Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

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·xNH₃ (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), one-dimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH₄)₂·xNH₃ (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·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·xNH₃ (x = 7-5, 4) releases NH₃ stepwise during thermal treatment, while mainly H₂ is released for x ≤ 3. In contrast, only NH₃ is released from RE(BH₄)₂·xNH₃ due to the lower charge density on the RE²⁺ ion and higher stability of RE(BH₄)₂. The thermal stability of RE(BH₄)₃·xNH₃ 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.

Beyond Hydrogen Storage—Metal Hydrides as Multifunctional Materials for Energy Storage and Conversion

Following the E-MRS (European Materials Research Society) fall meeting 2019, Symposium L, this Special Issue of Inorganics, entitled “Beyond Hydrogen Storage—Metal Hydrides as Multifunctional Materials for Energy Storage and Conversion”, is dedicated to the wide range of emerging energy-related inorganic hydrogen-containing materials [...]