Thermodynamic properties of epsilon-Fe with thermal electronic excitation effects on vibrational spectra

[摘要] The thermodynamic properties of hexagonal-close-packed iron (epsilon-Fe) are essential for investigating the internal structure and dynamic properties of planetary cores. Despite their importance to planetary sciences, experimental investigations of epsilon-Fe at relevant conditions are still challenging. Therefore, ab initio calculations are crucial to elucidating the thermodynamic properties of this system. Here, we use a free energy calculation scheme based on the phonon gas model compatible with temperature-dependent phonon frequencies. We investigate the effects of electronic thermal excitations, which introduces a temperature dependence on phonon frequencies, and the implication for the thermodynamic properties of epsilon-Fe at extreme pressure (P) and temperature (T) conditions. We disregard phonon-phonon interactions, i.e., anharmonicity and their effect on phonon frequencies. Nevertheless, the current scheme is also applicable to T-dependent anharmonic frequencies. We conclude that the impact of thermal electronic excitations on vibrational properties is not significant up to similar to 4000 K at 200 GPa but should not be ignored at higher temperatures or pressures. However, the static free energy F-st must always include the effect of thermal excitation fully in a continuum of T. Our results for isentropic equations of state show good agreement with data from recent ramp compression experiments up to 1400 GPa conducted at the National Ignition Facility.

[关键词] DENSITY-OF-STATES;MOLECULAR-DYNAMICS;SOUND VELOCITIES;HCP-FE;IRON;PRESSURE;TRANSITION;METALS;ENERGY;ALLOY