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

Abstract

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 ∼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.

Keywords:

• phosphide
• equation of state
• bulk modulus
• planetary cores
• barringerite
• schreibersite

Acknowledgements

We gratefully acknowledge Y. Al-Khatatbeh and K. Lee for access to experimental beamtime and P. Dera for helpful discussions. We also thank S. Kentner and K. Crispin for experimental assistance. Use of the HPCAT facility was supported by U.S. Department of Energy (DOE)-Basic Energy Sciences, DOE-National Nuclear Security Administration, NSF, Department of Defense-Tank-Automotive and Armaments Command and the W.M. Keck Foundation. GSECARS is supported by the National Science Foundation – Earth Sciences (EAR-0217473), Department of Energy – Geosciences (DE-FG02-94ER14466) and the State of Illinois. Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund, for partial support of this research (HPS), and to the IUSB SMART program (NB).