Decoding the hydrogen storage signature of LaMgHn n = 1–24 clusters: Atomic-scale insights for rare-earth modified magnesium hydrides

The rational design of high-capacity hydrogen storage materials demands a fundamental understanding of hydride formation at bimetallic active sites. We present a comprehensive first-principles investigation of LaMgHn n = 1–24 clusters (where n denotes the number of hydrogen atoms), the simplest rare-earth–magnesium diatomic model. Unbiased global structure search combined with density functional theory reveals that the rigid La-Mg dimer forces homogeneous hydrogen distribution on both metal centers, a behavior that contrasts sharply with the site-specific adsorption in Sc-Mg systems. Energetic analysis identifies pronounced odd–even stability oscillations and several magic clusters (LaMgH5, LaMgH7, LaMgH17) whose exceptional stability is electronically encoded by optimal La charge and even-electron closed-shell configurations favored by odd-n electron counting. Leveraging Quantum Theory of Atoms in Molecules, we establish a three-level hydrogen storage classification: (1) anchored isolated H, (2) anchored H2, and (3) unanchored physisorbed H2. This physically grounded framework shows that all hydrogen up to n = 16 belongs to Levels I and II, giving LaMgH16 a theoretical gravimetric density of 8.99 wt% at 0 K. However, finite-temperature RRHO thermodynamic analysis reveals that only n = 8 among the Level 2-containing clusters n = 6–16 retains its anchored H2 at room temperature (ΔG(300 K) > 0), while all others, including LaMgH16, spontaneously desorb their Level 2H2 above their respective Tdes (ranging from < 50 K to 329 K). This framework effectively separates usable hydrogen from weakly bound species. For n ≥ 17, unanchored H2 emerges, defining a practical capacity ceiling. By integrating energetic, electronic, and topological descriptors, this work delivers an atomic-scale characterization for the La-Mg core, offering useful insights for guiding the development of rare-earth-modified magnesium hydrides. It is important to emphasize that this study is based on isolated gas-phase clusters (0D model) and the reported gravimetric densities are theoretical limits for individual molecules, not directly achievable capacities for bulk solid-state materials. The results provide fundamental insights into metal–hydrogen interactions at the atomic level, which may inform the design of nanostructured or amorphous hydrides where such diatomic motifs could be stabilized, but direct extrapolation to bulk crystalline hydrides would be inappropriate.