Stabilizing $\gamma$-MgH$_2$ at Nanotwins in Mechanically Constrained Nanoparticles

TL;DR

This paper presents a nanoparticle design that stabilizes metastable gamma-MgH2 during hydrogenation, improving dehydrogenation properties for hydrogen storage and plasmonics, by leveraging volume expansion-induced stress and twinning mechanisms.

Contribution

It introduces a novel nanoparticle approach that intrinsically forms and stabilizes gamma-MgH2 through stress-induced twinning during hydrogenation.

Findings

Gamma-MgH2 forms via twinning in confined nanoparticles.

Residual compressive stress stabilizes gamma-MgH2 nanolamellas.

The design enhances cycle stability for hydrogen storage applications.

Abstract

Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH$_2$ system a prime candidate for solid state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable $\gamma$-MgH$_2$. A nanoparticle (NP) design, where $\gamma$-MgH$_2$ forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed.Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of $\beta$-MgH$_2$ via (301) $\beta$ twinning. It is proposed that these twins nucleate $\gamma$-MgH$_2$ nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle-stable, Mg-based dynamic plasmonic and hydrogen-storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform $\gamma$-MgH$_2$ during each rehydrogenation