Equation of state and high-pressure stability of Fe3P-schreibersite:: Implications for phosphorus storage in planetary cores

We have collected in situ X-ray diffraction patterns of end-member Fe3P-schreibersite in a diamond anvil cell to pressures of 30 GPa at 300 K. Some samples of Fe3P were also laser heated at high pressure to temperatures of similar to 2000 K and examined following thermal quench. Below 8 GPa, variation of the schreibersite unit cell delineates a smooth pressure-volume curve corresponding to a second order Birch-Murnaghan equation of state with bulk modulus, K-0T, of 159( 1) GPa. Above 8 GPa, however, the schreibersite structure shows substantial stiffening, and an unidentified structure develops between 17 and 30 GPa; upon decompression to ambient pressure the schreibersite structure returns. Therefore, although ubiquitous in iron-rich meteorites, it is unlikely that schreibersite is the stable phosphorus-bearing phase in deep planetary cores.