Trends in the Series of Ammine Rare-Earth-Metal Borohydrides: Relating Structural and Thermal Properties

Ammine metal borohydrides display extreme structural and compositional diversity and show potential applications for solid-state hydrogen and ammonia storage and as solid-state electrolytes. Thirty-two new compounds are reported in this work, and trends in the full series of ammine rare-earth-metal borohydrides are discussed. The majority of the rare-earth metals (RE) form trivalent RE(BH4)(3)center dot xNH(3) (x = 7-1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions (x = 5-7), to molecular structures (x = 3, 4), onedimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH4)(3)center dot xNH(3) (x = 4, 2, 1) compounds are observed for RE2+ = Sm, Eu, Yb, with structures varying from molecular structures (x = 4) to two-dimensional layered (x = 2, 1) and three- dimensional structures (Yb(BH4)(2)center dot NH3). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH3 coordinates to the metal, while BH4- has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH4)(3)center dot xNH(3) (x = 7-5, 4) releases NH3 stepwise during thermal treatment, while mainly H-2 is released for x <= 3. In contrast, only NH3 is released from RE(BH4)(3)center dot xNH(3) due to the lower charge density on the RE2+ ion and higher stability of RE(BH4)(2). The thermal stability of RE(BH4)(3)center dot xNH(3) increase with increasing cation charge density for x = 5, 7, while it decreases for x = 4, 6. For x = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH4- group, making it more reactive toward NH3. This research provides a large number of novel compounds and new insight into trends in the crystal chemistry of ammine metal borohydrides and reveals a correlation between the local metal coordination and the thermal stability.

Automated summary

Amine metal borohydrides exhibit extreme structural and compositional diversity, showing potential applications for solid-state hydrogen and ammonia storage as well as solid-state electrolytes. The research reports 32 new compounds and discusses trends in the full series of rare-earth metal borohydrides. The crystal structure and composition of the compounds are dependent on the volume of the rare-earth ion, and ammonia coordinates to the metal while BH4- acts as a flexible coordination.