The influence of elastic anisotropy on the X-ray topographic image width of pure screw dislocations

The X-ray topographic image widths of pure screw dislocation lines along twofold axes of orthorhombic ammonium hydrogen oxalate hemihydrate, lithium formate monohydrate, thiourea and monoclinic potassium hydrogen oxalate have been studied. The widths observed on topographs made with imaging planes perpendicular to the dislocation line vary with the orientation of the incidence plane. This effect results from the elastic anisotropy, which gives rise to a varying slope of the helical surface formed by the distorted lattice planes. The displacements, strains, and effective misorientations of the imaging planes are quantitatively analysed for screw dislocations along a twofold axis or perpendicular to a mirror plane, taking into account elastic anisotropy. The image widths for various orientations of the incidence plane are calculated. The ratio of maximum and minimum widths is W_max/W_min = c₄₄/c₅₅, with c₄₄ and c₅₅ the maximum and minimum shear resistance, respectively, in the plane perpendicular to the dislocation line. Observed and calculated ratios are in good agreement.