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Crystal Structure Prediction and Dehydrogenation Mechanism of LiMg(BH 4 ) 3 (NH 3 ) 2

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The synthesis and dehydrogenation performance of purified NaZn(BH4)3 with a new phase and its novel ammine metal borohydride, NaZn(BH4)3·2NH3, were first reported. Structure analysis shows that NaZn(BH4)3·2NH3 crystallizes in an orthorhombic structure with lattice parameters of a = 7.2965(2) Å, b = 10.1444(2) Å and c = 12.9714(3) Å and space group P21nb, in which the Zn atoms are located in a tetrahedral coordination environment with two NH3 molecules and two BH4− units, presenting a novel 3D framework comprised of isolated BH4−1 units and [NaZn(BH4)2(NH3)2]+ complexes. Dehydrogenation results showed that the ZnCl2 assisted NaZn(BH4)3·2NH3 is able to release 7.9 wt% hydrogen at 110 °C without the concomitant release of undesirable gases such as ammonia and/or boranes, thereby demonstrating the potential of the ammoniated Zn-based borohydrides to be used as solid hydrogen storage materials.
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Mg(BH4)2 and Ca(BH4)2 with 14.9 and 11.6 mass% hydrogen, respectively, are among the most promising materials for mobile hydrogen storage, but until now very little has been known about their hydrogen desorption properties. In this work the materials have been studied by time-resolved in situ synchrotron powder X-ray diffraction, thermal desorption spectroscopy and energy dispersive X-ray spectroscopy, and details of the phase transitions and decomposition routes are reported.
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This article describes recent technical developments that have made the total-energy pseudopotential the most powerful ab initio quantum-mechanical modeling method presently available. In addition to presenting technical details of the pseudopotential method, the article aims to heighten awareness of the capabilities of the method in order to stimulate its application to as wide a range of problems in as many scientific disciplines as possible.
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A new ammine dual-cation borohydride, LiMg(BH(4))(3)(NH(3))(2), has been successfully synthesized simply by ball-milling of Mg(BH(4))(2) and LiBH(4)·NH(3). Structure analysis of the synthesized LiMg(BH(4))(3)(NH(3))(2) revealed that it crystallized in the space group P6(3) (no. 173) with lattice parameters of a=b=8.0002(1) Å, c=8.4276(1) Å, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH(4) units. Strong N-H···H-B dihydrogen bonds exist between the NH(3) and BH(4) units, enabling LiMg(BH(4))(3)(NH(3))(2) to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH(4))(3)(NH(3))(2)/LiBH(4) composite is able to release over 8 wt% hydrogen below 200 °C, which is comparable to that released by Mg(BH(4))(3)(NH(3))(2). More importantly, it was found that release of the byproduct NH(3) in this system can be completely suppressed by adjusting the ratio of Mg(BH(4))(2) and LiBH(4)·NH(3). This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.
Article
Alkali and alkali– earth metal hydrides have high hydrogen storage capacity, but high operation temperature hinders their use. The alanates and borohydrides of alkali and alkali– earth metals are widely studied because of their light weight and high hydrogen content. Borohydrides are highly stable and decompose only at elevated temperatures while alanates decompose in two steps. A detailed study of the properties of these hydrides is required for further understanding of their stability. A lot of thermodynamic information can be derived from investigation of materials under pressure or temperature. Structural measurements on the hydride compounds at ambient and high P–T conditions result in a better understanding of the stability of the hydride structures and will assist us in the design of suitable storage materials with desired thermodynamic properties. The structural data are potential source of information regarding inter-atomic forces which determine pressure/temperature induced changes. During the past few years, structural stability of hydrides is widely investigated under pressure and temperature, both experimentally and theoretically. In this review we discuss structural phase transition and decomposition behavior of light metal hydrides, borohydrides and alanates of the elements that belong to first and second group in the periodic table.
Article
A general method, suitable for fast computing machines, for investigating such properties as equations of state for substances consisting of interacting individual molecules is described. The method consists of a modified Monte Carlo integration over configuration space. Results for the two-dimensional rigid-sphere system have been obtained on the Los Alamos MANIAC and are presented here. These results are compared to the free volume equation of state and to a four-term virial coefficient expansion. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.
Article
We present the first comprehensive comparison between free energies, based on a phonon dispersion calculation within density functional theory, of theoretically predicted structures and the experimentally proposed ? (P61) and ? (Fddd) phases of the promising hydrogen storage material Mg(BH4)2. The recently proposed low-density ground state is found to be thermodynamically unstable, with soft acoustic phonon modes at the Brillouin zone boundary. We show that such acoustic instabilities can be detected by a macroscopic distortion of the unit cell. Following the atomic displacements of the unstable modes, we have obtained a new F 222 structure, which has a lower energy than all previously experimentally and theoretically proposed phases of Mg(BH4)2 and is free of imaginary eigenmodes. A new meta-stable high-density I41/amd structure is also derived from the phase. Temperatures for the decomposition are found to be in the range of 400?470?K and largely independent of the structural complexity, as long as the primary cation coordination polyhedra are properly represented. This opens a possibility of using simple model structures for screening and prediction of finite temperature stability and decomposition temperatures of novel borohydride systems.
