Increasing Hydrogen Density with the Cation-Anion Pair BH4−-NH4+ in Perovskite-Type NH4Ca(BH4)3

A novel metal borohydride ammonia-borane complex Ca(BH4)2·NH3BH3 is characterized as the decomposition product of the recently reported perovskite-type metal borohydride NH4Ca(BH4)3, suggesting that ammonium-based metal borohydrides release hydrogen gas via ammonia-borane-complexes. For the first time the concept of proton-hydride interactions to promote hydrogen release is applied to a cation-anion pair in a complex metal hydride. NH4Ca(BH4)3 is prepared mechanochemically from Ca(BH4)2 and NH4Cl as well as NH4BH4 following two different protocols, where the synthesis procedures are modified in the latter to solvent-based ball-milling using diethyl ether to maximize the phase yield in chlorine-free samples. During decomposition of NH4Ca(BH4)3 pure H2 is released, prior to the decomposition of the complex to its constituents. As opposed to a previously reported adduct between Ca(BH4)2 and NH3BH3, the present complex is described as NH3BH3-stuffed α-Ca(BH4)2

Investigations on the structure and properties of novel mixed-metal borohydrides

This thesis deals with structural topologies of different dimensionalities in novel complex hydrides based on the tetrahydroborate anion. While classical applications of hydrides such as mobile hydrogen storage are discussed, the use of hydrogen-storage incompatible heavy metals, in especial lanthanides, yields new structural features and functionalities in borohydride chemistry. In this context, the photophysical properties as well as extensive structural dynamics provide means of venturing into new fields such as solid state phosphors and solid state electrolytes. Extensive characterizations are presented to correlate physical behaviour with the crystal structure. These include synchrotron X-ray diffraction, quasi-elastic neutron scattering and vibrational spectroscopies as well as optical spectroscopy and thermal analyses. In particular, di-hydrogen contacts are revealed to play a dominant role in phase transition mechanisms and it is shown that the tetrahydroborate anion can be utilized to engineer both lattice instabilities and physical properties in perovskite type complex hydrides

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

Complex Hydrides – When Powder Diffraction needs Help

'Real life' energy-related materials such as solid-state hydrogen storage compounds or components of electrochemical cells are usually polycrystalline, poorly crystallized, highly reactive and dynamic systems. Powder diffraction at modern high brilliance X-ray sources is the most useful tool to investigate such systems because it is easy, fast and extremely versatile with respect to measurement conditions as well as in situ setups. However, it is in the nature of these systems that they undergo processes that cannot be investigated by diffraction alone. The central role in hydrogen storage materials is played by hydrogen itself, the worst X-ray scatterer in the periodic system. Gas release, the purpose of a hydrogen storage material, is not detected by diffraction. Amorphous components are badly characterized. We want to show how a complementary approach combining different methods allows to overcome limitations imposed on powder diffraction by the sample nature of 'real' hydrogen storage materials

Role of the Li⁺ node in the Li-BH ₄ substructure of double-cation tetrahydroborates

The phase diagram LiBH4-ABH4 (A = Rb,Cs) has been screened and revealed ten new compounds LiiAj(BH4)i+j (A = Rb, Cs), with i, j ranging between 1 and 3, representing eight new structure types amongst homoleptic borohydrides. An approach based on synchrotron X-ray powder diffraction to solve crystal structures and solid-state first principles calculations to refine atomic positions allows characterizing multi-phase ball-milled samples. The Li-BH4 substructure adopts various topologies as a function of the compound's Li content, ranging from one-dimensional isolated chains to three-dimensional networks. It is revealed that the Li+ ion has potential as a surprisingly versatile cation participating in framework building with the tetrahydroborate anion BH4 as a linker, if the framework is stabilized by large electropositive counter-cations. This utility can be of interest when designing novel hydridic frameworks based on alkaline metals and will be of use when exploring the structural and coordination chemistry of light-metal systems otherwise subject to eutectic melting

Fast ion conduction in garnet-type metal borohydrides Li₃K₃Ce₂(BH₄)₁₂ and Li₃K₃La₂(BH₄)₁₂

Complex hydrides are a family of compounds which have attracted a lot of attention in the last decade for various clean energy-related purposes, from solid state hydrogen storage to materials suitable in Li-ion batteries. We present two new garnet-type borohydride materials suitable as solid state electrolytes. Li3K3Ce2(BH4)12 and Li3K3La2(BH4)12 show unexpectedly high room temperature Liþ ionic conductivity (compared to the reported isostructural garnet oxide Li-conductor) of sigma_Li 3 x 10**7 and 6 x 10**7 S/cm with corresponding activation energies of Ea = 0.79 and Ea = 0.67 eV, respectively, which result from large bottleneck windows in the conduction path. The effect of heterovalent cation substitution is investigated as means of tailoring ionic conductivity. Doping with divalent Sr2+ and Eu2+ shows that sigma_Li can be increased by one order of magnitude in the whole temperature range measured

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

Structural investigations on mixed-metal borohydrides have been the subject of powder diffraction since the discovery that hydrogen-release temperatures can be tailored by using more electronegative metals. The lack of producing suitable samples for single crystal X-ray diffraction has defined powder diffraction as the method of choice which, however, is less sensitive to the structural details of the compounds in question. Here we show how to overcome this limitation by developing a flux-assisted single crystal growth procedure to lower the melting point of mixed-metal compounds that are thermally unstable and usually decompose before melting, or are unstable in the melt. We prove the validity of this principle on a member of the recently reported perovskite-type class of borohydrides and show that the defined approach is easily generalized. Interesting structural details are revealed that stand in contrast to the results obtained from samples produced by mechano-chemistry. The differences in lattice instabilities are discussed and put into context with the discovered epitaxial relationships between rocksalt-type ABH4 and perovskite-type ACa(BH4)3 (A = alkaline metal). In this context, the preliminary results provide a valuable scheme that can be made use of when physical deposition of metal borohydrides reaches its working stage

Superionic Conduction of Sodium and Lithium in Anion-Mixed Hydroborates Na₃BH₄B₁₂H₁₂ and (Li_0.7Na_0.3)₃BH₄B₁₂H₁₂

Novel mixed-anion compounds Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12 show superionic conduction corresponding to highly disordered cations and continuous migration paths in the structure. Na-rich Na3BH4B12H12 with a room-temperature conductivity of 0.5 × 10−3 S cm−1 provides a light-weight solution for the electrolyte in Na-based all solid-state batteries. (Li0.7Na0.3)3BH4B12H12 favors ionic conductivity for both Li and Na cations, but only above 500 K