This review paper focuses on dislocations and plastic deformation in magnesium oxide crystals. MgO is an archetype ionic ceramic with refractory properties which is of interest in several fields of applications such as ceramic materials fabrication, nano-scale engineering and Earth sciences. In its bulk single crystal shape, MgO can deform up to few percent plastic strain due to dislocation plasticity processes that strongly depend on external parameters such as pressure, temperature, strain rate, or crystal size. This review describes how a combined approach of macro-mechanical tests, multi-scale modeling, nano-mechanical tests, and high pressure experiments and simulations have progressively helped to improve our understanding of MgO mechanical behavior and elementary dislocation-based processes under stress.

A hierarchical multi-scale model was used to study the effect of high pressure on the critical shear stresses of MgO. The two main slip systems, ½⟨110⟩{110} and ½⟨110⟩{100}, were considered. Based on a generalised Peierls–Nabarro model, it is shown that the core structure of ½⟨110⟩ screw dislocations is strongly sensitive to pressure. Mostly planar and spread in {110} at ambient pressure, the core of screw dislocations tends to spread in {100} with increasing pressure. Subsequently, an inversion of the easiest slip systems is observed between 30 and 60 GPa. At high pressure, the plasticity of MgO single crystals is expected to be controlled by ½⟨110⟩{100} slip systems, except at high temperature where both slip systems become active. Pressure is also found to increase the critical resolved shear stresses and to shift the athermal temperature toward higher temperatures. Under high pressure, MgO is thus characterised by a significant lattice friction on both slip systems.