The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.

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Four novel bimetallic borohydrides have been discovered, K2M(BH4)4 (M = Mg or Mn), K3Mg(BH4)5, and KMn(BH4)3, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH4, α-Mg(BH4)2, and α-Mn(BH4)2 ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K2M(BH4)4, most importantly concerning the extent to which the complex anion [M(BH4)4]2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K3Mg(BH4)5 is ascribed to the motion of BH4 groups inducing hydrogen repulsive effects, and the dynamics of K3Mg(BH4)5 are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH4)3, the first perovkite type borohydride reported to date

A new alkaline transition-metal borohydride, NaSc(BH 4 ) 4 , is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH 4 ) 4 forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl 3 . A new ternary chloride Na 3 ScCl 6 (P2 1 /n, a ) 6.7375), isostructural to Na 3 TiCl 6 , was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH 4 ) 4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl 3 /NaBH 4 of 1:2. NaSc(BH 4 ) 4 crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a ) 8.170(2) Å, b ) 11.875(3) Å, c ) 9.018(2) Å, V ) 874.9(3) Å 3 ). The structure of NaSc(BH 4 ) 4 consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH 4 ) 4 ] -, located inside slightly distorted trigonal Na 6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc 3+ is tetrahedrally surrounded by four BH 4 tetrahedra with a 12-fold coordination of H to Sc, while Na + is surrounded by six BH 4 tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na + cations and [Sc(BH 4 ) 4 ] -anions in NaSc(BH 4 ) 4 is a deformation variant of the hexagonal NiAs structure type. NaSc(BH 4 ) 4 is stable from RT up to ∼410 K, where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH 4 ) 4 between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.