Crystallographic changes in an initially-pure Pd-wire cathode were monitored in situ as D was electrochemically deposited on the surface and diffused radially into the wire. A constant electrolysing current density of 1mA/cm2 was maintained, and D diffused slowly into the wire. As the D concentration increased, the wire transformed firstly from pure Pd into the a phase, and finally into the ß phase. This reversible phase transformation began on the surface, and progressed radially inwards. During the experiment, X-ray diffraction data were collected from a volume element of about 180pl. This volume element was moved systematically, in 50µ steps, from the edge to the center of a 1mm-diameter Pd wire. During the experiment, the bulk value of x in PdDx - as deduced from simultaneous measurements of the electrical resistivity - increased from 0 to about 0.72. For each setting of the volume element, a monotonic increase in the volume of the a-phase was observed until the material entered the 2-phase region. When the ß-phase appeared, the volumes of both phases decreased slightly with continued loading. The integrated intensities of diffraction peaks from each phase were used, together with the known phase diagram, to estimate the rate of compositional change within the volume element. The diffusion rate of the solute atoms was estimated to be 57nm/s, based upon the temporal and spatial dependences of the integrated intensities of the diffraction peaks from each phase. These data were also used to determine the time-dependence of the concentration of the solute atoms, and their diffusivity. The former increased linearly, from 6.2 x 10-5/s at the surface to 7.6 x 10-5/s midway into the wire. The diffusivity was calculated to be equal to 3.1 x 10-11m2/s when the transition began at a radius of 250µ, but 2ks later it had decreased to 2.1 x 10-11m2/s. This was suggested to be due to the fact that the volume of the ß lattice also decreased during this period.

In situ Monitoring of Crystallographic Changes in Pd Induced by Diffusion of D. E.F.Skelton, P.L.Hagans, S.B.Qadri, D.D.Dominguez, A.C.Ehrlich, J.Z.Hu: Physical Review B, 1998, 58[22], 14775-9