spectrum and X-ray diffraction pattern.
Besides experiment we investigated these phenomena also by computer simulations. These were performed by PhD student Ondrej Tóth in the research group of Prof. Roman Martoňák at the Department of Experimental Physics of the Faculty of Mathematics, Physics and Informatics at the Comenius University in Bratislava. Simulations on the national supercomputing facility Aurel agree very well with experiment and allowed us to understand on the atomistic level what is going on in the system at high pressure. Molecules SO2 have multiple bonds and when they approach each other upon compression, they may create single covalent bonds and form a polymeric chain. Our calculations predicted how such chain looks in a perfectly ordered crystal (Fig. 3 A). It would be infinitely long and straight. In our case, however, the starting point is not an ordered molecular crystal, but amorphous state consisting of disordered molecules. The latter upon compression also form chains, but these are now distorted (Fig. 3 B). On Fig. 3 C,D,E one can see how the system of disordered molecules (C) transforms to system of distorted chains (D) and comes back upon release of pressure (E).
Our study discovered a new type of reversible transformation between two amorphous forms, molecular and polymeric, which takes place in a system of simple molecules. SO2 is thus another example that even disordered matter may exist in several structurally distinct forms. Our results suggest that a similar transition might exist also in liquid phase of SO2, but to pursue this hypothesis further experiments are necessary.
Pressure-induced amorphization and existence of molecular and polymeric amorphous forms in dense SO2
Huichao Zhang, Ondrej Tóth, Xiao-Di Liu, Roberto Bini, Eugene Gregoryanz, Philip Dalladay-Simpson, Simone De Panfilis, Mario Santoro, Federico Aiace Gorelli, and Roman Martoňák, PNAS 2020