Dynamical properties of lithium borohydride – ammine composite LiBH4·NH3: A nuclear magnetic resonance study

• Published in: Journal of alloys and compounds 2022-02, Vol.894, p.162446, Article 162446
• Main Authors: Skoryunov, R.V., Babanova, O.A., Soloninin, A.V., Grinderslev, J.B., Skripov, A.V., Jensen, T.R.
• Format: Article
• Language: English
• Subjects: A. Energy storage materials; Activation energy; Ammonia; Anions; Borohydrides; C. Diffusion; Crystal structure; D. Nuclear resonances; Lithium isotopes; Magnetic properties; Molecular structure; NMR; Nuclear magnetic resonance; Spin-lattice relaxation; Symmetry; Tetrahedra
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2021.162446

•Four types of BH4 reorientational jump motion are revealed in LiBH4·NH3.•For the fastest reorientations, the jump rate reaches ~108 s−1 at T = 95 K.•The activation energies for different types of reorientations range from 78 to 297 meV.•No signs of diffusive Li+ jumps at the NMR frequency scale are found up to 293 K. The lithium borohydride – ammine composite LiBH4·NH3 exhibits a high hydrogen density and combines [BH4]− anions and neutral NH3 molecules within the common crystal structure. To study the dynamical properties of this compound, we have measured the 1H, 11B, and 7Li nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 18 – 293 K. Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. One of these processes corresponds to extremely fast BH4 reorientations with the jump rate reaching ~108 s−1 already at 95 K; this fast process is characterized by the activation energy of 78(2) meV. The local coordination of [BH4]− anions in LiBH4·NH3 suggests that the fastest process is represented by rotations around a single 3-fold symmetry axis of the BH4 tetrahedron. The slower processes characterized by the activation energies of 152(5) meV, 216(7) meV, and 297(10) meV may be attributed to reorientations around other symmetry axes of the BH4 tetrahedron. In the studied temperature range up to 293 K, we have not found any signs of diffusive Li+ jumps in LiBH4·NH3 at the frequency scale of ~104 s−1 or higher.