I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[17433]
Abstract: Octahydridoborate, i.e. [B3H8]− containing compounds, have recently attracted interest for hydrogen storage. In the present study, the structural, hydrogen storage, and ion conductivity properties of KB3H8 have been systematically investigated. Two distinct polymorphic transitions are identified for KB3H8 from a monoclinic (α) to an orthorhombic (α′) structure at 15 °C via a second-order transition and eventually to a cubic (β) structure at 30 °C by a first-order transition. The β-polymorph of KB3H8 displays a high degree of disorder of the [B3H8]− anion, which facilitates increased cation mobility, reaching a K+ conductivity of ∼10−7 S cm−1 above 100 °C. β-KB3H8 starts to release hydrogen at ∼160 °C, simultaneously with the release of B5H9 and trace amounts of B2H6. KBH4 and K3(BH4)(B12H12) are identified as crystalline decomposition products above 200 °C, and the formation of a KBH4 deficient structure of K3−x(BH4)1−x(B12H12) is observed at elevated temperature. The hydrogen-uptake properties of a KB3H8–2KH composite have been examined under 380 bar H2, resulting in the formation of KBH4 at T ≥ 150 °C along with higher metal hydridoborates, i.e. K2B9H9, K2B10H10, and K2B12H12.
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May 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[15781]
Abstract: Synthesis, crystal structures, and thermal and magnetic properties of the complete series of halide-free rare-earth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behavior, and magnetic properties for the entire series of RE(BH4)x are compared and discussed. The RE(BH4)x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE3+-ion with increasing atomic number, which correlates linearly with the unit cell volume of the α- and β-RE(BH4)3 polymorphs and the solvated complexes α-RE(BH4)3·S(CH3)2. The thermal analysis reveals a one-step decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH4)3 except Lu(BH4)3, which follows a three-step decomposition pathway. In contrast, the RE(BH4)2 decompose at higher temperatures in the range 306 to 390 °C due to lower charge density on the rare-earth ion. The RE(BH4)3 show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie–Weiss paramagnetic behavior down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with those of isolated 4f ions. Some of the RE(BH4)x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH4)3, indicating superexchange through a borohydride group.
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Apr 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[14015]
Abstract: A new potassium tetraamidoboranealuminate, K[Al(NH2BH3)4], has been synthesized by a mechanochemical reaction between KAlH4 and NH3BH3. The compound, K[Al(NH2BH3)4], crystallizes in a triclinic unit cell with space group symmetry P−1. The crystal structure consists of [K(NH2BH3)6]5− octahedra which facilitate the bridging between K+ in 1D chains, while also bridging K+ to Al3+ to connect the 1D chains in a 3D network. Thermal analysis reveals that K[Al(NH2BH3)4] decomposes in two exothermic steps at T ∼ 94 and 138 °C and releases primarily hydrogen. The total gas release amounts to ∼6.0 wt% H2. The decomposition products are investigated ex situ by powder X-ray diffraction, infrared spectroscopy, and 11B and 27Al NMR and identified as KBH4 and amorphous phases, possibly BN3, N2BH, and/or NBH2 whereas aluminum is found in four-, five-, and six-fold coordination. Unfortunately, the decomposed sample shows no hydrogen absorption at T = 260 °C and p(H2) = 110 bar.
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Dec 2017
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[14015]
Abstract: A new synthesis method of samarium borohydride, Sm(BH4)2, using tetrahydrofuran borane, THF-BH3, and samarium hydride, SmH2, was demonstrated and verified. The synthesised Sm(BH4)2 was mechanochemically treated with MBH4, M = K, Rb, Cs. Initially, formation of KSm(BH4)3 is observed while subsequent heat treatment is necessary to form MSm(BH4)3, M = Rb, Cs. The new compounds crystallise in orthorhombic unit cells adopting perovskite-type 3D frameworks containing distorted [Sm(BH4)6] octahedra. In-situ X-ray diffraction studies reveal two second-order polymorphic transitions of α-CsSm(BH4)3 via a tetragonal intermediate, α’-CsSm(BH4)3, into a cubic high-temperature polymorph, β-CsSm(BH4)3, resembling the ideal perovskite structure. The new compounds, MSm(BH4)3, are thermally stable to T ~ 280 °C after which they decompose into mainly MBH4, SmH2 and possibly SmB6 and SmB12H12. Finally, after three cycles of hydrogen release and uptake, the storage capacity was 1.0 or 0.84 wt% H2 for KSm(BH4)3 or RbSm(BH4)3 and CsSm(BH4)3, respectively.
