In order to better understand properties of iron and iron mixtures under earth core conditions. Linear Muffin-Tin Orbital (LMTO) band structure calculations for iron, sulfur, oxygen, and iron-sulfide (in CsCl, WC, and AuCu3 structures) were undertaken. The computationally efficient LMTO method has proven accurate in determining equation-of-state behavior; even for iron (where predicted properties represent near worse case examples) the deviations between theoretical and measured densities in the 100-GPa pressure regime are found to be less than 2%. Sulfur and oxygen have differing high-pressure behavior. Sulfur experiences a marked s-p state to d state electronic transition with increasing compression that leads to an atomic volume compatible with ideal mixing in close-packed iron-dominated structures. In the iron-sulfide system, sulfur and iron have overlapping bands that further enhance solid solution behavior. In contrast, oxygen, with no closed-shell p electrons does not become a d band metal at high pressure. Its smaller atomic volume excludes alloy solid solution behavior with iron.

Electronic contributions to specific heat are accurately estimated under ambient conditions by the LMTO calculations. Under Earth core and high-pressure Hugoniot conditions, the electronic contribution to specific heat is 2/3 as large as the lattice contribution. Hugoniot temperatures for ϵ iron at 200 GPa determined through integration of thermodynamic quantities are found to be within experimental errors of results based on radiative measurements of shocked iron interfaces. Based on Hugoniot measurements, the melting temperature for iron at 243 GPa is 5600¿500 K. The difference in density between the solid inner core and liquid outer core may be larger than that expected for chemistry dominated by an iron-sulfur solid solution series. In contrast, the expected eutectic behavior between iron and FeO may be consistent with the seismic model, if core chemistry lies to the iron-rich side of the eutectic composition. Under this assumption, a core mantle boundary temperature of 3200 K is estimated. ¿ American Geophysical Union 1990