Lattice Parameter of Alpha Iron by Divergent Beam Diffraction

Abstract

The divergent beam diffraction technique has been used to determine the lattice parameter of single crystals of alpha iron made by vapor deposition techniques. An electron beam focused to approximately 10 μ was used to produce a source of K series characteristic radiation within the samples. All unique Kossel conics which emerged from the specimens were recorded on flat photographic films. The back-reflection geometry was employed. Three different crystal orientations were investigated: a fourfold symmetry axis normal to the film plane, a threefold symmetry axis normal to the film plane, and mirror—mirror symmetry of the crystal normal to the plane of the film. Data obtained from the latter photographs have been used to demonstrate the application of the method of ratios. These analytical techniques were applied to Kossel conics that were continuous and interrupted. The lattice parameter was established by five different series of calculations. The most accurate value of the lattice parameter obtained in this investigation was 2860.6±0.1 xu. Stereographic projections have been made for three different orientations of bcc crystals. The value of the dimensionless parameter (λ/a) is 0.675 in these representations. All Kossel conic intersections required by symmetry considerations are indicated.