First-Principles Calculated Phase Diagram for Nanoclusters in the Na−Al−H System: A Single-Step Decomposition Pathway for NaAlH4

Abstract

We present first-principles calculations of the phase diagram of small clusters composed of Na, Al, and H and determine their decomposition pathways as functions of Na:Al ratio and cluster size up to eight formula units. We consider ionic clusters of AlH3, NaH, and NaAlH4 and include as decomposition products metallic clusters of Na, Al, and mixed Na−Al. Small clusters of Na3AlH6 are found to be unstable because of a Jahn−Teller distortion that destabilizes the constituent [AlH6]3− anions and causes them to split into two H− ions and [AlH4]−. Cluster geometries for ionic clusters were obtained by relaxing prototype electrostatic ground state (PEGS) structures using density-functional theory calculations; vibrational free energy was also calculated for each of the clusters. We find that small clusters of AlH3 increase in stability with smaller cluster size from enthalpies of around 51 to 160 kJ/mol H2 for eight and one formula unit (fu), respectively. In contrast, small clusters of NaH have an enthalpy of ...