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Aug 2017
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[12714]
Abstract: Mechanochemical treatment (ball-milling) of NaAlH4-Ca(BH4)2 mixtures leads to partial formation of NaBH4 and Ca(AlH4)2 by a metathesis reaction. The reaction proceeds to different extents depending on the applied ball-milling times, which is confirmed by powder X-ray diffraction and infrared spectroscopy. Additionally, an in-situ synchrotron radiation powder X-ray diffraction study reveals that the metathesis reaction continues due to thermal treatment while the data also supports a two-step decomposition of the formed Ca(AlH4)2. Finally, the reactive hydride composite system was investigated by mass spectrometry and Sieverts' measurement, which reveal release of ∼6 wt% H2 at T < 375 °C.
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May 2017
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I11-High Resolution Powder Diffraction
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Abstract: Three new perovskite-type bimetallic alkali metal strontium borohydride compounds, α-MSr(BH4)3 (M = K, Rb, Cs), have been synthesized and investigated by in-situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectrometry and Sievert’s measurements. The bimetallic borohydrides were synthesized via an addition reaction between Sr(BH4)2 and MBH4 (M = K, Rb, Cs) by mechanochemical treatment. The Sr(BH4)2 – NaBH4 system, which was treated in a similar manner, did not undergo reaction. All three α-MSr(BH4)3 compounds crystallize in the orthorhombic crystal system at room temperature: KSr(BH4)3 (P21cn), a = 7.8967(6), b = 8.2953(7), and c = 11.508(1) Å (V = 753.82(12) Å3). RbSr(BH4)3 (Pbn21), a = 8.0835(3), b = 8.3341(4), and c = 11.6600(5) Å (V = 785.52(6) Å3). CsSr(BH4)3 (P22121), a = 8.2068(9), b = 8.1793(9), and c = 6.0761(4) Å (V = 407.87(7) Å3). All three compounds are perovskite-type 3D framework structures built from distorted [Sr(BH4)6] octahedra. High-temperature polymorphs are identified to form at 258, 220 and 150 °C for MSr(BH4)3, M = K, Rb and Cs, respectively. The new compounds are thermally stable and decompose at T > 360 °C into SrB6, SrH2 and MBH4 (M = K, Rb, Cs).
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Nov 2015
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[9031]
Abstract: A metal borohydrideammonia borane complex, Mg(BH4)2(NH3BH3)2 was synthesized via a solid-state reaction between Mg(BH4)2 and NH3BH3. Different mechanochemical reaction mechanisms are observed, since Mg(BH4)2(NH3BH3)2 is obtained from α-Mg(BH4)2, whereas a mixture of Mg(BH4)2(NH3BH3)2, NH3BH3, and amorphous Mg(BH4)2 is obtained from γ-Mg(BH4)2. The crystal structure of Mg(BH4)2(NH3BH3)2 has been determined by powder X-ray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 Å), which may contribute to the high melting point of Mg(BH4)2(NH3BH3)2 found to be ∼48 °C in contrast to those for other molecular metal borohydrides. Precise values for the 11B quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH3BH3 sites and two BH4 sites in Mg(BH4)2(NH3BH3)2 from 11B MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The 11B quadrupole coupling parameters agree excellently with the electric field gradients for the 11B sites from the DFT calculations and suggest that a more detailed structural model is obtained by DFT optimization, which allows evaluation of the dihydrogen bonding scheme.
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Jun 2014
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