Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8

Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH4)2

The thermodynamic properties of key compounds Mg(B₃H₈)₂, MgB₂H₆, MgB₁₀H₁₀, Mg(B₁₁H₁₄)₂, Mg₃(B₃H₆)₂, and MgB₁₂H₁₂, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH₄)₂ and the uptake of hydrogen by MgB₂, have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H₂) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H₂). Although our treatment lowers the calculated formation energy of Mg(B₃H₈)₂, it is still too high to explain the experimental observation of B₃H₈^-. We discuss possible reasons for this disparity and propose that the formation of B₃H₈^- and H^- in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B₃H₈)₂ and MgH₂ considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH₄)₂, Mg(B₃H₈)₂, Mg(B₁₁H₁₄)₂, MgB₁₀H₁₀, and MgB₁₂H₁₂ provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB₂H₆ and the proposed trianion species Mg₃(B₃H₆)₂ that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH₄)₂ without the formation of MgH₂ as a coproduct which could phase separate and inhibit rehydrogenation.

A safe and efficient synthetic method for alkali metal octahydrotriborates, unravelling a general mechanism for constructing the delta B3 unit of polyhedral boranes

A safe and efficient synthetic method for MB₃H₈ (M = Na, K, Rb and Cs) has been developed with excellent yields by directly reacting the corresponding MBH₄ with the dimethyl sulfide borane complex (DMS·BH₃). A general mechanism for constructing B₃H₈^-, a basic unit for building polyhedral boranes, has been unravelled.

Experimental and computational characterization of phase transitions in CsB3H8

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B₃H₈^- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH₄^- (3BH₄^-↔ B₃H₈^- + 2H₂), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB₃H₈ prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB₃H₈) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsB₃H₈), and then into a face-centered cubic phase, here named γ-CsB₃H₈, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsB₃H₈ were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsB₃H₈, a significant disorder in B₃H₈^- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.

Thermal Conversion of Unsolvated Mg(B3H8)2 to BH4 - in the Presence of MgH2

In the search for energy storage materials, metal octahydrotriborates, M(B₃H₈) _n , n = 1 and 2, are promising candidates for applications such as stationary hydrogen storage and all-solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B₃H₈)₂ to BH₄ ^- as-synthesized and in the presence of MgH₂. The conversion of our unsolvated Mg(B₃H₈)₂ starts at ∼100 °C and yields ∼22 wt % of BH₄ ^- along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH₂ to unsolvated Mg(B₃H₈)₂ drastically increases the thermal conversion to 85-88 wt % of BH₄ ^- while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH₂ substantially decreases the formation of (closo-hydro)borates and provides the necessary H₂ for the B₃H₈-to-BH₄ conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH₄ ^- under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB₃H₈-MgH₂ system very promising for energy storage applications.

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

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.