Spectroscopic and Structural Characterization of Thermal Decomposition of γ‑Mg(BH₄)₂: Dynamic Vacuum versus H₂ Atmosphere

Magnesium borohydride [Mg­(BH₄)₂] attracts a particular interest as a material for hydrogen storage because of its high gravimetric capacities and being suggested as a rehydrogenable compound. Although extensively studied, besides the whole decomposition process, a large debate is still present in the literature about the temperatures leading to the different (and in many cases, unknown) products. In this paper, an ad hoc designed thermogravimetric study, together with a critical review of literature data, allowed us to identify the products for low reaction rates. Two reaction environments have been considered: dynamic vacuum and hydrogen atmosphere. In order to guarantee quasi-equilibrium conditions, the samples were obtained after 20 h isotherms in the room temperature to 400 °C range. The decomposition of γ-Mg­(BH₄)₂ has been here characterized by adopting a new approach and by X-ray diffraction (XRD) and medium-infrared spectroscopy, together with experimental techniques used for the first time for this process (far-infrared and UV–vis–near-infrared spectroscopies). Density functional calculations were performed to help the identification of the amorphous products. A possible process mechanism was delineated and in particular that (a) Mg­(BH₄)₂ decomposition starts at 200 °C; (b) MgB₄H₁₀ is proposed, for the first time, as the phase responsible for its reversibility for <i>T</i> < 270 °C, which would implicitly restrict the Mg­(BH₄)₂ reversible capacity to 3.7 mass %

Investigation on the Decomposition Enthalpy of Novel Mixed Mg_{(1–<i>x</i>)}Zn_<i>x</i>(BH₄)₂ Borohydrides by Means of Periodic DFT Calculations

The combination of Mg­(BH₄)₂ and Zn­(BH₄)₂ compounds has been theoretically investigated as a possible mixed borohydride prone to give an enthalpy of decomposition around 30 kJ/mol_H₂, that is, suitable for a dehydrogenation process close to room temperature and pressure. The total energy of pure compounds and solid solutions has been computed by means of periodic DFT calculations. To generate the Mg_{(1–<i>x</i>)}Zn_<i>x</i>(BH₄)₂ solid solutions, the α-phase of Mg­(BH₄)₂ (space group <i>P</i>6₁22) has been considered in which Mg²⁺ ions have been progressively replaced with Zn²⁺, without lowering the symmetry of the crystalline structure. A charge density topological analysis is reported to better understand the chemical bonding in the pure and mixed metal borohydrides. The decomposition enthalpy of the mixed borohydrides according to two different reaction paths that lead to MgH₂, Zn, H₂, and α-B or B₂H₆, respectively, as products has been estimated. As regards the former, a value of about 30 kJ/mol_H₂ has been predicted for a Mg_{(1–<i>x</i>)}Zn_<i>x</i>(BH₄)₂ solid solution with <i>x</i> = 0.2–0.3

Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides

An integrated experimental–theoretical approach for the solid-state NMR investigation of a series of hydrogen-storage materials is illustrated. Seven experimental room-temperature structures of groups I and II metal hydrides and borohydrides, namely, NaH, LiH, NaBH₄, MgH₂, CaH₂, Ca­(BH₄)₂, and LiBH₄, were computationally optimized. Periodic lattice calculations were performed by means of the plane-wave method adopting the density functional theory (DFT) generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Quantum ESPRESSO package. Projector augmented wave (PAW), including the gauge-including projected augmented-wave (GIPAW), methods for solid-state NMR calculations were used adopting both Rappe–Rabe–Kaxiras–Joannopoulos (RRKJ) ultrasoft pseudopotentials and new developed pseudopotentials. Computed GIPAW chemical shifts were critically compared with the experimental ones. A good agreement between experimental and computed multinuclear chemical shifts was obtained

