Ab initiostudies of phonon softening and high-pressure phase transitions ofα-quartzSiO2

Abstract

Density functional perturbation theory calculations of α-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.

First-principles calculations of the high-temperature phase transformation in yttrium tantalate

“…In Refs. [11][12][13][14] , expressions for the Landau free energy expansion for the high symmetry stishovite phase were successfully derived and applied to explain the observed spontaneously macroscopic strains and elastic behaviors. The transformation under pressure of another oxide, the ScVO 4 compound was studied by Panchal et al 15 by Raman scattering and density-functional perturbation theory (DFPT) calculations.…”

A first principles based polarizable O(N) interatomic force field for bulk silica

“…Our results are 5.56 within the LDA and 4.43 with the PBE-GGA, to be compared with the experimental value of BЈ = 5.99. 13 In general, the PBE-GGA functional is less accurate at predicting quartz elastic properties, 13,15 which supports our decision to use the LDA to reparametrize our potential. The elastic constants predicted by the new potential are in excellent agreement with both DFT LDA and experiment.…”

“…[13][14][15][16][17][18] The LDA consistently underestimates the equilibrium volume of quartz compared with experiment, whereas the PBE-GGA overestimates by a slightly larger factor, leading to increased errors in the elastic properties with respect to experiment. 13 This point is further illustrated by considering a third-order fit to the Murnaghan equation of state.…”

“…[13][14][15][16][17][18][19] The phase diagram of silica is very complex, with a large number of high pressure crystalline polymorphs. 20 In this work, our motivation is to develop a new potential suitable for simulations of crystalline and amorphous silica at low temperatures and pressures.…”

High-pressure structural and elastic properties of Tl2O3

“…47 PIA due to these instabilities usually occurs when the crystalline solid cannot undergo a phase transition to a HP crystalline phase at a smaller pressure than that of amorphization. The hindrance of the pressure-driven phase transition between two crystalline phases is usually due to the presence of kinetic barriers between the low-and highpressure structures.…”