Nuclear magnetic resonance study of hydrogen dynamics in Al(BH4)4-based hypersalts M[Al(BH4)4] (M = Na, K, Rb, Cs)

• Published in: Journal of alloys and compounds Vol. 745; pp. 179 - 186
• Main Authors: Babanova, O.A., Skoryunov, R.V., Soloninin, A.V., Dovgaliuk, I., Skripov, A.V., Filinchuk, Y.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-05-2018
• Subjects: A. Energy storage materials; C. Diffusion; D. Nuclear resonances; A. Energy storage materials; C. Diffusion; D. Nuclear resonances
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2018.02.201

In order to study the dynamical properties of the novel series of complex hydrides M[Al(BH4)4] (M = Na, K, Rb, Cs), we have measured the 1H and 11B spin-lattice relaxation rates and the 1H nuclear magnetic resonance spectra in these compounds over broad temperature ranges (6–384 K) and resonance frequency ranges (14–90 MHz). For all the studied compounds, the behavior of the spin-lattice relaxation rates is governed by the reorientational motion of BH4 groups. For Na[Al(BH4)4], the temperature dependences of the measured 1H and 11B spin-lattice relaxation rates suggest a coexistence of two reorientational processes with different characteristic jump rates and the activation energies of 186(7) and 262(9) meV. For K[Al(BH4)4], Rb[Al(BH4)4], and Cs[Al(BH4)4], the relaxation data are satisfactorily described by the model with a Gaussian distribution of the activation energies and the average activation energies of 393(6), 360(5), and 353(5) meV, respectively. The barriers for reorientational motion in M[Al(BH4)4] are discussed on the basis of changes in the local environment of BH4 groups.
•BH4 reorientations in M[Al(BH4)4] (M = Na, K, Rb, Cs) are studied by NMR.•For Na[Al(BH4)4], two types of reorientational motion are revealed.•For compounds with M = K, Rb, and Cs, distributions of H jump rates are found.•Non-monotonic dependence of the activation energies on the cation radius.