发表机构
Tohoku University; Hiroshima University; The University of Tokyo（东北大学; 广岛大学; 东京大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究针对氨化硼氢化锂“固-液-固”反常相变的微观机制谜题，通过结构预测、从头算模拟及实验验证，揭示锂配位环境演化驱动相变的规律，为富氢材料的流变性与刚性切换提供理论框架。
AI 中文摘要
氨（$\mathrm{NH_3}$）的吸收会驱动$\mathrm{LiBH_4\!\cdot\!xNH_3}$发生可折返的“固-液-固”相变：$\mathrm{LiBH_4\!\cdot\!NH_3}$是结构明确的固态氨化物，组分接近$\mathrm{LiBH_4\!\cdot\!2NH_3}$的体系呈类液态或部分液化状态，而$\mathrm{LiBH_4\!\cdot\!3NH_3}$则回归更具刚性的非液态氨化物态。然而，这种反直觉现象的微观起源一直是长期未解的谜题。本研究揭示了其背后的机制。跨数据库分析表明，硼氢化物是一类物态多样性极强、对组分变化高度敏感的材料家族。结构预测与从头算分子模拟显示，$\mathrm{NH_3}$会逐步取代锂配位壳层中的$\mathrm{BH_4^-}$。类液态并非出现在$\mathrm{NH_3}$负载量最高时，而是出现在接近$x\approx2$的组分处——此时Li-N与Li-B配位模式高度混合，配位记忆效应最弱，采样得到的Li-N/N$\cdots$B配位图景最宽泛。进一步氨化会形成以Li-N为主的配位结构，减缓$\mathrm{BH_4^-}/\mathrm{NH_3}$的接触更新，由此带来的网络持续性提升伴随刚性氨化物态的恢复。压力-组分等温线、$^{1}\mathrm{H}$与$^{11}\mathrm{B}$核磁共振以及拉曼测量结果，均支持这种非单调的物态演化及相关的$\mathrm{BH_4^-}/\mathrm{NH_3}$重构。这些发现将氨诱导液化从一种经验性相变反常，转化为原生网络破坏、混合配位阻挫与配体构建网络重构之间的竞争机制，为富氢材料中在利于传输的流态与利于稳定的刚性之间实现化学切换提供了理论框架。
英文摘要
Ammonia (NH$_3$) absorption drives LiBH$_4\cdot x$NH$_3$ through a re-entrant ``solid--liquid--solid'' transition: LiBH$_4\cdot$NH$_3$ is a well-defined solid ammoniate, compositions near LiBH$_4\cdot 2$NH$_3$ are liquid-like or partially liquefied, whereas LiBH$_4\cdot 3$NH$_3$ 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 $x \approx 2$, these contributions are most balanced among the simulated compositions, and the sampled Li--N/N$\cdots$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, $^1$H and $^{11}$B 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.
Comments24 pages, 4 figures
