THE EFFECT OF FERROMAGNETISM ON THE EQUATION OF STATE OF FE3C STUDIED BY FIRST-PRINCIPLES CALCULATIONS

Abstract

First-principles calculations have been used to determine the equation of state of Fe3C in both its low-pressure magnetically ordered and high-pressure non-magnetically ordered states; at 0 K the ferromagnetic transition was found to occur at about 60 GPa. In the high pressure, non-magnetically ordered regime at 0 K the material may be described by a Birch-Murnaghan third-order equation of state with V0=8.968(7) Α3 per atom, K0=316.62(2) GPa and K'=4.30(2). At atmospheric pressure the ferromagnetic phase transition in Fe3C occurs at ~483 K; preliminary measurements of the thermal expansion by powder neutron diffraction show that this transition produces a large effect on thermoelastic properties. The volumetric thermal expansion coefficient in the paramagnetic phase was found to be 4.34x10-5 K-1 at T~550 K. By applying a thermal expansion correction to the calculated equation of state at 0 K, predicted values for the density and adiabatic incompressibility of this material at core pressures and temperatures were obtained. These results appear to be sufficiently different from seismological data so as to preclude Fe3C as the major inner core-forming phase.