Structure and behavior of the barringerite Ni end-member, Ni₂P, at deep Earth conditions and implications for natural Fe-Ni phosphides in planetary cores

High-pressure and high-temperature behavior of synthetic Ni₂P has been studied in a laser-heated diamond anvil cell up to 50 GPa and 2200 K. Incongruent melting associated with formation of pyrite-type NiP₂ and amorphous Ni-P alloy was found at an intermediate pressure range, between 6.5 and 40 GPa. Above 40 GPa, Ni₂P melts congruently. At room conditions, Ni₂P has hexagonal C22-type structure, and without heating it remains in this structure to at least 50 GPa. With a bulk modulus .K₀ = 201(8) GPa and K' = 4.2(6 Ni₂P is noticeably less compressible than hep Fe, as well as all previously described iron phosphides, and its presence in the Earth core would favorably lower the core density. In contrast to Fe₂P, the c/a ratio in Ni₂P decreases on compression because of the lack of ferromagnetic interaction along the c direction. Lack of the C22→C23 transition in Ni₂P rules out a stabilizing effect of Ni on the orthorhombic phase of natural (Fe_1-xNi_x)P allabogdanite.