Diffusion of Hydrogen in the γ Phase of Titanium Hydride

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Abstract
The diffusion of hydrogen in titanium hydride has been examined in terms of a simple electrostatic model. The energy of a hydrogen atom at various positions in the lattice has been calculated from screened Coulombic potentials, neglecting repulsive terms of other origins. This gives Ea, the activation energy for hydrogen diffusion, as a function of the net electronic charge on the hydrogen atom. Agreement with the experimental results for Ea is obtained from a value for the electronic charge on the hydrogen which is consistent with other evidence. The model exhibits the distinct preference of the hydrogen atom for the accommodation in tetrahedral rather than octahedral interstices in the γ phase. It predicts the decrease observed in Ea for titanium hydrides containing less than the stoichiometric proportion of hydrogen. Moreover, the model provides an explanation of the tendency for hydrogen in the γ phase to distribute itself randomly and uniformly throughout the lattice rather than to form aggregates.