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

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 Si to 160 kJ/mol H-2 for eight and one formula unit (fu), respectively. in contrast, small clusters of NaH have an enthalpy of decomposition of about 70 kJ/mol H-2 and show no destabilization with size until the cluster is two formula units or smaller when they spontaneously decompose. Clusters of NaAlH4 also show increasing stability with decreasing cluster size with an enthalpy of decomposition that increases from 80 kJ/mol H-2 for eight fu clusters to 150 kJ/mol H-2 for 1 fu clusters. NaAlH4 clusters are found to decompose directly into mixed Al-Na clusters in one step, which is in agreement with recent experimental work on NaAlH4 in nanoporous carbons. Finally, we predict destabilized reactions involving simple hydride (NaH and AlH3) and complex hydride (NaAlH4) nanoclusters that have lower critical temperatures than either of the reactants.