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.