Low-Energy Polymeric Phases of Alanates

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.

Figure 1

Low-energy spectra of ${\mathrm{LiAlH}}_4$, ${\mathrm{NaAlH}}_4$, ${\mathrm{KAlH}}_4$, $\mathrm{Mg}({\mathrm{AlH}}_4{)}_2$, $\mathrm{Ca}({\mathrm{AlH}}_4{)}_2$, and $\mathrm{Sr}({\mathrm{AlH}}_4{)}_2$. Dashed-blue and solid-red lines represent the isolated and polymeric phases. Energies are given in units of $\mathrm{kJ}/[\mathrm{mol}{\mathrm{H}}_2]$ with respect to those of the reference phases (see the text).

Figure 2

Most stable polymeric phases of ${\mathrm{LiAlH}}_4$, ${\mathrm{NaAlH}}_4$, ${\mathrm{KAlH}}_4$, $\mathrm{Mg}({\mathrm{AlH}}_4{)}_2$, $\mathrm{Ca}({\mathrm{AlH}}_4{)}_2$, and $\mathrm{Sr}({\mathrm{AlH}}_4{)}_2$. Green spheres represent metal cations while octahedra are formed by [${\mathrm{AlH}}_6$] complexes. Space groups are also given.

Figure 3

Energetic ordering and space groups of the structures obtained by substituting the cations of the alanates within a group. For each of the two groups ($M=\mathrm{Li}$, Na, K and $M=\mathrm{Mg}$, Ca, Sr) structures with identical colors and patterns have the same origin.

Figure 4

Densities of phonon states (partial and total) for 2 f.u. of $\mathrm{Mg}({\mathrm{AlH}}_4{)}_2$ in $P{2}_1$ and $P\overline{3}m1$ phases.

Figure 5

Free energy of the lowest polymeric structures of the alanates, given with respect to that of the corresponding isolated reference structures.

Figure 6

Enthalpies as a function of pressure for the lowest polymeric structures with respect to the corresponding isolated reference phases.