A chemical reaction strongly dependent upon the degree of order of an alloy: The absorption of hydrogen by Pd₃Fe

The magnitudes of the interstitial hydrogen content of Pd3Fe alloys have been investigated as a function of pressure. Since the alloy, Pd3Fe, does not absorb hydrogen at ≤ 1 atm. (25°c), high pressures were needed to introduce measurable amounts of hydrogen. It was found, for example, that a Pd3Fe alloy with long-range order attains a value of 3·5 for the ratio of its electrical resistance to its hydrogen-free value at 1000 atm. H2, whereas this ratio for the disordered alloy is 1·1 (25°c). The respective hydrogen contents (lower limits), expressed as H-to-metal (atomic ratio), corresponding to this pressure are 0·167 and 0·035. This marked difference in the behaviour of the alloys is attributed to the occupation of a unique interstitial site in the ordered alloy; this site is at the centre of the f.c.c. unit cell and has only palladium atoms as its nearest neighbours.