Nature Materials reports: Two Steps to Densify …

Nature Materials reports: Two Steps to Densify …

Figure 1 (Left) Figure 2 (right)

… and a 15-year old mystery solved. Using simulations UM researcher Kieffer and his postdoctoral research associates Huang and Durandurdu have uncovered a ubiquitous mechanism by which certain inorganic compounds densify under pressure. This finding is important for understanding the response of materials under severe mechanical impacts and the deformation of rocks in the Earth’s interior, and is therefore relevant in contexts ranging from materials design to earthquakes, and possibly, the flow of glaciers. The observed mechanism is also fascinating from a purely scientific point of view. As revealed by the group’s concurrent molecular dynamics simulations and first-principles calculations, materials that have polyhedral network structures under normal conditions, compact according to a two-step process. First, a compact high-symmetry anion sub-lattice forms, governed by the strong repulsion between large anions. Subsequently, cations redistribute onto interstices of this sub-lattice. These steps are illustrated for the case of cristobalite in Fig. 1.
The UM researchers observe this for two different polymorphs of silica, i.e., quartz and cristobalite. The intriguing aspect of this process is that in both cases the first step results in the formation of a highly symmetric close-packed oxygen sub-lattice (bcc in the case of quartz and hcp in the case of cristobalite). These findings are reported in the December 2006 issue of Nature Materials (Vol. 5, p. 977 "Transformation Pathways of Silica under High Pressure," by Liping Huang, Murat Durandurdu, and John Kieffer). Note that polymorphs of ice, namely ice II and ice Ic have the same space groups as quartz and cristobalite, respectively. Hence, this process may be taking place at the very moment we chew the ice from our beverages.
As an added point of interest, for cristobalite the completion of the first stage in this two-step process is manifest by the formation of a metastable phase, which corresponds to an elusive new phase (X-I silica) that has been observed in experiments over a decade ago [Nature 347, 267 (1990)]. The nature of this phase has so far remained elusive because the corresponding diffraction patterns were too ambiguous to be indexed. By reproducing the compression experiment in the computer Kieffer and coworkers now have identified the structure of this new polymorph, and furthermore, they explain the role it plays in the densification of silica, i.e., X-I corresponds to the structural state where the hcp anion packing is achieved (Fig. 2). This is an illustrative example of how computer simulations can enhance experiments. The importance of this demonstration has been described by T. Yagi, the author of the 1990 Nature paper, in a News & Views article that accompanies the paper by Huang et al. in the December issue of Nature Materials.