A two-phase equation of state for Aluminum

By well-established theoretical means, it is now possible to calculate thermodynamic properties of elemental crystals and liquids to an accuracy matching that of available low-pressure experiments; it is also known that the neglect of anharmonicity and electron-phonon interactions introduces errors smaller than 5% at low pressures. To check this theoretical framework and the validity of these approximations at higher pressures, we have constructed a Helmholtz free energy for crystal and liquid Aluminum and compared the Hugoniot it predicts with experiments up to pressures of 10 Mbar, densities of 7 g/cm3, and temperatures of roughly 6 eV. We use standard lattice dynamics for the crystal phase and Wallace’s liquid dynamics for the liquid. We find excellent agreement of theory and experiment through the entire range, not just at low pressures, and we find that the inclusion of melting is important for achieving this agreement.