![]() The most recent thermodynamic calculations of the Fe phase diagram including solid phases and liquid were performed in a few studies 18, 19, 20, 21 using the CALPHAD thermodynamic formalism 10, 22. The bcc-Fe and fcc-Fe phases are stable with liquid at the first triple point at Р = 5.2 GPa and T = 1991 K 7, whereas hcp-Fe and fcc-Fe were found in equilibrium with liquid at the second triple point, whose location is variable in different works (e.g. There are two triple points along the melting line of Fe. However, bcc-Fe is also suggested as a reliable candidate in the inner core 15, 16. It was argued that hcp-Fe is likely a stable phase in the inner core of the Earth 12, 13, 14. At 1185–1667 K the crystal structure of iron changes to a face-centred cubic (fcc) cell (γ-Fe or fcc-Fe), however, above 1667 K and up to the melting temperature of 1811 K, iron again has the bcc structure (δ-Fe).Īt 10.5 GPa and 753 K 7 (or by more precise and recent measurements at 8.2 GPa and 678 K 11), there is a triple point between bcc-Fe, fcc-Fe, and the high-pressure phase hcp-Fe, which has the hexagonal close-packed (hcp) structure (hcp-Fe). At this transition, the heat capacity of Fe has a characteristic λ-shape form with a maximum at T C 9, 10. The Curie temperature ( T C) of 1043 K marks the transition to the paramagnetic state with the same structure. At the standard conditions ( T = 298.15 K and P = 1 bar), iron is a ferromagnet and has a body-centred cubic (bcc) structure (α-Fe or bcc-Fe). The phase diagram of Fe is relatively complex due to the existence of several polymorphic modifications 7, 8. Iron is a major component of the Earth’s core, therefore knowledge of its P–V–T relations and thermodynamic properties is extremely important 1, 2, 3, 4, 5, 6. We determined the density deficit of hcp-Fe at the inner core boundary ( T = 5882 K and P = 328.9 GPa) to be 4.4%. ![]() The density, adiabatic bulk modulus and P-wave velocity of liquid Fe calculated up to 328.9 GPa at adiabatic temperature conditions started from 5882 K (outer/inner core boundary) were compared to the PREM seismological model. At high-pressure conditions, the metastable bcc–hcp curve is located inside the fcc-Fe or liquid stability field. Therefore, the hcp–liquid curve overlaps the metastable fcc–liquid curve at pressures of about 160 GPa. At conditions near the fcc–hcp–liquid triple point, the Clapeyron slope of the fcc–liquid curve is d T/d P = 12.8 K/GPa while the slope of the hcp–liquid curve is higher (d T/d P = 13.7 K/GPa). ![]() The calculated triple points at the phase diagram have the following parameters: bcc–fcc–hcp is located at 7.3 GPa and 820 K, bcc–fcc–liquid at 5.2 GPa and 1998 K, and fcc–hcp–liquid at 106.5 GPa and 3787 K. The equations of state for solid (with bcc, fcc, and hcp structures) and liquid phases of Fe were defined via simultaneous optimization of the heat capacity, bulk moduli, thermal expansion, and volume at room and higher temperatures.
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