Line-Breadth in Electron Diffraction

Abstract

ONE of the major factors determining line-breadth in electron diffraction powder patterns has been shown^1–3 to be the refraction of the electron beam resulting from the finite inner potential of the crystals. Striking differences in line-breadth occur in patterns from crystals of regular habit, such as the cubes of magnesium oxide and cadmium oxide smokes and the elongated hexagonal prisms of zinc oxide smoke. Certain reflexions, for example, h00 in magnesium and cadmium oxides, and 00l in zinc oxide, are sharply defined, and it has been shown by Cowley and Rees² that the refraction components lie along these rings and do not contribute to the line-breadths. Moreover, any elongation of reflexions arising from the non-spherical symmetry of the scattering function about the reciprocal lattice points for zinc oxide of this habit will lie along these rings also. Nevertheless, these breadths are still greater than can be accounted for by limitation of crystal dimensions and the finite breadth of the beam.