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Acta Cryst. (2014). A70, C73
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A series of complex hydrides based on the highly dynamic tetrahydroborate anion BH4- and crystallizing in the ABX3 type lattice has recently been discovered. They present a rare case of a family of iono-covalent hydrides that has a genuine tunable host lattice, making them an interesting new class of host compounds for not only the design of hydrogen storage materials but also hydride-properties related to heavy metals. Amongst these, preliminary results on REE-based luminescence will be discussed in the neat and doped compounds, the Ln2+ excited states surprisingly not being subject to significant quenching by B-H vibrations. Unlike oxide- or halide-perovskites some members of the AB(BH4)3 group do not evolve to higher symmetries as a function of temperature. We show by means of in-situ synchrotron X-ray powder diffraction, vibrational spectroscopy and ab initio calculations in the solid state, that temperature-induced structural distortions in perovskite-type ACa(BH4)3 (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts of repulsive nature. Coupling between internal B-H vibrations and phonons results in lattice distortions that are identical in symmetry to well-known instabilities (soft modes) in perovskites, which generally condense to lower temperatures. Anion-substitution BH4- <-> X- (X = Halide) calculated on ordered models can relax distortions caused by repulsive effects. High temperature phase-transitions in ACa(BH4)3 can be of first or second-order, including 2-fold superlattices, simple cubic-cubic transitions accompanied by volume expansion or complex modulated superstructures accompanied by negative volume expansion, as is the case in RbCa(BH4)3. Close di-hydrogen contacts may be suggested as a tool to tailor the crystal symmetry in complex hydride perovskites in the future.

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Acta Cryst. (2014). A70, C944
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An increasing number of novel single, double and triple cation borohydrides has been structurally characterized in the last few years as hydrogen storage materials and solid state electrolytes for battery applications [1,2]. It is interesting to note that the majority of these novel metal borohydrides resemble structures of various metal oxides. This is not altogether surprising considering the fact that [BH4]- and O2- anions are isoelectronic. The particularity of the borohydride-oxide analogy is the double negative charge of O2- vs. the simply charged borohydride [BH4]-, this providing a superior structural flexibility in oxides with respect to mixed valence compounds. Though somewhat handicapped by its lower charge regarding this issue, the borohydride anion obtains its versatility as a building block due to being a non-spherical anion with a tetrahedral shape as opposed to the oxide, which is approximately spherical. This commonly results in a lower symmetry of the borohydride when compared to the related oxide, the prototype very often showing a close packing of a readily polarisable soft oxide anion. We will show by means of in-situ synchrotron X-ray powder diffraction and ab initio calculations in the solid state that structural distortions in metal borohydrides compared to oxide prototypes have their origin in close hydridic di-hydrogen contacts of repulsive nature. These contacts may be suggested as a tool to tailor the crystal symmetry in complex metal hydrides in the future. Nearly twice as big as the oxide, the borohydride anion allows for connectivities of the coordination polyhedra rarely observed among the oxides such as tetrahedral edge sharing. We will show how the borohydride-oxide isomorphism, and cationic heterovalent substitution allow the prediction and design of novel borohydrides with high hydrogen content or high cation mobility.
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