Image formation from ordered alloys in the field-ion microscope

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Abstract
It is possible to analyse the field-ion image contrast from solid solution alloys, and thereby to obtain the distribution of the two atomic species. Such an analysis depends on recognizing the relative contributions to the image contrast from two distinct sources : the stability of the species towards field evaporation (selective evaporation) and their relative response to field ionization (selective ionization). Here this scheme is extended to the case of ordered alloys, and illustrated by reference to the image contrast from PtCo and Pt3Co. It is shown that the cobalt atoms will be evaporated preferentially with respect to the platinum, and that this fully accounts for the experimental observation that the cobalt atoms are not imaged. Considerable experimental evidence is cited to substantiate this point. While calculations have been performed only for the platinum–cobalt alloys, the procedure would be identical for other Li0 and Ll2 superlattices, while requiring only slight modification for other structures. In every instance the selective evaporation behaviour must be the controlling factor, and only in those cases where both species are predicted to remain on the specimen surface will selective ionization determine the contrast.

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