Allotropy of Palladium

An examination of the heating and cooling curves of palladium in vacuo and in hydrogen led Andrew and Holt to the conclusion that palladium is dimorphous, the stability of the two forms depending upon the temperature. The rate of change of the one form into the second is very slow, and the two forms do not absorb hydrogen with equal rapidity. This is borne out by a study of the rate of solution of hydrogen in palladium foil.

The assumption that palladium can thus exist in allotropic forms receives interesting support from the work of Sieverts. This investigator points out that palladium black upon ignition yields palladium sponge, and that there is a simultaneous increase in the amount of hydrogen absorbed at temperatures below 600° C. Above this temperature the absorption of hydrogen by all physical varieties of palladium is practically the same. The explanation seems to be that both platinum black and sponge are mixtures of amorphous and crystalline palladium, the former of which has a greater power of absorbing hydrogen than the crystalline variety. On heating the palladium black, there is an alteration induced in the relative proportions of amorphous and crystalline metal, and consequently a variation in the amount of hydrogen absorbed is observed. Sieverts suggests that the absorption of hydrogen by the amorphous metal is a case of simple adsorption, whilst the hydrogen is actually dissolved in the crystalline variety.