Figure 2. Structure and compressibility of cristobalite X-I.

(a) Third-order Birch-Murnaghan Equation of State fit to the P–V data obtained in experiment (blue line) and in ab initio calculations (red line). Exact unit-cell parameters obtained experimentally and theoretically are given in Supplementary Tables 3 and 4. (b) [010] projection of the structure of cristobalite X-I as obtained experimentally. The experimental structure contains half-occupied octahedral positions (Si3) shown by red spheres, and fully occupied octahedra (Si1 and Si2) represented by all other spheres, see Table 1. The blue and the green octahedra represent different levels in the structure. A row of three fully occupied octahedra surrounded by two half-occupied octahedra at each side form the ‘5 × 2 × 2 skeleton'. (c) The ordered structure used for model in ab initio calculations. The theoretical structure contains 4 × 4 arrangement of the fully occupied octahedra (Supplementary Table 6). The blue and the green octahedra represent different levels in the structure.