STRUCTURAL TRANSFORMATION OF QUARTZ AT HIGH-PRESSURES

THE behaviour of alpha-quartz, one of the most common tetrahedrally coordinated silica polymorphs, at high pressures has been the subject of several experimental studies. At room temperature, gradual pressure-induced amorphization is observed (at about 25-35 GPa) 1,2, followed at higher pressures (above 60 GPa) by a transformation to a crystalline octahedrally coordinated 'rutile-like' structure 3. The driving force for these transformations is not well understood. We report here first-principles calculations of the electronic and structural properties of the high-pressure structure of alpha-quartz, which show a pressure-induced transformation of the oxygen sublattice to a body-centred cubic structure. The main features of cubic packing are present at 30 GPa, and the ideal form is achieved at about 60 GPa. The formation of the cubic sublattice facilitates structural transformations involving a change in Si coordination. In the oxygen body-centred cubic lattice, a small displacement of Si ions along the empty channels of the structure is sufficient to transform alpha-quartz either to a structure with mixed fourfold/sixfold coordination or, for a larger silicon displacement, to a purely sixfold-coordinated structure. This description is consistent with the evidence of intermediate crystalline phases with mixed Si coordination above the amorphization pressure in molecular-dynamics simulations 4, and can also explain the transformation to a sixfold crystalline structure at higher pressure.