Structure and diffusion in liquid complex hydrides via ab initio molecular dynamics

We have used density functional theory based ab initio molecular dynamics (AIMD) to study NaAlH₄, LiBH₄, LiNH₂, and Li₂BNH₆ across a range of temperatures, above and below the experimental melting temperature. We have elucidated the structure, vibrational, and diffusion characteristics of these four materials. We find: (i) In all cases, the liquid state remains a mixture of the ions found in the solid state. (ii) The anions remain intact on average but undergo large deformations across the range of temperatures. (iii) In the case of LiNH₂, there is evidence that the Li⁺ sublattice melts before the anionic sublattice. (iv) We find a connection between increased anion-anion ordering and reduced anion mobility even above the experimental melting point, due to long range Coulombic interactions between anions. (v) Finally, we find the liquid has the same major vibrational modes as the solid, though the lower frequency vibration and rotation modes become more prominent with increasing temperature.