I11-High Resolution Powder Diffraction
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Abstract: Ammonium borohydride, NH4BH4, has the highest gravimetric and volumetric hydrogen density among known inorganic compounds and a fascinating rock salt type crystal structure composed of H disordered tetrahedral complexes, NH4+ and BH4−, which are interlinked by a dense network of dihydrogen bonds. Here we report the synthesis, structure and properties of solid solutions in the binary systems, NH4BH4–MBH4 (M = K, Rb, Cs), which are investigated by in situ synchrotron radiation powder X-ray diffraction and thermal and photographic analysis. Full solubility and formation of (NH4)xM1−xBH4, is observed upon cryo-mechanochemical treatment. The solid solutions stabilize NH4BH4 from T ∼68 to ∼96 °C, alter the decomposition pathway and suppress the fierce decomposition of NH4BH4. However, for increased amounts of NH4BH4 in the solid solutions, the decomposition gradually shows more resemblance to that of pristine ammonium borohydride, and the thermal stability of the solid solutions appears to decrease down the group of the alkali metal ions, i.e. decreasing from K+, Rb+ and to Cs+.
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Nov 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[29197]
Abstract: Metal closo-boranes have recently received significant attention as solid-state electrolytes due to their high thermal and electrochemical stability, and the weak interaction between the cat- and anion, facilitating fast ionic conductivity. Here we report a synthesis method for obtaining a novel mixed closo-carborane compound, [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08]. The crystal structures are investigated for [NH(CH3)3][CB9H10] and [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08], revealing that the latter forms a solid solution isostructural to [NH(CH3)3][CB9H10]. The compounds exhibit polymorphism as a function of temperature, and we report the discovery of four polymorphs of [NH(CH3)3][CB9H10] and four isostructural solid solution [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08], along with a high-temperature decomposition intermediate of the latter. The α-polymorph is an ordered structure, with increasing amounts of disorder for the β- and γ-polymorphs, while the high temperature δ- and ε-polymorphs at T > 476 K are fully disordered on both the cation and anion site. These new compounds may be used as precursors for new types of solid-state ionic conductors.
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Sep 2022
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I11-High Resolution Powder Diffraction
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Abstract: Metal closo-borates are attractive electrolytes for solid state batteries. Here we present a detailed investigation of the polymorphism and thermal and electrochemical properties of Li2B10H10 and Li2B12H12 and their composites, (1 − x)Li2B12H12–xLi2B10H10. A new polymorph, β-Li2B10H10, is identified with a cubic structure, a = 9.567(1) Å (Fm[3 with combining macron]m), with dynamically disordered B10H102− anions. A sub-stoichiometric compound denoted as γ-Li2B10H10−y is prepared by thermal treatment (380 °C, 1 hour) in hydrogen and can be indexed to a cubic face-centered unit cell with a = 9.9224(5) Å. The 7Li MAS NMR spectra along with spin–lattice relaxation (T1) measured by 7Li saturation-recovery NMR experiments clearly reveal a high degree of dynamics assigned to increasing amounts of γ-Li2B10H10−y, which is in accordance with the measured Li+ ionic conductivity. Thermal treatment (380 °C, 1 hour) of Li2B12H12 in argon reveals the highest degree of dynamics and Li+ conductivity. The (1 − x)Li2B12H12–xLi2B10H10 composites are found to be physical mixtures of γ-Li2B10H10−y and Li2B12H12 with minor amounts of α-Li2B10H10, and their Li+ conductivities are proportional to the amount of γ-Li2B10H10−y. The highest Li+ conductivity is observed for γ-Li2B10H10−y: σ(Li+) = 7.6 × 10−6 S cm−1 at 30 °C and 7.2 × 10−3 S cm−1 at T = 330 °C. Cyclic voltammetry of γ-Li2B10H10−y reveals an oxidative stability up to 2.8 V vs. Li/Li+, and a stable plating and stripping of lithium.
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Jul 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[26090]
Abstract: Solid-state inorganic magnesium batteries are considered as potential high energy storage devices for the future. Here we present a series of magnesium borohydride tetrahydrofuran (THF) composites, Mg(BH 4 ) 2 · x THF(−MgO), 0 ≤ x ≤ 3, as solid-state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH 4 ) 2 ·2/3THF ( Cc ), α-Mg(BH 4 ) 2 ·2THF ( P2 1 /c ) and β-Mg(BH 4 ) 2 ·2THF ( C2 ), and the detailed structures of α− and β−Mg(BH 4 ) 2 ·2THF are presented. The magnesium ionic conductivity of composites formed by these compounds were several orders of magnitude higher than that of the distinct compounds, x = 0, 2/3, 2, and 3. The nanocomposite stabilized by MgO nanoparticles (~50 nm), Mg(BH 4 ) 2 ·1.5THF−MgO(75 wt%), displayed the highest Mg 2+ conductivity, σ(Mg 2+ ) ~10 -4 S cm -1 at 70 °C, a high ionic transport number of t ion = 0.99, and cyclic voltammetry revealed an oxidative stability of ~1.2 V vs. Mg/Mg 2+ . The electrolyte was stable towards magnesium electrodes, which allowed for stable Mg plating/stripping for at least 100 cycles at 55 °C with a current density of 0.1 mA cm -2 . Finally, a proof-of-concept rechargeable solid-state magnesium battery was assembled with a magnesium metal anode, a TiS 2 cathode providing a maximum discharge capacity of 94.2 mAh g -1 , which corresponds to y = 0.2 in Mg y TiS 2.
