Role of the Li+node in the Li-BH4substructure of double-cation tetrahydroborates

Structural Diversity and Trends in Properties of an Array of Hydrogen-Rich Ammonium Metal Borohydrides

Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH₄BH₄, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H₂). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH₄⁺ countercations. Dihydrogen interactions between complex NH₄⁺ and BH₄^- ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH₄⁺ to BH₄^-, and the nearest coordination sphere of NH₄⁺ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.

Insights into the Rb-Mg-N-H System: an Ordered Mixed Amide/Imide Phase and a Disordered Amide/Hydride Solid Solution

The crystal structure of a mixed amide-imide phase, RbMgND₂ND, has been solved in the orthorhombic space group Pnma ( a = 9.55256(31), b = 3.70772(11) and c = 10.08308(32) Å). A new metal amide-hydride solid solution, Rb(NH₂) _xH_{(1- x)}, has been isolated and characterized in the entire compositional range. The profound analogies, as well as the subtle differences, with the crystal chemistry of KMgND₂ND and K(NH₂) _xH_{1- x} are thoroughly discussed. This approach suggests that the comparable performances obtained using K- and Rb-based additives for the Mg(NH₂)₂- 2LiH and 2LiN H₂-MgH₂ hydrogen storage systems are likely to depend on the structural similarities of possible reaction products and intermediates.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides

A series of organic derivatives of magnesium borohydride, including Mg(BH4)2·1.5DME (DME = 1,2-dimethoxyethane) and Mg(BH4)2·3THF (THF = tetrahydrofuran) solvates and three mixed-cation borohydrides, [Cat]2[Mg(BH4)4], [Cat] = [Me4N], [nBu4N], [Ph4P], have been characterized. The phosphonium derivative has been tested as a precursor for synthesis of inorganic mixed-metal borohydrides of magnesium, Mx[Mg(BH4)2+x], M = Li-Cs, via a metathetic method. The synthetic procedure has yielded two new derivatives of heavier alkali metals M3Mg(BH4)5 (M = Rb, Cs) mixed with amorphous Mg(BH4)2. Thermal decomposition has been studied for both the organic and inorganic magnesium borohydride derivatives. Amorphous MgB2 has been detected among the products of the thermal decomposition of the solvates studied, together with organic and inorganic impurities.

The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides

The crystal structures of inorganic homoleptic metal borohydrides are analysed with respect to their structural prototypes found amongst metal oxides in the inorganic databases such as Pearson's Crystal Data [Villars & Cenzual (2015). Pearson's Crystal Data. Crystal Structure Database for Inorganic Compounds, Release 2014/2015, ASM International, Materials Park, Ohio, USA]. The coordination polyhedra around the cations and the borohydride anion are determined, and constitute the basis of the structural systematics underlying metal borohydride chemistry in various frameworks and variants of ionic packing, including complex anions and the packing of neutral molecules in the crystal. Underlying nets are determined by topology analysis using the program TOPOS [Blatov (2006). IUCr CompComm. Newsl. 7, 4-38]. It is found that the Pauling rules for ionic crystals apply to all non-molecular borohydride crystal structures, and that the latter can often be derived by simple deformation of the close-packed anionic lattices c.c.p. and h.c.p., by partially removing anions and filling tetrahedral or octahedral sites. The deviation from an ideal close packing is facilitated in metal borohydrides with respect to the oxide due to geometrical and electronic considerations of the BH4(-) anion (tetrahedral shape, polarizability). This review on crystal chemistry of borohydrides and their similarity to oxides is a contribution which should serve materials engineers as a roadmap to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials.

Flux-assisted single crystal growth and heteroepitaxy of perovskite-type mixed-metal borohydrides

Single crystals of mixed-metal perovskite-type borohydride KCa(BH₄)₃ are prepared by using an easily generalized flux melting procedure based on eutectic borohydride systems.