Low-Energy Polymeric Phases of Alanates

Abstract

Low-energy structures of alanates are currently known to be described by patterns of isolated, nearly ideal tetrahedral [${\mathrm{AlH}}_{4}$] anions and metal cations. We discover that the novel polymeric motif recently proposed for ${\mathrm{LiAlH}}_{4}$ plays a dominant role in a series of alanates, including ${\mathrm{LiAlH}}_{4}$, ${\mathrm{NaAlH}}_{4}$, ${\mathrm{KAlH}}_{4}$, $\mathrm{Mg}({\mathrm{AlH}}_{4}{)}_{2}$, $\mathrm{Ca}({\mathrm{AlH}}_{4}{)}_{2}$, and $\mathrm{Sr}({\mathrm{AlH}}_{4}{)}_{2}$. In particular, most of the low-energy structures discovered for the whole series are characterized by networks of corner-sharing [${\mathrm{AlH}}_{6}$] octahedra, forming wires and/or planes throughout the materials. Finally, for $\mathrm{Mg}({\mathrm{AlH}}_{4}{)}_{2}$ and $\mathrm{Sr}({\mathrm{AlH}}_{4}{)}_{2}$, we identify two polymeric phases to be lowest in energy at low temperatures.