Characterization of nanocrystalline palladium by x-ray atomic density distribution functions

We present results of x-ray scattering experiments on a set of nanocrystalline Pd samples, produced by inert-gas condensation, with different consolidation, aging, and annealing parameters. Atomic density distribution functions indicate that the number of atoms irt the coordination shells of nanocrystalline Pd is substantially lower than in the coarse-grained polycrystalline lattice. The results suggest that, as a function of grain size and/or age, the samples have two different atomic structures. In coarser-grained samples aged for several months at room temperature and in annealed samples, practically all atoms are located on crystal lattice sites, and the reduction in nearest-neighbour coordination number is only an effect of the finite size of the crystallites. In finer-grained samples examined immediately after preparation, about 10% of the atoms are located on non-lattice sites with little or no atomic short-range order.