The phase diagram of NiSi under the conditions of small planetary interiors

•Phase diagram of NiSi to 60GPa.•NiSi is much more complex than FeSi, with 4 stable structures and 2 metastable structures.•At conditions of Earth’s core NiSi will likely have the CsCl structure and form a solid solution with FeSi.•At conditions of the cores of smaller terrestrial planets NiSi will likely be FeSi-structured and form a eutectic melt with FeSi.