High-resolution images of bent crystals having rutile-type structures: Tin dioxide

High-resolution images of SnO₂, crystals, oriented into the [001] zone, show 4·7 × 4·7 Ų detail produced by 100 and 010 reflections. Similar images are obtained at both 200 and 500 kV. Since these reflections are both kinematically and dynamically forbidden, for symmetrical zone orientations, the image contrast is not predicted using usually-acceptable computer-simulation techniques. On the other hand convergent-beam diffraction patterns are quite consistent with the expectations of dynamical scattering theory, assuming the accepted structure and space group. It is suggested that the apparently anomalous behaviour of the images is due to breakdown in symmetry caused by elastic bending, when the screw-axis operations, responsible for extinction of 100 and 010, will be lost. Image calculations for a simple bent crystal show that radii of curvature of 10–20 μm, extending over lateral extent 50–100 Å, are more than sufficient to destroy the screw operations effectively as far as dynamical scattering of electrons is concerned. The implications of these results for the interpretation of high-resolution images are discussed in terms of the spatial coherence of both the crystal structure and the electron probe. Differences in effective lateral coherence of convergent-beam and high-resolution illumination conditions make the latter more sensitive to local bending.