Electronic structure and optical properties of lightweight metal hydrides

Abstract

We study the dielectric functions of the series of simple hydrides LiH, NaH, $\mathrm{Mg}{\mathrm{H}}_{2}$, and $\mathrm{Al}{\mathrm{H}}_{3}$, and of the complex hydrides ${\mathrm{Li}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$, ${\mathrm{Na}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$, $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_{4}$, $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4}$, and $\mathrm{Mg}{(\mathrm{Al}{\mathrm{H}}_{4})}_{2}$, using first-principles density-functional theory and $GW$ calculations. All compounds are large gap insulators with $GW$ single-particle band gaps varying from $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in $\mathrm{Al}{\mathrm{H}}_{3}$ to $6.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_{4}$. Despite considerable differences between the band structures and the band gaps of the various compounds, their optical responses are qualitatively similar. In most of the spectra the optical absorption rises sharply above $6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and has a strong peak around $8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The quantitative differences in the optical spectra are interpreted in terms of the structure and the electronic structure of the compounds. In the simple hydrides the valence bands are dominated by the hydrogen atoms, whereas the conduction bands have mixed contributions from the hydrogens and the metal cations. The electronic structure of the aluminium compounds is determined mainly by aluminium hydride complexes and their mutual interactions.