SANS measurements of deuteride (hydride) formation in single crystal Pd

In situ small-angle neutron scattering (SANS) measurements of deuteride precipitation in well-annealed, undeformed single crystal Pd have been performed at room temperature to observe the morphology of the precipitate as the deuterium fraction was increased over a range of 0.005 to 0.097 [D]/[Pd]. Supporting measurements were made of the room temperature deuterium solubility isotherm in the single crystal material and of the progressive lattice mosaic broadening by gamma-ray diffraction analysis. The use of single crystal material eliminated the effect of grain boundaries on the precipitation process and allowed the orientation of the deuteride precipitates relative to the host Pd lattice to be established. The SANS data for deuterium loading beyond the solid solution phase are dominated by a dΣ/dΩ to approximately Q^-2 scattering response well fit by a model cross section of small plate-like precipitates with a thickness of 25-30 angstroms. This response showed little indication of a preferred habit plane and the plate thickness was independent of the total deuterium loading. The volume fraction of these small plates grew uniformly with increased deuterium loading, but was surprisingly small, reaching only 8×10^-5 at 0.060 [D]/[Pd]. This means that almost all of the deuterium resides in some other precipitate structure. The SANS data in the lowest Q region showed a second, anisotropic scattering component that can be interpreted as the Porod tail from plate-like particles of grossly larger dimensions. These plates favor a (001̄)_α habit plane, normal to the elastically soft [001̄]_α direction. Post-measurement visual inspection of the sample surfaces showed the presence of an oriented, quasi-periodic one-dimensional array of plates with thicknesses of approximately 0.1 to 0.2 mm and with an orientation in agreement with the anisotropy of the SANS data. We conclude that most of the deuteride phase precipitates in these large structures.