Why Ammoniated Lithium Borohydrides Liquefy and Resolidify?
Abstract
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 increasing NH3 loading increases the direct Li--N coordination number while progressively decreasing BH4--associated contacts in the local Li environment. Near x ≈ 2, these contributions are most balanced among the simulated compositions, and the sampled Li--N/N⋯B coordination landscape is broadest. Further ammoniation produces Li--N-dominant coordination and slower BH4-/NH3 contact renewal, accompanying recovery of a more rigid ammoniate state. Pressure--composition isotherm, 1H and 11B nuclear magnetic resonance, and Raman measurements support this non-monotonic state evolution and associated BH4-/NH3 reorganization. These findings transform ammonia-induced liquefaction from an empirical phase anomaly into a competition between 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.
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BibTeX
@misc{wang2026ammoniated,
title = {{Why Ammoniated Lithium Borohydrides Liquefy and Resolidify?}},
author = {Qian Wang and Zixin Xu and Ryuhei Sato and Yiming Lu and Hiroki Miyaoka and Takayuki Ichikawa and Eric Jianfeng Cheng and Shin-ichi Orimo and Fangqin Guo and Hao Li},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.01266},
eprint = {2608.01266},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.01266},
}
RIS
TY - UNPB
TI - Why Ammoniated Lithium Borohydrides Liquefy and Resolidify?
AU - Qian Wang
AU - Zixin Xu
AU - Ryuhei Sato
AU - Yiming Lu
AU - Hiroki Miyaoka
AU - Takayuki Ichikawa
AU - Eric Jianfeng Cheng
AU - Shin-ichi Orimo
AU - Fangqin Guo
AU - Hao Li
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2608.01266
UR - https://arxiv.org/abs/2608.01266
AB - Ammonia (NH_3) absorption drives LiBH_4· xNH_3 through a re-entrant ``solid--liquid--solid'' transition: LiBH_4·NH_3 is a well-defined solid ammoniate, compositions near LiBH_4· 2NH_3 are liquid-like or partially liquefied, whereas LiBH_4· 3NH_3 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 increasing NH_3 loading increases the direct Li--N coordination number while progressively decreasing BH_4^--associated contacts in the local Li environment. Near x ≈ 2, these contributions are most balanced among the simulated compositions, and the sampled Li--N/N⋯B coordination landscape is broadest. Further ammoniation produces Li--N-dominant coordination and slower BH_4^-/NH_3 contact renewal, accompanying recovery of a more rigid ammoniate state. Pressure--composition isotherm, ^1H and ^11B nuclear magnetic resonance, and Raman measurements support this non-monotonic state evolution and associated BH_4^-/NH_3 reorganization. These findings transform ammonia-induced liquefaction from an empirical phase anomaly into a competition between 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.
ER -