It was noted that a discrepancy between the low-temperature variation of the diffusion activation energy as a function of the H/metal ratio, and that found at higher temperatures, could not be explained by so-called volume contributions to the activation energy. It was concluded that, in spite of published data concerning the effects of the H concentration in the Pd-H binary system, more attention should be paid to H-H interactions and to the effects of changes, in the H/metal ratio, upon tunnelling.

R.B.McLellan, L.Yang: Scripta Metallurgica et Materialia, 1995, 32[7], 1015-7