Iron at earth-core conditions

Abstract

We have carried out electronic structure calculations for iron under high pressure using pseudopotential plane wave methods. There is a controversy regarding the structure of iron at moderate pressures (30-100 GPa) and temperatures (1000-2400 K), with different experiments suggesting different structures, such as orthorhombic, double hexagonal close packed (dhcp), etc. Our earlier calculations using the linear muffin-tin orbital method within atomic-sphere-approximation had argued against the stability of the orthorhombic phase. The more accurate calculations presented here predict qualitatively the same results. We have additionally studied the stability of various phases of Fe at different compressions by calculating phonon frequencies. These rule out the stability of the orthorhombic phase. To validate our zero-temperature electronic-structure results at finite temperature, we have compared the shock Hugoniot and melting properties of iron with the results of our electronic structure calculations. Though we have used simplified models for these estimates, our predictions compare well with the experimental data. We thus propose that these models can be used to obtain the information about the high-pressure melting curves of planetary materials.