Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Chemistry and Pharmacology
A method of determining the long-range order parameter S in alloys with a high accuracy from thickness fringes in electron micrographs is proposed and applied to Cu3Au and Fe3Al. The extinction distance ξg and the intensity of a g (superlattice) reflexion at the exact 2g Bragg position in the systematic condition are calculated as a function of S at various accelerating voltages with the many-beam dynamical theory. In the case of Cu3Au the variation of ξg with S is predicted to be very large in the range 200-1000 kV, and the intensity of the g reflexion decreases with increase of accelerating voltage. In the case of Fe3A1 with a B2-type superlattice, on the other hand, S dependence on ξg and the intensity of the g reflexion are not sufficient to determine the value of S with high accuracy at around 200 kV, but they are sufficient at around 1000 kV. An experiment on Cu3Au reproduces the theoretical prediction and indicates that this method is useful for determining the long-range order parameter, especially in local regions.
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