Molten iron-hydrogen alloys are likely an important constituent of planetary cores, making the material properties of iron hydride liquids relevant across the universe. Yet, they are underexplored relative to other iron-light element alloys. This thesis aims to holistically characterize molten iron-hydrogen alloys at the high pressures and temperatures of rocky planetary interiors using ab initio molecular dynamics simulations.
First, I describe how the properties of the melts vary as function of hydrogen concentration, pressure, and temperature. The addition of hydrogen pushes iron atoms apart, and at high hydrogen contents, H2 molecules can form in the iron framework. Hydrogen lubricates the melts, as atomic diffusivity increases, and viscosity decreases with hydrogen content. Parameterizing a pressure-volume-temperature-composition equation of state, I quantify how hydrogen increases the compressibility, decreases the molar volume, and increases the thermal expansivity of the melts. Next, I investigate the stability of liquid mixtures along the Fe-H2 binary. Noteworthy in this study is the identification of H2 exsolution from iron hydride melt not only by examining the chemical speciation in the atomistic simulation, but also by using the Gibbs free energy of mixing attained from thermodynamic integration. I identify conditions at which iron hydride and H2 melts are immiscible, quantifying how the solubility limit of H2 in iron hydride melt increases as a function of both pressure and temperature. Low solubility at lower pressures and temperatures may limit the amount of hydrogen that small bodies can have in their cores, whereas iron and hydrogen are fully miscible at the core conditions of larger planets. Finally, in an exploration of the iron-hydrogen-oxygen and iron-hydrogen-sulfur systems, I demonstrate how binary phase relations are fundamental to understanding and predicting the stability of higher-order multicomponent systems.