Crystal Structures of Silica Polymorphs and their Phase Transitions.

Abstract

Silica occurs as minerals involving more than 20 different phases. Most silica mineral structures can persist up to high temperatures, where thermal expansion coefficients are nearly null or rather negative. Quartz, well known by its unparalleled applications of piezoelectricity and, of course, as the most common silica mineral, shows the α-β structure transition, which was found by Le Chatelier in the end of the 19th century and has continuously drawn attention of a wide variety of researchers. However, the structural problems such as high temperature structures and structure transformations in many silica minerals have been yet established. Tridymite, for example, shows successive displacive-structure transitions with varying temperature among disordered states, but the theoretical aspect of its mechanism has remained unknown so far. In this article, we review the recent structural problems in quartz, tridymite and cristobalite, all low-pressure polymorphs, and then introduce an example of successful application of molecular dynamics simulation to quartz, stressing the importance of unified application of diffraction crystallography and computer simulation to the complicated problems in silicas.