Directions of dislocation lines in crystals of ammonium hydrogen oxalate hemihydrate grown from solution

The defect structures in elastically highly anisotropic crystals of ammonium hydrogen oxalate hemihydrate, NH₄HC₂O₄.½H₂O, space group Pnma, have been studied by X-ray topography. Most of the dislocation lines visible on the topographs are straight and show clearly defined directions which depend on Burgers vector and on growth direction. Burgers vectors parallel to [100], [010], [001], (101) and (011) have been observed. For these Burgers vectors, the variation of energy factor K with direction and the preferential directions of the corresponding dislocation lines in various growth sectors have been calculated. In most cases, observed and calculated directions agree well. The directions of a few dislocations tend to align parallel to low-indexed (symmetry) directions. The influence of the elastic anisotropy on the directions of the dislocation lines as well as the possibility of determining the Burgers vectors from observed preferential directions is discussed.