Unbiased crystal structure prediction of NiSi under high pressure

On the basis of an unbiased structure prediction, it is shown that the stable form of NiSi under pressures of 100 and 200 GPa is the Pmmn structure. Furthermore, a new stable phase has been discovered: the deformed tetragonal CsCl-type structure with a = 2.174 Å and c = 2.69 Å at 400 GPa. Specifically, the sequence of high-pressure phase transitions is the following: the Pmmn structure below 213 GPa, the tetragonal CsCl type in the range 213–522 GPa, and cubic CsCl higher than 522 GPa. As the CsCl-type structure is considered as the model structure of the FeSi compound at the conditions of the Earth's core, this result implies restrictions on the Fe–Ni isomorphic miscibility in FeSi.