Elastic Instabilities and Amorphization of Crystalline Silica Under Pressure
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Elastic Instabilities and Amorphization of Crystalline Silica Under Pressure James R. Chelikowsky and Nadia Binggeli Department of Chemical Engineering and Materials Science, Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455
ABSTRACT Solid state amorphization can occur when a crystalline phase is compressed at a sufficiently low temperature to inhibit kinetically the transformation to a stable high pressure crystalline phase. An example of such a vitrification transformation occurs in a-quartz, the most stable phase of SiO 2 at standard temperature and pressure conditions. Under pressure at room temperature a-quartz gradually transforms to an amorphous form in the range of 25-30 GPa. The driving force for this amorphization is not clear, and speculation has centered on mechanical instabilities of the quartz crystal under pressure. The elastic properties of a-quartz are studied as a function of pressure using both classical interatomic potentials, and ab initio pseudopotentials. In both cases, we find that the ca-quartz structure becomes mechanically unstable at about 30GPa. This finding supports a picture in which the amorphization of quartz is triggered by the onset of a lattice shear-instability. The microscopic origin of this elastic softening is intimately related to the presence of an oxygen close-packed cubic arrangement in the quartz high pressure structure.
INTRODUCTION Order-disorder transformations are of fundamental importance in understanding the solid state. Also, amorphous solids are commonly employed in technological applications. As such, controlling and understanding their synthesis is of some importance. The recent observation of pressure induced amorphization of silica is of some significance[l]. Silica can serve as an archetypical noncrystalline solid. It can be synthesized by melt-quenching, static high pressures, comminution, and shock compression[2]. These amorphous states appear similar with respect to x-ray diffraction patterns and vibrational spectra[3J. Therefore, a microcsopic knowledge of the pressure induced amorphization of quartz may have application to other vitrification processes. From a theoretical perspective, the role of pressure induced transformations is more easily handled than temperature induced transformations. The role of pressure in crystalline transformations can be reproduced accurately by ab inijio pseudopotential calculations[4,5]. Applications have been made to a number of semiconducting and ionic crystals. Perhaps the most significant such application has been the prediction of superconducting forms of silicon under pressure[4]. The most stable form of Si0 2 at room temperature and at pressures less than 3 GPa is aquartz. At higher pressures, quartz exists as a metastable state; vitrification occurs gradually and is complete in the 25-30 GPa range[l]. A similar amorphization under hydrostatic pressure occurs for other crystalline oxides with the quartz structure such as AIPO 4 and GeO 2 [6,7]. The driving force for amorphization u
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