Electronic energy bands of metal hydrides — palladium and nickel hydride

Electronic energy band calculations for transition metal hydrides have provided a new theoretical understanding of these materials. The calculations show when the proton model has relevance and when the anion model is more appropriate, the essential feature being the structure of the hydride. The structure in turn is determined by the lowest energy state of the solid. The relative positions of the electronic energy levels are found to be a strong function of the interatomic distances. The relative stability and structures of the metal hydrides can be understood in terms of their band structures. The calculations lead to a unifying new model, the “band model”, for interpreting and understanding the electronic properties of these materials. Energy band calculations of the palladium‐hydrogen system for various structures have been carried out and illustrate the above conclusions. The calculations were carried out with the hydrogen atoms occupying various interstitial positions in the face centered cubic palladium lattice to represent varying hydrogen contents for PdH x (specifically, for x = 0.25, 0.75, 1.0, and 2.0). In contrast to the simple proton model for this system, the calculations are able to show why β‐phase PdH saturates at higher hydrogen concentrations than the known concentration of holes in palladium d bands. Calculations for nickel hydride also show why the simple bandfilling concept fails. This result is in agreement with recent experimental electronic specific heat measurements. Both systems demonstrate that the rigid band model is not applicable.