The structural role of sodium dithionate impurity in the habit modification of sodium chlorate single crystals

A detailed study into the crystal habit modification of the NaClO3 / Na2S2O6 (host / impurity) system is presented. Ordinary morphology of NaClO3 present faces of {001}, {110} and {111} types. The presence of impurity Na2S2O6 has led to rapid development of new faces of {111} type on the NaClO3 crystals grown from solution. Above 70ppm doping concentration, the morphology of NaClO3 crystal is dominated by {111} faces. Crystal twinning occurred under 800ppm doping concentration and above. X-ray topography was used to investigate the growth history and defect configuration of the pure and doped crystals. Lattice distortions at various lattice planes within the pure and doped crystals were determined using X-ray multiple-wave diffraction (XRMD). The local structure of S2O6 2- in NaClO3 crystal was determined using X-ray absorption fine structure (XAFS). Molecular modelling was applied to investigate the molecular similarity between the impurity and the host. Strong impurity incorporation in the {111} sectors was revealed by X-ray topography. The growth history of doped crystal was reconstructed and interpreted with respect to the inhibiting effect of S2O6 2-. Disturbance in lattice planes of doped crystals was investigated, which was attributed to the incorporation of S2O6 2- on { 111} faces. It also revealed different types of local strain on the { 111 } faces along two different directions. The three-dimensional orientation and the actual structure of S2O6 2- impurity on the {111} faces of NaClO3 crystal were obtained. A structural model for the impurity incorporation was established, showing good consistency with the experimental results. In addition, the segregation coefficient of the impurity was determined by elemental mapping, indicating strong impurity incorporation on the {111} faces rather than others. The incorporated S2O6 2- was concluded to be capable of disrupting the proper packing structure of the {111} faces, obstructing the generation and propagation of growth steps, and decreasing of driving force for crystal growth.