Iron phases at high pressures and temperatures: Phase transition and melting

Based on an in situ X-lay study of Fe and a review of the available data, w e propose the following triple-points in the Fe phase diagram-the hexagonal closest packed (HCP), face-centered cubic (FCC), and the beta phase: P = 4 0 (4) GPa at T = 1550 (100) K, the beta-phase-FCC-melt; P = 60 (10) GPa at T = 2600 (100) K. We define the stability of beta phase from a combination of new X-ray results on externally heated Fe between pressures of 37 to 300 GPa, as well as the previous data on externally heated and laser-heated sa mples. X-ray data on externally heated Fe, without any pressure medium, con firms the double hexagonal closest packed (DHCP) structure for the beta pha se. The HCP-beta phase boundary has a very small negative dP/dT indicating the similarity of physical properties (molar volume. thermal expansion, and bulk modulus) between the two phrases, but a higher entropy and enthalpy f or the beta phase.