Using molecular-dynamics simulations we have examined a quartzlike polymorph of silica, the structural analog of ice XII recently found in applied-field computer simulations of water [I. M. Svishchev and P. G. Kusalik, Phys. Rev. B 53, 8815 (1996)]. This form of silica is built from continuous zig-zag chains of the ${\mathrm{SiO}}_4$ tetrahedra similar to those found in the familiar hexagonal quartz structure and it retains the same valent Si-O distance as in hexagonal quartz, roughly 1.7 Å. Yet, the linkage pattern of the coordination polyhedra in the structure is such that it results in an orthorhombic, Pnc2, symmetry with 12 ${\mathrm{SiO}}_2$ per unit cell. Under ambient conditions the density of the orthorhombic phase is roughly 5% higher than that of hexagonal quartz. This orthorhombic form of silica is found to be stable over a wide range of temperatures and pressures, up to 1500 K and 10 GPa, respectively.

DOI:https://doi.org/10.1103/PhysRevB.55.721