Real Space Imaging of Structurally and Compositionally Complex Materials

High-Angle-Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF/STEM) is a technique uniquely suited for detailed studies of the structure and composition of complex materials and allows us to speed up structural investigations in comparison to powder diffraction. The Mo-V-Nb-(Sb,Te)-O system has been identified as a commercially viable catalysts for the selective oxidation and ammoxidation of small alkanes to make about 25% of important organic chemicals and intermediates used in industry. The difficulty plaguing conventional crystallographic refinements of such a complex material is that the model has up to ~200 crystallographic parameters, requiring very high quality data and suitable starting models with imposed bond distance and angle constraints to reduce parameter space and ensure a stable and converging refinement. The availability of aberration-corrected electron microscopes has transforming imaging at the nanometer scale. Recent work also significan tly advanced the complexity of “frozen-phonon” calculations now possible to simulate HAADF-STEM images using massively parallel computations. In-situ heating in a STEM has revealed very unique thermal behavior and imaging at various temperatures helps us understand the operation of these catalysts near operando conditions .