Structure and behavior of the barringerite Ni end-member, Ni_2P, at deep Earth conditions and implications for natural Fe-Ni phosphides in planetary cores

摘要

High-pressure and high-temperature behavior of synthetic Ni_2P 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 NiP_2 and amorphous Ni-P alloy was found at an intermediate pressure range, between 6.5 and 40 GPa. Above 40 GPa, Ni_2P melts congruently. At room conditions, Ni_2P has hexagonal C22-type structure, and without heating it remains in this structure to at least 50 GPa. With a bulk modulus K_0 = 201(8) GPa and K' = 4.2(6), Ni_2P is noticeably less compressible than hcp 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 Fe_2P, the c/a ratio in Ni_2P decreases on compression because of the lack of ferromagnetic interaction along the c direction. Lack of the C22→C23 transition in Ni_2P rules out a stabilizing effect of Ni on the orthorhombic phase of natural (Fe_(1-x)Ni_x)_2P allabogdanite.