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(BH)·NH ( = 7-1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions ( = 5-7), to molecular structures ( = 3, 4), one-dimensional chains ( = 2), and structures built from two-dimensional layers ( = 1). Divalent RE(BH)·NH ( = 4, 2, 1) compounds are observed for RE = Sm, Eu, Yb, with structures varying from molecular structures ( = 4) to two-dimensional layered ( = 2, 1) and three-dimensional structures (Yb(BH)·NH). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH coordinates to the metal, while BH has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH)·NH ( = 7-5, 4) releases NH stepwise during thermal treatment, while mainly H is released for ≤ 3. In contrast, only NH is released from RE(BH)·NH due to the lower charge density on the RE ion and higher stability of RE(BH). The thermal stability of RE(BH)·NH increase with increasing cation charge density for = 5, 7, while it decreases for = 4, 6. For = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH group, making it more reactive toward NH. 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.