Understanding Chemical Changes across the α-Cristobalite to Stishovite Transition Path in Silica

The interplay of structural, energetic, and chemical bonding features across the pressure-induced α-cristobalite → stishovite phase transition in SiO2 is fully characterized by first principles calculations and topological analysis of the electron localization function. Under a martensitic approach, the transition mechanism is theoretically modeled at the thermodynamic transition pressure using the α-cristobalite P41212 space group. A soft and symmetric transition path is determined with an activation barrier lower than 100 kJ/mol. No bond breaking is found, but a synchronous bonding formation process leading to Si 6-fold and O 3-fold coordinations. Only when the third coordination sphere of oxygens approaches a given Si at a distance close to 2.0 A do new Si–O bonding basins appear. This situation occurs at a transformation stage well beyond the maximum in the energetic profile of the transition path.