P-V equation of state for Fe2P and pressure-induced phase transition in Fe3P

As part of our ongoing investigations of elasticity and high-pressure stability in the Fe-P system, we have measured the room-temperature bulk modulus (K-0T) of Fe2P, barringerite, to 8 GPa using in situ synchrotron X-ray diffraction and diamond anvil cells. A second-order fit (i.e. dK/dP fixed at 4) to our experimental data using the Birch-Murnaghan equation of state produces a K-0T of 165 +/- 3 GPa. This value is similar to 4% less than the experimental values for Fe3P. For comparison with the experimental data, we have also performed first-principle theoretical calculations on this phase. For ferromagnetic Fe2P at zero pressure, we find that the magnetic moments increase rapidly for a Hubbard U > 1 eV and are significantly higher than observed experimentally. Thus, our results support previous findings that magnetism in Fe2P is largely itinerant with at most a minor component due to on-site correlation in the iron-3d shell. Additionally, we present new high-pressure diffraction data for a natural Fe3P, schreibersite, sample which conclusively demonstrate that a first-order phase transformation occurs between 15 and 20 GPa.