Synthesis and Structural Investigation of Zr(BH₄)₄

Zirconium tetraborohydride, Zr­(BH₄)₄, was synthesized by a metathesis reaction between LiBH₄ and ZrCl₄ using high-energy ball milling. Initially, a white powder was produced, and during storage at −30 °C in a closed vial, transparent rectangular single crystals formed under the lid by vapor deposition. Single-crystal X-ray diffraction data revealed a cubic unit cell (<i>a</i> = 5.8387(4) Å, space group <i>P</i>-43<i>m</i>, determined at <i>T</i> = 100 K), which consists of neutral Zr­(BH₄)₄ molecules. The BH₄^– anions coordinate to Zr via the tetrahedral faces (η₃). The shortest distance between neighboring molecules in the solid is defined by B–H2<b>···</b>H2–B interactions of 2.77 Å. DFT calculations, based on the experimental structure, have been performed with the CRYSTAL code. A phonon instability in the Γ point was observed for space-group symmetry <i>P-</i>43<i>m</i>, which can be eliminated by a symmetry reduction to the cubic space group <i>P</i>23. Computed IR spectra for the two structural models turned out to be very similar. Synthesis and decomposition was further investigated using in situ synchrotron radiation powder X-ray diffraction

Synthesis and Structural Investigation of Zr(BH₄)₄

Zirconium tetraborohydride, Zr­(BH₄)₄, was synthesized by a metathesis reaction between LiBH₄ and ZrCl₄ using high-energy ball milling. Initially, a white powder was produced, and during storage at −30 °C in a closed vial, transparent rectangular single crystals formed under the lid by vapor deposition. Single-crystal X-ray diffraction data revealed a cubic unit cell (<i>a</i> = 5.8387(4) Å, space group <i>P</i>-43<i>m</i>, determined at <i>T</i> = 100 K), which consists of neutral Zr­(BH₄)₄ molecules. The BH₄^– anions coordinate to Zr via the tetrahedral faces (η₃). The shortest distance between neighboring molecules in the solid is defined by B–H2<b>···</b>H2–B interactions of 2.77 Å. DFT calculations, based on the experimental structure, have been performed with the CRYSTAL code. A phonon instability in the Γ point was observed for space-group symmetry <i>P-</i>43<i>m</i>, which can be eliminated by a symmetry reduction to the cubic space group <i>P</i>23. Computed IR spectra for the two structural models turned out to be very similar. Synthesis and decomposition was further investigated using in situ synchrotron radiation powder X-ray diffraction

Halide Substitution in Magnesium Borohydride

The synthesis of halide-substituted Mg­(BH₄)₂ by ball-milling, and characterization with respect to thermodynamics and crystal structure, has been addressed. The ball-milled mixture of Mg­(BH₄)₂ and MgX₂ (X = Cl, Br) has been investigated by in situ/ex situ synchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), and infrared and Raman spectroscopy. High resolution SR-PXD patterns reveal that the unit cell volume of β-Mg­(BH₄)₂ in milled and annealed mixtures of Mg­(BH₄)₂ with MgCl₂/MgBr₂ is smaller than that of pure β-Mg­(BH₄)₂. This is due to substitution of BH₄^– by Cl^–/Br^– ions which have ionic radii smaller than that of BH₄^–. For comparison, ab initio calculations were run to simulate Cl substitution in α-Mg­(BH₄)₂. The α-polymorph was used rather than the β-polymorph because the size of the unit cell was more manageable. Electronic energy data and thermodynamic considerations confirm the miscibility of MgCl₂ and Mg­(BH₄)₂, both in α- and β-polymorphs

Li₅(BH₄)₃NH: Lithium-Rich Mixed Anion Complex Hydride

The Li₅(BH₄)₃NH complex hydride, obtained by ball milling LiBH₄ and Li₂NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters <i>a</i> = 10.2031(3), <i>b</i> = 11.5005(2), and <i>c</i> = 7.0474(2) Å at 77 °C. The crystal structure of Li₅(BH₄)₃NH has been solved in space group <i>Pnma</i>, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH₄:2Li₂NH ball-milled and annealed sample. Solid-state nuclear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li⁺ cation is coordinated by three BH₄^– and one NH^2– anion in a tetrahedral configuration. The room-temperature ionic conductivity of the new orthorhombic compound is close to10^–6 S/cm at room temperature, with activation energy of 0.73 eV