The critical-voltage effect in convergent-beam high-voltage electron diffraction

The critical-voltage effect has been combined with the convergent-beam electron diffraction technique using a high-voltage electron microscope. The method allows the critical voltage V_c to be detected to within approximately ± 1 kV, compared with ± 5 kV in previous methods. This accuracy is achievable throughout the 100 to 1000 kV accelerating-voltage range of the high-voltage microscope used. V_c has also been determined by varying the specimen temperature at constant voltage, which has the advantage that all electron-optical parameters are kept constant: thus the 'transfer function' of the microscope is constant. The critical voltage is easily identified experimentally by the appearance of a characteristic 'dark band' in the Bragg-satisfied second-order convergent-beam disc. Changes in the asymmetry of the Kikuchi line within the dark band enable the precise localization of V_c to ± 1 kV to be made. The improved precision of this new method considerably increases its application to the determination of scattering factors in pure materials and to ordering and electron-transfer effects in certain alloys. The method is illustrated by applying it to Cu and Cu-Al alloys.