Pressure and temperature effects on the degree of symmetry and chirality of the molecular building blocks of low quartz

We establish quantitative correlations between pressure and temperature (PT) changes, and the degree of symmetry and of chirality of the main molecular building blocks of low quartz that these PT changes induce. The distortion from ideal tetrahedral symmetry, from helicity (deviation from C₂ symmetry), and the level of chirality are evaluated quantitatively using the continuous-symmetry and chirality-measures approach. These measures are global and reflect all changes in bond angles and bond lengths. The specific molecular building blocks analyzed are the SiO₄ elementary building block (which is found to be chiral!), the Si(OSi)₄ unit, the second-shell SiSi₄ tetrahedron [composed of the five Si atoms of Si(OSi)₄] and the four-tetrahedra helix fragment, -O(SiO₃)₄-. The temperature and pressure effects on symmetry and chirality were found to mirror each other in all building blocks. By employing this quantitative approach to symmetry and chirality we were able to combine the pressure effects and temperature effects into a unified picture. Furthermore, the global nature of the symmetry measure allows the comparison of the behavior of isostructural materials such as germania and quartz. For these crystals it has been shown that the symmetry/chirality behavior of germania at low pressures is a predictor for the behavior of these structural properties in quartz at higher pressures. Finally, given that the rigid SiO₄ unit undergoes only minor structural changes, it has been a useful observation that the symmetry/chirality of the small SiSi₄ tetrahedron is a very sensitive probe for the symmetry and chirality changes in quartz as a whole.