Iron Phase Diagram at High Pressures and Temperatures

Abstract

Diamond-anvil cell (DAC) high-pressure technique with in situ laser heating of a sample permits the determination of iron phase diagram to pressures reaching planetary cores. The DAC technique in combination with in situ x-ray study of iron has revealed the presence of at least one additional state of iron with possibly a double hexagonal closest packed (DHCP) structure given the name β. A review of the available data, indicates that in addition to the previously known triple points, the BCC( body-centered cubic)-HCP (hexagonal closest packed)-FCC (face centered cubic) and the β-BCC-FCC-melt, the following triple-points may exist in the iron phase diagram: the HCP-FCC-β: pressure (P) = 40 (4) GPa at temperature (T) = 1550 (100) K, the β-FCC-melt: P = 60 (10) GPa at T = 2600 (100) K. We define the stability of β-phase between pressures of 37 to 300 GPa at high temperatures. The HCP-β phase boundary has a small negative dP/dT indicating the similarity of physical properties (molar volume, thermal expansi n and bulk modulus) between the two but a higher entropy and enthalpy for the β-phase. The melting curve of iron has been determined quite reliably with the laser heated sample in the DAC to a pressure of about 80 gigapascal (GPa). The pressure range of melting has been extended to as high as 200 GPa but is not supported by shock-wave data and requires further studies for confirmation.