Structure and diffusion in liquid complex hydrides viaab initiomolecular dynamics

Abstract

We have used density functional theory based ab initio molecular dynamics (AIMD) to study NaAlH${}_{4}$, LiBH${}_{4}$, LiNH${}_{2}$, and Li${}_{2}$BNH${}_{6}$ 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 LiNH${}_{2}$, 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.