Hydrogen Occupancy, Site Hierarchy, and Hydride-Transformation Pathways in BCC High-Entropy Alloys

Body-centered cubic (BCC) high-entropy alloys (HEAs) are among the most promising HEA-based solid-state hydrogen-storage materials, yet their behavior is still too often discussed through composition, average phase label, or storage capacity alone. This Perspective argues that such descriptions remain incomplete because hydrogen accommodation in BCC HEAs is governed by the interplay among local interstitial accessibility, site hierarchy, and hydrogen-induced structural evolution. We therefore recast the problem around three linked questions: where hydrogen resides first, how the relative accessibility of tetrahedral and octahedral environments evolves with loading, and how that evolving occupancy redirects the host lattice toward specific hydride-transformation pathways. Recent experimental and computational studies show that hydrogen occupation in BCC HEAs is mixed, selective, and concentration-dependent, rather than fixed to a single ideal interstitial type. They also show that direct BCC-to-FCC/BCT-type hydrogenation routes, as well as pathway failure in structurally unstable BCC-related systems, are best understood from this occupancy-centered viewpoint. On this basis, we suggest that future design of BCC HEA hydrides should move beyond composition screening toward an occupancy-informed framework in which local site hierarchy, pathway integrity, and hydrogen-induced phase switching are treated as central design variables.