Thermal expansion of MgO, periclase, is approximated by contributions from a quasi‐harmonic perfect crystal and its equilibrium high temperature/pressure defects. We demonstrate that the influence of thermal defects on high pressure thermal expansion is very small and may be neglected at lower mantle conditions. Following that, a simple relationship has been obtained between high pressure thermal expansion and the thermal expansion of a quasi‐harmonic perfect crystal at zero pressure. Our estimation of MgO thermal expansion at the core‐mantle boundary is 9.36 × 1
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