Crystal structure ofCa(AlH4)2predicted from density-functional band-structure calculations

Abstract

Calcium alanate $[\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}]$ 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 $\mathrm{Ca}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$. 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 $\mathrm{Ca}{(\mathrm{Ba}{\mathrm{F}}_{4})}_{2}$ and was in the orthorhombic space group $Pbca$ with lattice constants $a=1337$, $b=928$, and $c=891\phantom{\rule{0.3em}{0ex}}\mathrm{pm}$. The electronic density of states reveals an insulator with a band gap of around $4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.