2026/08/02 by Qian Wang, Zixin Xu, Ryuhei Sato +6
Physics and Astronomy · #cond-mat.mtrl-sci
21 pages, 4 figures
arxiv created 2026/08/02 · arxiv updated 2026/08/04
Ammonia (NH3) absorption drives LiBH4 ⋅ xNH3 through a re-entrant "solid-liquid-solid" transition: LiBH4⋅ NH3 is a well-defined solid ammoniate, compositions near LiBH4⋅ 2NH3 are liquid-like or partially liquefied, whereas LiBH4⋅ 3NH3 returns to a more rigid, non-liquid ammoniate state. However, the microscopic origin of this unintuitive response remains a long-lasting mystery. Here, we uncover its mechanism. Cross-database analysis identifies borohydrides as a particularly state-diverse and composition-responsive material family. Structure prediction and ab initio molecular simulations reveal that NH3 progressively replaces BH4- in the Li coordination shell. The liquid-like state emerges not at the highest NH3 loading but near x ≈ 2, where Li-N and Li-B coordination modes are strongly mixed, coordination memory is weakest, and the sampled Li-N/N⋯B coordination landscape is broadest. Further ammoniation produces Li-N-dominant coordination and slows BH4-/NH3 contact renewal, resulting in increased network persistence and recovery of a rigid ammoniate state. Pressure-composition isotherm, 1H and 11B nuclear magnetic resonance, and Raman measurements support this non-monotonic state evolution and the associated BH4-/NH3 reorganization. These findings transform ammonia-induced liquefaction from an empirical phase anomaly into a competition among native-network disruption, mixed-coordination frustration, and ligand-built network reconstruction, providing a framework for chemically switching between transport-favouring fluidity and stability-favouring rigidity in hydrogen-rich materials.