Article
The strategy of using double-cations to tune the temperature and purity of dehydrogenation of ammine borohydrides is reported. The first double-cation ammine borohydride, Li(2)Al(BH(4))(5)·6NH(3), which forms a novel structure with ordered arrangement of Al(NH(3))(6)(3+) ammine complexes and Li(2)(BH(4))(5)(3-) complex anions, is found to release over 10.0 wt % hydrogen below 120 °C with favorable kinetics and high H-purity (>99%).
Article
Al(BH4)3⋯6aNH3 has been prepared by a convenient route and its composite is able to release more than 10 wta% hydrogen below 140a°C with favorable kinetics by a weakly exothermic combination of N-H and B-H bonds. The high hydrogen capacity, favorable dehydrogenation, and relative stability to air make Al(BH4) 3⋯6NH3 an advanced solid-state hydrogen-storage candidate.
Article
The potential energy surface of LiBH4 is investigated by a ground-state search method based on simulated annealing and first-principles density functional theory calculations. A new stable orthogonal structure with Pnma symmetry is found, which is 9.66  kJ/mol lower in energy than the proposed Pnma structure by Soulié et al. [J. Alloys Compd. 346, 200 (2002)]. For the high-temperature structure, we suggest a new monoclinic P2/c structure, which is 21.26  kJ/mol over the ground-state energy and shows no lattice instability.
Article
Amminelithium borohydride, LiBH(4) x NH(3) which has two temperature sensitive chemical bonds N:-->Li(+) and N-H...H-B, is shown to release hydrogen at low temperatures by stabilizing the ammonia and promoting the recombination of the NH...HB bond.
Article
Mix and match: A novel series of alkali-metal zinc borohydrides, LiZn 2(BH4)5 (see picture), NaZn2(BH 4)5, and NaZn(BH4)3, with fascinating structures are presented. An interpenetrated network structure, containing a [Zn2(BH4)5]-. ion, is observed for the first time for a borohydride. A three-dimensional framework containing a polymeric [{Zn(BH4)3}n] n- ion is also identified.
Article
Store it up: The ammoniate, Li(NH3)BH4, has been prepared and fully characterized by Raman and NMR spectroscopy and X-ray powder diffraction. The potential for its use as an ammonia store and a comparison to other ammonia storage materials is discussed. Lithium borohydride absorbs anhydrous ammonia to form four stable ammoniates; Li(NH3)nBH4, mono-, di-, tri-, and tertraammoniate. This paper focuses on the monoammoniate, Li(NH3)BH4, which is readily formed on exposure of LiBH4 to ammonia at room temperature and pressure. Ammonia loss from Li(NH3)BH4 commences around 40 °C and the compound transforms directly to LiBH4. The crystal structure of Li(NH3)BH4 is reported here for the first time. Its close structural relationship with LiBH4 provides a clear insight into the facile nature and mechanism of ammonia uptake and loss. These materials not only represent an excellent high weight-percent ammonia system but are also potentially important hydrogen stores.
Article
We present a unified scheme that, by combining molecular dynamics and density-functional theory, profoundly extends the range of both concepts. Our approach extends molecular dynamics beyond the usual pair-potential approximation, thereby making possible the simulation of both covalently bonded and metallic systems. In addition it permits the application of density-functional theory to much larger systems than previously feasible. The new technique is demonstrated by the calculation of some static and dynamic properties of crystalline silicon within a self-consistent pseudopotential framework.
Article
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Article
An empirical method to account for van der Waals interactions in practical calculations with the density functional theory (termed DFT-D) is tested for a wide variety of molecular complexes. As in previous schemes, the dispersive energy is described by damped interatomic potentials of the form C6R(-6). The use of pure, gradient-corrected density functionals (BLYP and PBE), together with the resolution-of-the-identity (RI) approximation for the Coulomb operator, allows very efficient computations for large systems. Opposed to previous work, extended AO basis sets of polarized TZV or QZV quality are employed, which reduces the basis set superposition error to a negligible extend. By using a global scaling factor for the atomic C6 coefficients, the functional dependence of the results could be strongly reduced. The "double counting" of correlation effects for strongly bound complexes is found to be insignificant if steep damping functions are employed. The method is applied to a total of 29 complexes of atoms and small molecules (Ne, CH4, NH3, H2O, CH3F, N2, F2, formic acid, ethene, and ethine) with each other and with benzene, to benzene, naphthalene, pyrene, and coronene dimers, the naphthalene trimer, coronene. H2O and four H-bonded and stacked DNA base pairs (AT and GC). In almost all cases, very good agreement with reliable theoretical or experimental results for binding energies and intermolecular distances is obtained. For stacked aromatic systems and the important base pairs, the DFT-D-BLYP model seems to be even superior to standard MP2 treatments that systematically overbind. The good results obtained suggest the approach as a practical tool to describe the properties of many important van der Waals systems in chemistry. Furthermore, the DFT-D data may either be used to calibrate much simpler (e.g., force-field) potentials or the optimized structures can be used as input for more accurate ab initio calculations of the interaction energies.