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Jun 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[17433]
Open Access
Abstract: Fast Li-ion conductivity at room temperature is a major challenge for utilization of all-solid-state Li batteries. Metal borohydrides with neutral ligands are a new emerging class of solid-state ionic conductors, and here we report the discovery of a new mono-methylamine lithium borohydride with very fast Li + conductivity at room temperature. LiBH 4 ∙CH 3 NH 2 crystallizes in the monoclinic space group P 2 1 / c , forming a two-dimensional unique layered structure. The layers are separated by hydrophobic –CH 3 moieties, and contain large voids, allowing for fast Li-ionic conduction in the interlayers, σ(Li+) = 1.24∙10 -3 S/cm at room temperature. The electronic conductivity is negligible, and the electrochemical stability is ~2.1 V vs Li. The first all-solid-state battery using a lithium borohydride with a neutral ligand as the electrolyte, Li-metal as the anode and TiS 2 as the cathode is demonstrated.
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Jun 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[21804]
Abstract: Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(11BD4)3·3ND3 with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(11BD4)3·3ND3 was discovered. Furthermore, the series of Y(11BH4)3·xNH3 (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the 11BH4– and NH3 groups and unique insights into the hydrogen dynamics. Partial deuteration using ND3 in Y(11BH4)3·xND3 (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH3 and 11BH4– in Y(11BH4)3·xNH3, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(11BH4)3·xNH3 could roughly be divided into three regions: (i) below 55 meV, containing mainly 11BH4– librational motions, (ii) 55–130 meV, containing mainly NH3 librational motions, and (iii) above 130 meV, containing 11B–H and N–H bending and stretching motions.
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Jul 2021
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α′- and β-polymorphs. Furthermore, it was found that the [B3H8]− anion undergoes different reorientational motions in the three polymorphs α, α′, and β. In α-KB3H8, the [B3H8]− anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C2 symmetry axis of the [B3H8]− anion upon transitioning to α′-KB3H8. After transitioning to β-KB3H8, the [B3H8]− anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]− anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.
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Feb 2021
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I11-High Resolution Powder Diffraction
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Abstract: Ammine metal borohydrides display extreme structural and compositional diversity and show potential applications for solid-state hydrogen and ammonia storage and as solid-state electrolytes. Thirty-two new compounds are reported in this work, and trends in the full series of ammine rare-earth-metal borohydrides are discussed. The majority of the rare-earth metals (RE) form trivalent RE(BH4)3·xNH3 (x = 7–1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions (x = 5–7), to molecular structures (x = 3, 4), one-dimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH4)2·xNH3 (x = 4, 2, 1) compounds are observed for RE2+ = Sm, Eu, Yb, with structures varying from molecular structures (x = 4) to two-dimensional layered (x = 2, 1) and three-dimensional structures (Yb(BH4)2·NH3). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH3 coordinates to the metal, while BH4– has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH4)3·xNH3 (x = 7–5, 4) releases NH3 stepwise during thermal treatment, while mainly H2 is released for x ≤ 3. In contrast, only NH3 is released from RE(BH4)2·xNH3 due to the lower charge density on the RE2+ ion and higher stability of RE(BH4)2. The thermal stability of RE(BH4)3·xNH3 increase with increasing cation charge density for x = 5, 7, while it decreases for x = 4, 6. For x = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH4– group, making it more reactive toward NH3. This research provides a large number of novel compounds and new insight into trends in the crystal chemistry of ammine metal borohydrides and reveals a correlation between the local metal coordination and the thermal stability.
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Jan 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[12714, 14015]
Abstract: Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH4BH4, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H2). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH4+ countercations. Dihydrogen interactions between complex NH4+ and BH4– ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH4+ to BH4–, and the nearest coordination sphere of NH4+ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.
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Aug 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[15781, 17433]
Abstract: Metal closo-borates have recently received significant attention due to their potential applications as solid-state ionic conductors. Here, the synthesis, crystal structures, and properties of (NH4)2B10H10·xNH3 (x = 1/2, 1 (α and β)) and (NH4)2B12H12·xNH3 (x = 1 and 2) are reported. In situ synchrotron radiation powder X-ray diffraction allows for the investigation of structural changes as a function of temperature. The structures contain the complex cation N2H7+, which is rarely observed in solid materials, but can be important for proton conductivity. The structures are optimized by density functional theory (DFT) calculations to validate the structural models and provide detailed information about the hydrogen positions. Furthermore, the hydrogen dynamics of the complex cation N2H7+ are studied by molecular dynamics simulations, which reveals several events of a proton transfer within the N2H7+ units. The thermal properties are investigated by thermogravimetry and differential scanning calorimetry coupled with mass spectrometry, revealing that NH3 is released stepwise, which results in the formation of (NH4)2BnHn (n = 10 and 12) during heating. The proton conductivity of (NH4)2B12H12·xNH3 (x = 1 and 2) determined by electrochemical impedance spectroscopy is low but orders of magnitude higher than that of pristine (NH4)2B12H12. The thermal stability of the complex cation N2H7+ is high, up to 170 °C, which may provide new possible applications of these proton-rich materials.
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Jul 2020
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