Phase behavior of protoenstatite at high pressure studied by atomistic simulations

Structural phase transformations of MgSiO₃ protoenstatite at high pressures are studied by atomic scale simulation techniques. Molecular-dynamics simulations and electronic-structure calculations reveal two metastable polymorphs with space groups P2₁cn and Pbcn, respectively. They are related to protoenstatite by displacive transition mechanisms via subsequent change of the silicate chain rotations from O- to S-type. Metadynamics simulations in combination with molecular dynamics reveal possible mechanisms for the martensitic transition from protoenstatite to high-pressure clinoenstatite. Two different shear mechanisms in the (100) plane are activated during the transition. The first consists of four partial displacements in (100)[001] and (100)[010], whereas in a second step only a single shear in (100)[001] is observed.