Low-Energy Polymeric Phases of Alanates

Low-energy structures of alanates are currently known to be described by patterns of isolated, nearly ideal tetrahedral [AlH${}_4$] anions and metal cations. We discover that the novel polymeric motif recently proposed for LiAlH${}_4$ plays a dominant role in a series of alanates, including LiAlH${}_4$, NaAlH${}_4$, KAlH${}_4$, Mg(AlH${}_4$)${}_2$, Ca(AlH${}_4$)${}_2$ and Sr(AlH${}_4$)${}_2$. In particular, most of the low-energy structures discovered for the whole series are characterized by networks of corner-sharing [AlH${}_6$] octahedra, forming wires and/or planes throughout the materials. Finally, for Mg(AlH${}_4$)${}_2$ and Sr(AlH${}_4$)${}_2$, we identify two polymeric phases to be lowest in energy at low temperatures.