Elasticity and anisotropy of iron-nickel phosphides at high pressures

Abstract

Geochemical estimates indicate that around 90% of the planet's inventory of phosphorus is likely to be sequestered in the Earth's core. Iron phosphides such as scheirbisites (Fe 3 P) are commonly found in iron meteorites. Recently, melliniite (Fe,Ni) 4 P with 12.2 wt% phosphorus has been reported in iron-meteorites. Using static electronic structure calculations, we predict that Fe 4 P is unlikely to dissociate into Fe 3 P and hcp Fe at inner core conditions. Among the different structural varieties of Fe 4 P, we find the cubic polymorph with P2 1 3 space group symmetry to be stable over a wide range of geophysically relevant pressures. We have determined the equation of state and the full elastic constant tensor of the stable (Fe,Ni) 4 P phase at pressures up to 400 GPa. Upon compression, Fe 4 P undergoes a ferromagnetic (fm) to nonmagnetic (nm) transition at 80 GPa. In nonmagnetic (Fe,Ni) 4 P, nickel incorporation results in reduction of the P-and S-wave velocities. However, incorporation of nickel enhances the P-and S-wave anisotropy. Copyright 2011 by the American Geophysical Union.