STRUCTURAL CHARACTERIZATION OF NANOMETER-SIZED CRYSTALLINE PD BY X-RAY-DIFFRACTION TECHNIQUES

Quantitative x-ray-diffraction measurements of ultrafine-grained (nanocrystalline) Pd samples and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The intensity profiles of the Bragg reflections from the nanocrystalline samples were considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of intensity from the Bragg peaks was found in the tails of the reflections. The remaining intensity differed only slightly for different grain-sized materials, therefore, atomic relaxations in the vicinity of grain boundaries in nanocrystalline Pd must be small in magnitude and/or extremely localized. The results of the present work do not support the previously proposed existence of either a "gaslike" grain boundary phase, or large quantities of vacancies or voids within the grains of nanocrystalline Pd, which produce broadly distributed diffuse scattering. The broadening of the Bragg reflections was related to the small particle size of nanocrystalline Pd, and strain located in the grains and/or interfacial regions. Evidence was seen for anisotropic grain shapes preferentially elongated along the [111] direction. The Debye-Waller parameter of nanocrystalline Pd was observed to be larger than the literature value for coarse-grained Pd, which suggests larger displacements of the atoms from their ideal lattice locations in the nanocrystalline material than in the coarse-grained material.