Calcium alanate $\left[\mathrm{Ca}{\left(\mathrm{Al}{\mathrm{H}}_4\right)}_2\right]$ is a candidate material for high-density reversible solid-state hydrogen storage that thus far has been scarcely studied. This paper presents a scheme for solving the crystal structure of a compound based on only a few model structures from similar compounds and employs this to predict the crystal structure and electronic structure of $\mathrm{Ca}{\left(\mathrm{Al}{\mathrm{H}}_4\right)}_2$. By deliberately breaking the symmetry of the model structures down to $P1$, local minima may be avoided, and thus the number of required input models is smaller. Density-functional band-structure calculations within the generalized gradient approximation were used in the structural minimizations. The most stable structure was based on $\mathrm{Ca}{\left(\mathrm{Ba}{\mathrm{F}}_4\right)}_2$ and was in the orthorhombic space group $Pbca$ with lattice constants $a=1337$, $b=928$, and $c=891\mathrm{pm}$. The electronic density of states reveals an insulator with a band gap of around $4\mathrm{eV}$.

• Received 18 January 2005

DOI:https://doi.org/10.1103/PhysRevB.71.144111