Ab-initio Investigation of the Hydrogenation of Y

We present an <i>ab-initio</i> electronic structure study of a face centred (fcc) cubic Y lattice, which is subsequently filled with hydrogen in tetrahedral and octahedral sites. The stoichiometry of the calculated compounds is Y, YH₂^tetra, YH₃ and YH₁^octa. Upon placement of hydrogen in tetrahedral sites we note for the YH₂^tetra phase a new, fully occupied hydrogen related band and a redistribution in the d-band manifold at the Fermi level. Interestingly during the fcc Y → YH₂^tetra transition the overall topology of the Fermi surface (FS) is not changed, but we note a change in character from t_2g to e_g of the responsible band. It is this change to e_g character which allows subsequent H in octahedral sites to significantly interact with the FS, finally enabling the metal-insulator transition while reaching YH₃. A placement of hydrogen solely in octahedral sites does not affect significantly the t_2g d-band at the Fermi level.