Origin of the structural diversity of the alkaline metal borohydride MBH4 (M = Li, Na, K, Rb and Cs): Insights from first-principles calculations

Abstract Alkaline metal borohydrides are extensively studied as promising candidates for hydrogen storage. They have similar chemical composition, but their ground-state structures exhibit regular changes. The larger the alkali cations, the higher the structural symmetry, the lower the phase transition temperatures. This phenomenon has been discussed extensively in the literature, and the determination and understanding of its origin also caused considerable controversy. In this work, through comparative analysis of the electronic structures, mechanical properties, dynamic properties and thermodynamic properties of a series of alkaline metal borohydrides, the origin of the structural diversity, under the Pauling classification, are revealed and attributed to the coordination numbers of the alkaline cations, and the thermally activated reorientation of the anionic groups. The ionic radii are defined in this work as the Bader partition radius based on the ground state electron densities in specific model structures. The coordination number is 4 for the central cation Liq+ (tetrahedral coordination in Pnma) and 6 for other alkali cations, as the cation-anion radius ratio of LiBH4 is about 0.32 while those of NaBH4, KBH4, RbBH4 and CsBH4 are 0.44, 0.58, 0.63 and 0.69, respectively, which greater than 0.414 and less than 0.732. B forms strong polar covalent bonds with 4 surrounding H, with hydrogen slightly negatively charged. It was found that both the repulsions between the adjacent anions and the attractions between the anions and the cations favor the observed trend. The joint effect of the repulsive and attractive forces on the anionic groups in NaBH4 and KBH4 results in the Face-to-Face form (as in P42/nmc). The calculated rotation barriers of [BH4]q− at the equilibrium configuration of each species indicates that, in general, the larger the cation radius, the lower the activation barrier is. These interactions attenuate quickly with the increase of the distance, affected by the cation size and thermal activation. Therefore, NaBH4, KBH4 (at high temperatures), RbBH4 and CsBH4 stabilized in a superposition of P42/nmc, P43m and F43m, or in the Fm3m space group (a disordered face-centered cubic phase) if structural constraints are ignored. These conclusions are instructive to destabilize alkaline metal borohydrides.