Quasiparticle calculations for metal hydrides

The rare-earth hydrides YH_x and LaH _x are known to undergo both a conductive (metal to insulator) and optical transition (reflecting to transparent) as a function of hydrogen concentration for 2 < x < 3. The optical band gaps for each are estimated around 2 eV, yet band structures of the trihydrides calculated with standard ab-initio techniques are semi-metallic, with band overlaps as large as 1.5 eV. We have computed quasiparticle corrections to these band structures via the GW approximation to the self energy, using the implementation of Northrup, Hybertsen, and Louie [1,2]. These corrections are to first order and rely on the initial ab-initio wavefunctions and band structures found through solution of the Kohn Sham equations in the local density approximation (LDA). Corrections were also determined for various dielectric screenings calculated from shifted ab-initio band structures (insulator screening). Band gaps of approximately 0.8 eV and 0.1 eV were found for LaH ₃ and YH₃, respectively. Due to mixing of the wavefunctions near the overlap region in the original band structures, we found that the dipole selection rule between the H s valence bands and the metal d bands does not hold. This is reflected in the calculated absorption spectra.; A similar study of the alpha and gamma phases of MgH₂ was also performed. LDA structural parameters for both phases and the alpha phase absorption edge determined from quasiparticle band structures were found to be in remarkable agreement with experiment. We completed our work with an LDA structural study of the hydrogen storage material NaAlH₄.