Thermodynamics of Iron Phases at High Pressures and Temperatures

The data on iron melting temperatures at high pressure in a laser-heated diamond-anvil cell, along with the previously existing static pressure data on phase equilibrium reiations, have been used to obtain an internally consistent thermodynamic data base. With such a data base, it is not only possible to calculate phase equilibrium relations among the four well known phases (BCC, 6-BCC, FCC and HCP) but also to consider the effect of introducing additional possible phases yet to be verified. For the DHCP structure we have used a combination of recent X ray as well as laser-heating data to define an approximate field of stability. Similarly for explaining the disparity between the shock wave and static pressure data at pressures above 200 GPa, we have created a thermodynamically stable phase θ. This phase is stabilized because of high enthalpy and entropy; it melts at the core-mantle boundary at 6000 K or above which is much closer to the melting temperature from shock wave data than from the extrapolated static pressure data on melting of HCP (or DHCP).