Ab initio calculations and statistical mechanics are combined to elucidate the thermodynamics of H vacancies in ${\text{MgH}}_2$. A general method based on a grand-canonical ensemble of defect configurations is presented to model the exchange of hydrogen between crystalline ${\text{MgH}}_2$ and gas-phase ${\text{H}}_2$. We find that, even at the lowest hydrogen partial pressures at which the hydride phase is stable, ${\text{MgH}}_2$ is capable of accommodating only very small concentrations of hydrogen vacancies. These vacancies are mainly isolated rather than forming clusters, contrary to what is expected from a simple energetic analysis.

• Received 19 October 2009

DOI:https://doi.org/10.1103/PhysRevB.80.174117