ANOMALOUS DEFECT BEHAVIOR RESULTING FROM THE HYDRIDING OF NANOCRYSTALLINE PALLADIUM

X-ray diffraction measurements on a nanocrystalline Pd sample transformed in situ to PdH0.56 are described. Debye-Waller parameter measurements at several temperatures showed unexpected increases in both thermal and static atomic displacements caused by the hydriding. These increased displacements are associated either with the creation of new point defects on the Pd sublattice or with an increase in the magnitude of the strain fields of previously existing point defects. Substantial X-ray peak broadening due to the production of large numbers of dislocations is seen when coarse-grained Pd is hydrided. In contrast, measurable changes in the peak breadths were not observed for the nanocrystalline sample as a result of hydriding, indicating that dislocations with long-range displacement fields are not created when nanocrystalline Pd is hydrided. A preference to produce or modify short-range defects instead of dislocations during hydriding may occur as a result of the larger stresses required to generate dislocations when the grain size of the material is small.