Comparison of liquid-vapor relations and the critical points of CaO and MgO

Refractory materials play an important role in lunar formation as they are some of the first components to coalesce from vaporized material that resulted from a giant impact. CaO is chemically and structurally similar to MgO and together they make up over 40% of both pyrolite and lunar composition models. In this work, we explore the liquid-vapor coexistence region of CaO. We determine the critical point to be located between 6000 and 6500 K and between 0.3 and $0.7\phantom{\rule{0.28em}{0ex}}\mathrm{g}/{\mathrm{cm}}^{3}$, which puts it slightly below the one previously obtained for MgO. Structurally, both CaO and MgO consist mostly of sixfold and sevenfold coordinated cations at the higher density and threefold and fourfold ones at the lower end of the density studied. The vapor consists mainly of Ca, CaO, O, ${\mathrm{O}}_{2}$, ${\mathrm{Ca}}_{2}{\mathrm{O}}_{2}$, ${\mathrm{Ca}}_{2}\mathrm{O}$, and ${\mathrm{CaO}}_{2}$. The main difference with MgO is that the relative abundance of molecular oxygen is much lower in the vapor of CaO. Similarly to the MgO system, we find that the vapor appears to be close to neutral in charge, indicating an atomic vapor as opposed to an ionic one.