Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.

The pressure-induced phase transition in amorphous silicon ͑a-Si͒ is studied using ab initio constantpressure molecular-dynamic simulations. Crystalline silicon ͑c-Si͒ shows a phase transformation from diamond-to-simple hexagonal at 29.5 GPa, whereas a-Si presents an irreversible sharp transition to an amorphous metallic phase at 16.25 GPa. The transition pressure of a-Si is also calculated from the Gibbs free energy curves and it is found that the transformation takes place at about 9 GPa in good agreement with the experimental result of 10 GPa. We also study the electronic character of the pressure-induced insulator to metal transition.

The pressure-induced insulator-metal transition in amorphous GeSe 2 (a-GeSe 2 ) is studied using an ab initio constant pressure molecular-dynamic simulation. a-GeSe 2 transforms gradually to an amorphous metallic state under the application of pressure. The transition is reversible, and is associated with a gradual change from fourfold to sixfold Ge coordination, and from twofold to fourfold Se coordination. Pressure reduces the occurrence of chemical disorder up to 13 GPa. It is found that the optical gap decreases gradually, and the highly localized electronic and vibrational states of the glass at zero-pressure become extended with an increase of the pressure.

We report on the pressure-induced phase transition in amorphous Germanium (a-Ge͒ using an ab initio constant pressure-relaxation simulation. a-Ge exhibits a first-order polyamorphic phase transition at 12.75 GPa with a discontinuous volume change of ϳ19%. The transition pressure is also calculated from the Gibbs free-energy curves, and it is found that the transition occurs at 5.2 GPa in agreement, with the experimental result of 6 GPa. The pressure-induced delocalization of electronic and vibrational states is obtained.

The pressure-induced phase transition in silicon carbide is studied using a constant-pressure ab initio technique. The reversible transition between the zinc-blende structure and the rock-salt structure is successfully reproduced through the simulation. The transformation mechanism at the atomistic level is characterized, and it is found that the transition is based on a tetragonal and an orthorhombic intermediate state. The space groups of the intermediate states are determined as I4m2 and I mm2.

We study pressure-induced phase transitions in amorphous silicon and crystalline diamond silicon from Gibbs free energies considerations using ab initio total energy calculations. We predict a pressure-induced crystallization of the amorphous network at 2.5 GPa and a first order amorphous to amorphous phase transition at 9 GPa. Furthermore, we find a pressure-induced high density amorphization of crystalline diamond silicon around 15 GPa.

The pressure-induced phase transition in GaAs is studied using an ab initio constant-pressure relaxation simulation. GaAs undergoes a first-order phase transition to Cmcm at 54 GPa. Upon further increase of pressure a gradual phase change to Imm2 structure is seen at 57 GPa, which confirms an earlier experiment and clears some doubts about the existence and identity of Imm2. The transition pressures are also calculated from the Gibbs free energy, and it is found that the structural phase change occurs at 23.5 GPa for Cmcm and at 24 GPa for Imm2. The transformation path from Cmcm and Imm2 proceeds through sliding of some Cmcm planes and relatively large sliding yields a transition from Imm2 to simple hexagonal structure. We find that Cmcm and Imm2 phases are semimetals.

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