| 1. |
- Cerny, Radovan, et al.
(författare)
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NaSc(BH4)(4): A Novel Scandium-Based Borohydride
- 2010
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:2, s. 1357-1364
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Tidskriftsartikel (refereegranskat)abstract
- 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 tertiary chloride Na3ScCl6 (P2(1)/n, a = 6.7375(3) angstrom, b = 7.1567(3) angstrom, c = 9,9316(5) angstrom, beta = 90.491(3)degrees, V = 478.87(4) angstrom(3)), 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) angstrom, b = 11.875(3) angstrom, c = 9.018(2) angstrom, V = 874.9(3) angstrom(3)). The Structure of NaSc(BH4)(4) consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH4)(4)](-), located inside slightly distorted trigonal Na-6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1 degrees). 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(BH)(4))(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 similar 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.
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| 2. |
- Cerny, Radovan, et al.
(författare)
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Structure and Characterization of KSc(BH4)(4)
- 2010
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:45, s. 19540-19549
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Tidskriftsartikel (refereegranskat)abstract
- A new potassium scandium borohydride, KSc(BH4)(4), is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH4)(4), forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl3 together with a new ternary chloride K3ScCl6, which is also structurally characterized. This indicates that the formation of KSc(BH4)(4) differs from a simple metathesis reaction, and the highest scandium borohydride yield (similar to 31 mol %) can be obtained with a reactant ratio KBH4:ScCl3 of 2:1. KSc(BH4)(4) crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) angstrom, v = 936.4(8) angstrom(3) at RT, with the space group symmetry Prima. KSc(BH4)(4) has a BaSO4 type structure where the BH4 tetrahedra take the oxygen positions. Regarding the packing of cations, K+, and complex anions, [Sc(BH4)(4)](-), the structure of KSc(BH4)(4) can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH4)(4) in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH4)(4) with A = Li, Na. KSc(BH4)(4) is stable from RT up to similar to 405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460-500 K and 510-590 K. The hydrogen release involves the formation of KBH4, which reacts with K3ScCl6 and forms a solid solution, K(BH4)(1-x)Cl-x. The ternary potassium scandium chloride K3ScCl6 observed in all samples has a monoclinic structure at room temperature, P2(1)/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) angstrom, beta = 109.22(2)degrees, V = 1135.6(4) angstrom(3), which is isostructural to K3MoCl6. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 angstrom (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K3YCl6.
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| 3. |
- Ravnsbaek, Dorthe B., et al.
(författare)
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Thermal Polymorphism and Decomposition of Y(BH4)(3)
- 2010
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 49:8, s. 3801-3809
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Tidskriftsartikel (refereegranskat)abstract
- The structure and thermal decomposition of Y(BH4)(3) is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), B-11 MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH4 and YCl3 in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH4)(3), denoted beta-Y(BH4)(3), is discovered besides the Y(BH4)(3) polymorph previously reported, denoted alpha-Y(BH4)(3). beta-Y(BH4)(3) has a cubic crystal structure and crystallizes with the space group symmetry Pm (3) over barm and a bisected a-axis, a = 5.4547(8) angstrom, as compared to alpha-Y(BH4)(3), a = 10.7445(4) angstrom (Pa (3) over bar). beta-Y(BH4)(3) crystallizes with a regular ReO3-type structure, hence the Y3+ cations form cubes with BH4 anions located on the edges. This arrangement is a regular variant of (he distorted Y3+ cube observed in alpha-Y(BH4)(3), which is similar to the high pressure phase of ReO3. The new phase, beta-Y(BH4)(3) is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from alpha- to beta-Y(BH4)(3), at similar to 180 degrees C. Upon further heating, beta-Y(BH4)(3) decomposes at similar to 190 degrees C to YH3, which transforms to YH2 at 270 degrees C. An unidentified compound is observed in the temperature range 215-280 degrees C, which may be a new Y B H containing decomposition product. The final decomposition product is YB4. These results show that boron remains in the solid phase when Y(BH4)(3) decomposes in a hydrogen atmosphere and that Y(BH4)(3) may store hydrogen reversibly.
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