Hydrogen diffusion and radiationless electron transfer in metal hydrides

The activation energies of the radiationless electron transfer are calculated from the optical spectra of some Sc, Y, La, Lu, Nb, Pd, and Mg2Ni hydrides. These are practically the same with the values of the activation energies for hydrogen diffusion, pointing out to the importance of electron transfer from localized states to the conduction band in the mechanism of hydrogen diffusion. The exponential temperature dependence of the radiationless electron transfer probability together with vibrational tunneling at low temperatures can account for the temperature dependence of H diffusion in the entire temperature range. A comparison between the electronic structures of NiH and PdH qualitatively explains the higher activation energy of H diffusion in Ni. Isotopic effects in Pd hydrides and a treatment based on Frenkel defects also support the proposed model. (author)