High-pressure phase transitions and equations of state in NiSi. I. Ab initio simulations

First-principles calculations have been used to determine the equation of state and structural properties of NiSi up to pressures equivalent to that in the Earth's inner core. At atmospheric pressure, the thermodynamically stable phase is that with the MnP structure (as found experimentally). At high pressures, NiSi shows phase transformations to a number of high-pressure polymorphs. For pressures greater than similar to 250 GPa, the thermodynamically stable phase of NiSi is that with the CsCl structure, which persists to the highest pressures simulated (similar to 500 GPa). At the pressures of the Earth's inner core, therefore, NiSi and FeSi will be isostructural and thus are likely to form a solid solution. The density contrast between NiSi and FeSi at inner-core pressures is similar to 6%, with NiSi being the denser phase. Therefore, if a CsCl-structured (Fe, Ni) Si alloy were present in the inner core, its density (for the commonly assumed nickel content) might be expected to be similar to 1% greater than that of pure FeSi.