The effect of bond number on atom images in the field-ion microscope

Abstract

Abstract It is shown that (1) over a range of tip radii, the depth below a spherical surface where atoms of given coordination occur can be correlated with the thickness of the spherical shell which contains the atoms observed in a field ion image; (2) large changes in image intensity observed across certain zones correspond to abrupt asymmetrical changes in the atomic environment of the imaged atoms. The shortest possible bonds giving these changes correspond to first and fourth nearest neighbours in b.c.c. and third and fifth nearest neighbours in f.c.c.; (3) the changes in intensity across these zones become diffuse at small tip radii because a larger proportion of the image is derived from atoms with higher coordination. Their environment remains constant within a band extending some distance on either side of the zone.