Structural and Reorientational Dynamics of Tetrahydroborate (BH4-) and Tetrahydrofuran (THF) in a Mg(BH4)2𔅗THF Adduct: Neutron-Scattering Characterization

Published

January 7, 2020

Author(s)

Mirjana NMN Dimitrievska, Marina Chong, Mark E. Bowden, Hui Wu, Wei Zhou, Iffat Nayyar, Bojana Ginovska, Thomas Gennett, Tom Autrey, Craig M. Jensen, Terrence J. Udovic

Abstract

Metal borohydrides are considered as promising materials for hydrogen storage applications due to their high volumetric and gravimetric hydrogen densities. Recently, different solvents have been complexed with Mg(BH₄)₂ in efforts to improve its hydrogenation/dehydrogenation properties. Notably, Mg(BH₄)₂ &#8226xTHF adducts involving tetrahydrofuran (THF; C₄H₈O) have proven to be especially interesting. This work focuses on exploring structural, vibrational, and dynamical properties of the THF-rich Mg(BH₄)₂ &#82263THF and THF-poor Mg(BH₄)₂ &#82260.5THF adducts via various neutron-scattering methods and molecular DFT calculations. Structural analysis, based on neutron diffraction measurements, has confirmed lowering of the symmetry of Mg(BH₄)₂ &#82263TDF (D - deuterium) upon cooling, from monoclinic C2/c to P-1 via a triclinic distortion, around 200 K. Vibrational properties are strongly influenced by the THF environment; in particular, there is splitting in spectral features and a shift towards lower energies of the BH₄- modes with increase in THF content as a result of changes in the bond lengths, force constants and lowering of the overall symmetry. Interestingly, the orientational mobilities of the BH₄- anions obtained from quasielastic neutron scattering (QENS) are not particularly sensitive to the amount of THF present in the adducts and compare well with the mobilities of BH₄- anions in unsolvated Mg(BH₄)₂. The QENS data point to uniaxial 180° jump reorientations of the BH₄- anions around a preferred C₂ molecular symmetry axis over the whole temperature range measured. The THF rings are also found to be orientationally mobile in the THF-rich samples, undergoing 180 ° reorientational jumps around their C₂ molecular symmetry axis with jump frequencies about an order of magnitude lower than those for the BH₄- anions. In contrast, no dynamical behavior of the THF rings is observed in the case of THF-poor samples. These results are further confirmed by the DFT calculations, showing a decrease in the binding energy with each additional THF ring in the adduct. Based on the combined experimental and computational results, we propose that using fractions of THF to Mg(BH₄)₂ is beneficial in (i) preventing weakly bound THF from coming free from the magnesium cation and reducing the concentration of an unwanted impurity in the hydrogen and (ii) disrupting the stability of crystalline phase leading to a lower melting point and enhanced kinetics for any potential hydrogen storage applications.

Citation

Physical Chemistry Chemical Physics

Volume

22

Issue

1

Pub Type

Journals

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Keywords

hydrogen storage, magnesium borohydride, neutron powder diffraction, neutron scattering, neutron vibrational spectroscopy, quasielastic neutron scattering, reorientational jumps, rotational motion, terahydrofuran

Citation

Dimitrievska, M. , Chong, M. , Bowden, M. , Wu, H. , Zhou, W. , Nayyar, I. , Ginovska, B. , Gennett, T. , Autrey, T. , Jensen, C. and Udovic, T. (2020), Structural and Reorientational Dynamics of Tetrahydroborate (BH₄-) and Tetrahydrofuran (THF) in a Mg(BH₄)₂&#82263THF Adduct: Neutron-Scattering Characterization, Physical Chemistry Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=927630 (Accessed October 6, 2024)