Electronic structure of hydrogen storage materials

Abstract

The electronic structure of two types of hydrogen storage materials has been investigated by ab initio band structure methods. The author shows that the helium produced by the radioactive decay of tritium in a palladium matrix remains stable at the octahedral site of the fcc lattice for He/Pd ratio of at least 0.25. Such He production results in a lattice expansion of 2%. The experimentally observed increased stability of aged tritium in a palladium matrix is not entirely due to lattice expansion, which amounts to only 28% of the effect, but also to modifications of the electronic structure in the presence of helium. In addition, he shows that He leads to lattice decohesion. The effect of Ni substitution in LaNi{sub 5} by 3d and s-p elements on the electronic structure of the intermetallic and its hydrides has been also investigated. These substitutions are found to affect drastically the properties at the Fermi level and the filling of the Ni-d states. The Fermi level, E{sub F}, of LaNi{sub 4}M (M = Fe, Co, Mn) is found to lie in the narrow additional M-3d subband above the Ni-d states, leading to an increase in the density of states (DOS) at E{sub F}.more » In contrast, the substitution of Ni by an s element of the 3d series, Cu, or by an s-p element, Al or Sn, results in a progressive filling of the Ni-d bands and in a decrease of the DOS at E{sub F}. In all the substituted intermetallic compounds, he finds that the lattice expansion accounts for less than 50{degree} of the observed decreased stability. This shows the importance of chemical effects. He also discusses the factors that affect the electronic structure and the stability of the hydrides, and compares the results with available experimental data.« less