The Crystal Structures of the System Palladium-Hydrogen

Crystal structures of the system palladium-hydrogen.—Palladium in the form of a fine wire, 1/4 mm diam., or of a narrow strip 1/20 mm thick, was more or less saturated with hydrogen and used to diffract x-rays of known wave-length as in the well-known powder method. Two crystal structures were present, both having a face-centered cubic arrangement of atoms. One, that of pure palladium, has a parameter 3.900 × ${10}^{-8\mathrm{}}$ cm, while the other, that of hydrogen-saturated palladium, has a parameter varying between 4.000 and 4.039 × ${10}^{-8\mathrm{}}$ cm, depending on the degree of saturation. A value near the upper limit, say 4.036 × ${10}^{-8\mathrm{}}$ cm, probably corresponds to a compound ${\mathrm{Pd}}_{2}H$, with a density of 10.76 gm/${\mathrm{cm}}^3$. The arrangement of atoms may be as in ${\mathrm{Cu}}_2$O, but there is no x-ray evidence for the positions of the hydrogen atoms. Stability. All the evidence so far obtained indicates that in the absence of sufficient free atomic hydrogen, the saturated state is unstable or metastable and that return to the hydrogen-free condition once initiated in any crystal proceeds rapidly to the end; also pure palladium is unstable in the presence of atomic hydrogen.

Occlusion of hydrogen by palladium.—A necessary condition is the presence of atomic hydrogen which may be supplied by electrolysis or by surface dissociation of hydrogen at high temperatures. The penetration into the wire was shown to be relatively slow; in the case of the strip it was somewhat irregular, probably depending on the orientation of the crystals. The crystal evidence provides an explanation of the variations of electric resistance. A bibliography of 66 titles, mostly 1900-1921, is appended.