Why do compact grain boundary complexions prevail in rock-salt materials?

Over 40 years of studies of grain boundaries in ionic rock-salt materials have left the community divided. On one hand, numerical simulations systematically predict open, hollow structural units to be the ground state. On the other, most recent experiments report compact structures to be the norm. To reconcile modelling with experimental evidence, we investigate the stability of three high-angle symmetric tilt grain boundaries in magnesium oxide MgO with respect to the presence of charge-neutral vacancy pairs. We demonstrate that although open structural units are energetically the most favourable, they are easily destabilized by vacancies. It follows that open complexions can never be at equilibrium with the surrounding grains at finite temperature. On the contrary, compact structural units can accommodate a wide range of defects concentrations, and are much more resilient with respect to the absorption of vacancies. These results highlight the limitations of studies that consider only the ground state, and stress the importance of accounting for the presence of other defects when modelling grain boundaries in ionic materials.