Pressure- and temperature-dependent anharmonicity of MgO: Implications for the thermal conductivity of planetary mantles

Thermal conductivity of minerals composing planetary mantles plays a fundamental role in controlling the heat propagation and hence the dynamic history of a planet. Here we present ab-initio calculations of the lattice dynamics and thermal conductivity of MgO as a function of temperature and pressure, accounting for phonon scattering and the renormalization it induces. Our calculations, validated by measurements of phonon energies and linewidths, point to a complex interplay between pressure-induced and temperature-induced effects, which influences the predominance of 3- or 4-phonon contributions, the former leading to a reduction in energies, the latter to an increase. Thus, not only the relative magnitude but also the sign of the anharmonic corrections depends strongly on the planetary geotherms. Our study shows that calculations taking into account only 3phonon scattering underestimate thermal conductivity by 20-45% under the conditions of the Earth's lower mantle, while including anharmonic renormalization up to fourth order provides results in good agreement with high-pressure, high-temperature experiments. The predominance of 3-phonon interactions at core-mantle boundary (CMB) conditions significantly reduces phonon energies, leading to a thermal conductivity of 50.7 Wm(-1)K(-1), further reduced by extrinsic effects, including isotopic disorder, oxygen vacancies and iron inclusion. In particular, oxygen vacancies of up to 1% decrease the thermal conductivity of MgO at CMB conditions by similar to 40%, an effect that adds to that of Fe/Mg replacement. Our results indicate that mass disorder effectively reduces thermal conductivity of lower mantle minerals, contributing to the thermal blanketing that limits the heat flux from the core.