Why Ammoniated Lithium Borohydrides Liquefy and Resolidify?

Abstract

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.

24 pages, 4 figures