Fig. 1: High-pressure polymorphic phases of silica.
a Comparison of the energy-volume curves of various high-pressure polymorphs of silica. Lines correspond to the predictions by the ACE potential, while open points correspond to DFT (strongly-constrained and appropriately normed exchange-correlation functional61) results. The data of quartz, coesite, stishovite, seifertite and rosiaite-structured silica are taken from ref. 43. Since the silicon atoms of d-NiAs-type silica are randomly distributed in octahedral sites of the hcp arrangement of oxygen, we show the energy-volume curves of 10 different structures of d-NiAs-type silica with 3000 atoms each. Due to the system size, we show only the ACE result of d-NiAs-type silica. b Illustration of the direct transformation of strained quartz to rosiaite-structured silica, which was already suggested by Tsuchiya and Nakagawa40. Small black arrows indicate the displacements of the atoms necessary for the transition. Blue atoms indicate oxygen atoms, orange and red atoms indicate silicon atoms of different layers in the rosiaite-structured phase. c Various silica polymorphs based on an hcp arrangement of oxygen atoms. The different structures result from different distributions of silicon atoms in octahedral sites. In stishovite, the silicon atoms occupy octahedral sites that are arranged linearly along the c axis. In contrast, the silicon-filled octahedral sites in SnO2-, NaTiF4-, P21/c-type silica and seifertite are arranged in different zig-zag patterns. In rosiaite-structured silica, octahedral sites form layers with silicon occupancies of one third and two thirds alternating along the c axis. In d-NiAs-type silica, the silicon atoms are randomly distributed in octahedral sites with a probability of 50%. Color coding of the atoms is consistent with (b).
