Thermodynamics of hydrogen vacancies in MgH₂ from first-principles calculations and grand-canonical statistical mechanics

Abstract

Ab initio calculations and statistical mechanics are combined to elucidate the thermodynamics of H vacancies in MgH₂. A general method based on a grand-canonical ensemble of defect configurations is presented to model the exchange of hydrogen between crystalline MgH₂ and gas-phase H₂. We find that, even at the lowest hydrogen partial pressures at which the hydride phase is stable, MgH₂ 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.