Concentration-dependent diffusivity: Hydrogen percolation in WO3

Abstract

The diffusivity of H in a thin layer of WO3 was measured optically through the visible optical absorption of the H-containing substance. Microdensitometry and the photoresist technique made a 1-μm lateral resolution available. When the data were evaluated by Boltzmann’s method, the diffusion coefficient, as low as about Dm ≊10−11 cm2/sec at high H concentrations, was shown to increase sharply with decreasing H content. This behavior was quantitatively understood in terms of a percolation model modified by assuming H atoms to jam fast channels. The extreme low-concentration diffusion coefficient D0 was evaluated by virtue of this model and gave D0≊10−9 cm2/sec.