Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2

TL;DR

This study uses density functional perturbation theory to investigate phonon softening and phase transitions in alpha-quartz SiO2 under high pressure, revealing a new monoclinic C2 phase and insights into amorphization.

Contribution

It provides detailed computational analysis of high-pressure phase transitions and introduces a novel monoclinic C2 silica polymorph with mixed silicon coordination.

Findings

Phonon mode softening occurs above 32 GPa, indicating mechanical instability.

A new monoclinic C2 phase forms under uniaxial pressure, with mixed silicon coordination.

High pressure conditions influence amorphization and phase stability of quartz.

Abstract

Density functional perturbation theory calculations of alpha-quartz using extended norm conserving pseudopotentials have been used to study the elastic properties and phonon dispersion relations along various high symmetry directions as a function of bulk, uniaxial and non-hydrostatic pressure. The computed equation of state, elastic constants and phonon frequencies are found to be in good agreement with available experimental data. A zone boundary (1/3, 1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around the same pressure, studies of the Born stability criteria reveal that the structure is mechanically unstable. The phonon and elastic softening are related to the high pressure phase transitions and amorphization of quartz and these studies suggest that the mean transition pressure is lowered under non-hydrostatic conditions. Application of uniaxial pressure, results in a post-quartz crystalline monoclinic C2 structural transition in the vicinity of the K-point instability. This structure, intermediate between quartz and stishovite has two-thirds of the silicon atoms in octahedral coordination while the remaining silicon atoms remain tetrahedrally coordinated. This novel monoclinic C2 polymorph of silica, which is found to be metastable under ambient conditions, is possibly one of the several competing dense forms of silica containing octahedrally coordinated silicon. The possible role of high pressure ferroelastic phases in causing pressure induced amorphization in silica are discussed.