Low-Energy Polymeric Phases of Alanates

TL;DR

This paper reveals that polymeric structures, especially networks of corner-sharing [AlH6] octahedra, are dominant in low-energy phases of various alanates, challenging the traditional isolated tetrahedral model.

Contribution

It introduces a new polymeric motif involving [AlH6] octahedra as the lowest-energy structures in several alanates, expanding understanding of their structural diversity.

Findings

Polymeric [AlH6] octahedra networks are prevalent in low-energy alanate phases.

Most low-energy structures feature wires and planes formed by corner-sharing octahedra.

Two polymeric phases are identified as lowest in energy for Mg(AlH4)2 and Sr(AlH4)2 at low temperatures.

Abstract

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.