Stephan Rosenkranz
Argonne National Laboratory, USA
Title: Recent developments in single crystal diffuse scattering: Imaging nanoscale disorder in reciprocal space
Biography
Biography: Stephan Rosenkranz
Abstract
Correlated defects are responsible for the functional properties of many materials that underpin energy-related technologies. Single-crystal diffuse scattering using x-rays or neutrons offers a powerful probe of such short-range order in crystalline lattices, but its use has been limited by the experimental challenge of collecting data over a sufficiently large volume of reciprocal space and the theoretical challenge of modeling the results. However, instrumental and computational advances at both x-ray and neutron sources now allow the efficient measurement and rapid transformation of reciprocal space data into three-dimensional pair distribution functions, providing model-independent images of nanoscale disorder in real space. We discuss how these recent developments of efficient methods of measuring single crystal diffuse scattering provide new insights into cation disorder in electrode materials. Large volumes of measured diffuse scattering in reciprocal space are transformed into 3D difference pair distribution functions (3D-ΔPDF) that image defect-defect correlations in real space, allowing a model-independent view of short-range order. We demonstrate this with data on β-NaxV2O5 with x=0.2 and 0.4 over the temperature range 100K to 500K. The sodium intercalants partially occupy sites on two-rung ladders penetrating the framework of vanadium oxide pyramids and octahedra, with no long-range order at room temperature and above. However, at x=0.4, the length scale of sodium-sodium correlations increases significantly below 200K with the emergence of forbidden Bragg peaks below an order-disorder transition. The 3D-ΔPDF directly reveal that the sodium ions occupy alternate sites on each ladder rung, with a zig-zag configuration that is in phase with neighboring ladders. The growth in the length scale of sodium-sodium correlations with decreasing temperature is clearly seen in real space images that allow a quantitative determination of the interionic interactions that impede ionic mobility.
