Melting behavior of CaO at high temperature and pressure: a molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to accurately determine the melting behavior of calcium oxide (CaO) at high temperatures and pressures, resolving previous experimental discrepancies and providing new insights into its phase diagram.

Contribution

First ab-initio and classical MD calculations of CaO melting curve without assuming Clapeyron slope, revealing pressure-dependent overheating ratios and refining phase diagram models.

Findings

Melting temperature of CaO around 2940-3066 K.

Overheating ratio increases with pressure.

First calculation of high-pressure melting curve without Clapeyron slope assumptions.

Abstract

The thermodynamic behavior of calcium oxide (\ce{CaO}) under high temperature and pressure conditions is critical for understanding the physics of planetary interiors. This study employs molecular dynamics (MD) simulations, including both classical and ab-initio approaches, to investigate the melting behavior of CaO. We calculate the melting temperature of \ce{CaO} by the void-nucleated melting and two-phase coexistence techniques, aiming to resolve discrepancies in experimental data on the melting point, which range from 2843~K to 3223~K in different studies due to the high reactivity and vapor pressure of the substance. The obtained results are $T_f = 3066\pm12$~K and $T_f = 2940\pm65$~K using the void-nucleated melting and the two-phase coexistence method, respectively. Additionally, we calculate the enthalpy of fusion and the high-pressure melting curve, for the first time without making any assumption on the Clapeyron slope. This is extremely important since in experiments the Claperyon slope of the melting curve is estimated from low pressure measurements and the overheating ratio (i.e. $\eta=\frac{T_s}{T_f}-1$, where $T_s$ represents the thermal instability limit corresponding to the homogeneous melting temperature of the solid) is often assumed to be constant in simulations. Our MD results show that $T_s$ increases more rapidly with pressure than $T_f$ and thus that the overheating ratio sensibly depends upon pressure. These findings contribute to the accurate modeling of the CaO phase diagram, which is essential for geochemistry, cosmochemistry, and materials science.