The critical voltage effect in high voltage electron microscopy

Abstract

A number of new applications of the critical voltage effect are proposed and investigated both theoretically and experimentally. It is shown that this effect may be used not only for the accurate determination of atomic scattering factors but also for the determination of Debye temperatures and for measuring the degree of order of partially ordered alloys. In addition the effect is useful for increasing defect contrast and for detecting any inhomogeneity in a crystal, for example, due to segregation, which locally changes the lattice spacing or the electron density. The many-beam’ dynamical theory of the effect is given, and a simple approximate equation is derived for calculating approximate values of the critical voltage for a given material and reflection. Various experimental methods of determining the value of the critical voltage are assessed, and a table of results is given for a range of materials, together with the low-order atomic scattering factors which have been determined using these results.