ANOMALOUS DEFECT BEHAVIOR RESULTING FROM THE HYDRIDING OF NANOCRYSTALLINE PALLADIUM

X-ray diffraction measurements on a nanocrystalline Pd sample transfor med in situ to PdH0.56 are described. Debye-Waller parameter measureme nts at several temperatures showed unexpected increases in both therma l and static atomic displacements caused by the hydriding. These incre ased displacements are associated either with the creation of new poin t 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 disloca tions is seen when coarse-grained Pd is hydrided. In contrast, measura ble changes in the peak breadths were not observed for the nanocrystal line sample as a result of hydriding, indicating that dislocations wit h 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.