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STRUCTURE OF NANOCRYSTALLINE MATERIALS BY THE ATOMIC PAIR DISTRIBUTION FUNCTION TECHNIQUE

The approach of the atom ic pair distribution function (PDF) technique to study the structure of nanocrystalline materials is considered and successfully applied to V2O5.nH2O xerogel and Cs intercalated inside the pores of the zeolite ITQ-4. We find that V2O5.nH2O is an assembly of almost perfect bi-layers of VO5 units encapsulating the water molecules. Inside the nanopores of ITQ-4 the cesium ions are found to assemble in short-range ordered zig-zag chains. INTRODUCTION Nanocrystalline m aterials are being extensively studied because they often exhibit interesting and useful properties not occurring in perfect crystals. Examples include nanomagnets, nanocatalysts, nanocomposites with improved optical properties and nanodevices. Understanding and predicting the novel electronic and magnetic properties of all these materials requires detailed knowledge of the atomic ordering. Usually the 3-D structure of materials is obtained from the Bragg peaks in their diffraction patterns. However, materials constructed at the nanoscale often do not possess the translational symmetry and long-range order of conventional crystals. Instead they assume structures of reduced dimensionality and/or accommodate a large number of defects and local disorder. The diffraction patterns of such materials show a pronounced diffuse component and a few Bragg peaks, if any. This poses a real challenge to the traditional techniques for structure determination. The challenge can be met by employing the so-called atomic pair distribution function (PDF) technique. The atomic PDF gives the number of atoms in a spherical shell of unit thickness at a distance r from a reference atom. It peaks at characteristic distances separating pairs of atoms and thus describes the structure of materials. The PDF, G(r)=4πr(ρ(r)-ρo), is the sine Fourier transform of the so-called total scattering structure function, S(Q),