Structures and thermodynamics of the mixed alkali alanates

Abstract

The thermodynamics and structural properties of the hexahydride alanates $\left({\mathrm{M}}_2{\mathrm{M}}^{\prime }\mathrm{Al}{\mathrm{H}}_6\right)$ with the elpasolite structure have been investigated. A series of mixed alkali alanates (${\mathrm{Na}}_2\mathrm{LiAl}{\mathrm{H}}_6,$ ${\mathrm{K}}_2\mathrm{LiAl}{\mathrm{H}}_6$, and ${\mathrm{K}}_2\mathrm{NaAl}{\mathrm{H}}_6$) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 mol% $\mathrm{Ti}{\mathrm{Cl}}_3$) and uncatalyzed ${\mathrm{Na}}_2\mathrm{LiAl}{\mathrm{H}}_6$ exhibited an increase in kinetics, but no change in the bulk thermodynamics with the addition of a dopant. A structural analysis using synchrotron x-ray diffraction showed that these compounds favor the $Fm\overline{3}m$ space group with the smaller ion $\left({\mathrm{M}}^{\prime }\right)$ occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics of a number of materials with promising gravimetric hydrogen densities.