Formation of a two-dimensional layered structure in silica under shear stresses: An ab initio study

The behavior of ?-cristobalite under pure shear stresses is studied using an ab initio technique. The application of shear stress on the a-b planes yields a two-dimensional layered monoclinic phase with C2 space group via an orthorhombic phase having C2221 symmetry. In the layered structure, each silicon atom is fourfold coordinated. On the other hand, shearing of a-c and b-c planes causes first a transformation into monoclinic phases within P 21 symmetry and then structural failure. We also study the behavior of ? -cristobalite under the simultaneous application of shear stress and hydrostatic pressure. We consider six different loading conditions and find that the shear stresses have no influence on the densification mechanism until a first-order phase transformation occurs. On the other hand, the shear stresses play a significant role during the first-order phase transformation and result in the Cmcm, anataselike, and stishovite structures in our simulations. The phase transformation from ? -cristobalite to the Cmcm structure is due to the shear deformation on the a-b planes and proceeds via an orthorhombic intermediate phase having C2221 symmetry. © 2010 The American Physical Society