NaSc(BH4)4: A Novel Scandium-Based Borohydride

A new alkaline transition-metal borohydride, NaSc(BH4)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(BH4)4 forms at ambient conditions in ball-milled mixtures o...

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Bibliographic Details
Published in:Journal of physical chemistry. C 2010-01, Vol.114 (2), p.1357-1364
Main Authors: Černý, Radovan, Severa, Godwin, Ravnsbæk, Dorthe B, Filinchuk, Yaroslav, D’Anna, Vincenza, Hagemann, Hans, Haase, Dörthe, Jensen, Craig M, Jensen, Torben R
Format: Article
Language:English
Subjects:
C: Energy Conversion and Storage
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Summary:A new alkaline transition-metal borohydride, NaSc(BH4)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(BH4)4 forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl3. A new ternary chloride Na3ScCl6 (P21/n, a = 6.7375(3) Å, b = 7.1567(3) Å, c = 9.9316(5) Å, β = 90.491(3)°, V = 478.87(4) Å3), isostructural to Na3TiCl6, was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl3/NaBH4 of 1:2. NaSc(BH4)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) Å3). The structure of NaSc(BH4)4 consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH4)4]−, located inside slightly distorted trigonal Na6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc3+ is tetrahedrally surrounded by four BH4 tetrahedra with a 12-fold coordination of H to Sc, while Na+ is surrounded by six BH4 tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na+ cations and [Sc(BH4)4]− anions in NaSc(BH4)4 is a deformation variant of the hexagonal NiAs structure type. NaSc(BH4)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(BH4)4 between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.
ISSN:1932-7447
1932-7455
DOI:10.1021/jp908397w