Structure factor determination from critical voltages in electron diffraction

The principle of the critical voltage method in electron diffraction is an attractive one: a relation between structure factors can be determined with high precision from measurement of the condition for vanishing contrast of a contrast detail in the Kikuchi pattern or in the CBED pattern. In practice the method meets with some apparent and real limitations. The original, second order critical voltage in the systematic case (Watanabe, Uyeda and Fukuhara) depends on high accelerating voltage and can be applied mainly to strong low order structure factors from simple substances. Accurate additional information about other structure factors and temperature factors must be obtained from other methods. In order to increase the utility of the method a wider selection of configurations suitable for measurement has to be found. Several investigators have focussed on non-systematic cases: Gjønnes and Høier, Steeds.