Novel high pressure phase of silica

A transformation from a disordered to an ordered structure of silica has been observed at about 1 Mbar from molecular dynamics calculations. The packing of this high density phase is unique and may be important to other oxide structures. This new structure is monoclinic and is built from entirely sixfold coordinated SiO₆ octahedra arranged in alternating slabs of α-PbO₂-type structure where the octahedra share two edges inside the slab and three edges at the boundary. In comparison with the other two structures that have been proposed within this pressure range, the stability of the monoclinic phase is more favorable than the cubic Pa3 structure but is comparable to the orthorhombic CaCl₂ structure and therefore may likely coexist. The major features of the calculated diffraction pattern also fall in the same region as the CaCl₂ structure.