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

TL;DR

This study combines first-principles calculations with statistical mechanics to analyze hydrogen vacancy thermodynamics in MgH2, revealing limited vacancy concentrations mainly of isolated defects under practical conditions.

Contribution

It introduces a grand-canonical ensemble method for modeling hydrogen exchange in MgH2, providing new insights into vacancy behavior and thermodynamics.

Findings

MgH2 can only host very small hydrogen vacancy concentrations.

Vacancies mainly exist as isolated defects, not clusters.

The method accurately predicts vacancy thermodynamics under realistic conditions.

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 introduced to model the exchange of hydrogen between crystalline MgH2 and gas-phase H2. We find that, at temperatures and hydrogen partial pressures of practical interest, MgH2 is capable of accommodating only very small concentrations of hydrogen vacancies, which consist mainly of isolated defects rather than vacancy clusters, contrary to what is expected from a simple energetic analysis.