Thermal Diffuse Scattering from Nanocrystalline Systems

The thermal diffuse scattering (TDS) in X-ray diffraction (XRD) patterns contains significant information about the local lattice dynamic structure of nanocrystalline systems. Techniques such as the pair distribution function (PDF) are commonly employed to extract this information, where the correlated movement of atomic pairs remains encoded in the breadths of the PDF peaks. However, PDF techniques require a Fourier transformation of the experimental XRD data, orientationally averaging the local dynamic information, rendering it not readily distinguishable from the static component and crystallite size and shape effects. Herein, we explore the possibility of an analysis of local lattice dynamics based directly on XRD powder pattern modeling, where TDS is added to the structural model of the traditional Rietveld method. Allied with the whole powder pattern modeling approach, the crystallite shape and static components are simultaneously estimated. Two study cases of Pd nanocrystalline systems are analyzed: (i) in silico nanosphere powder simulations via molecular dynamics (MD) and (ii) synchrotron radiation XRD powder patterns of Pd nanocubes. In silico analysis points out that the TDS model provides the correct trends of the correlated atomic movement up to the ninth coordination shell. The experimental case shows that this TDS model correctly estimates the force mechanisms of the nanocrystalline Pd system. We believe that the established method and the obtained results in this study broaden the application scope of XRD for studying the dynamic properties of nanocrystalline materials.