Diffusion coefficient of hydrogen in the Pd3Mn compound as deduced from absorption measurements at high temperature

The hydrogen absorption rate has been investigated by the gas–solid reaction method in the disordered and ordered (L12) Pd3Mn compound. With the disordered material the measurements were carried out from 825 K up to 1038 K, with the ordered one from 485 K to 666 K. The H absorption data, analyzed according to the second Fick's equation, provided the H chemical diffusion coefficient Dc which exhibited an exponential temperature dependence. In the disordered state Dc was higher than in the ordered one in agreement with previous lower temperature data from Gorsky relaxation. This finding could be accounted for in terms of the different kinds of jumps between adjacent interstitial (I) octahedral sites that H is able to make in the two different states of order. In the disordered state H diffusion takes place through sequences of I6–I6, I6 ↔ I5 and I5–I5 jumps (sub-index indicates the number of Pd atoms in the first atomic shell of an octahedral interstitial site), while in the state of order L12 the involved jumps are I6 ↔ I4 and I4–I4.