Research of crystal habit modification by impurity provides not only useful applications to achieve optimize crystal morphology for industrial crystallisation process, but also practical approaches to the fundamental aspects of surface chemistry and mechanisms involved in crystal growth. Sodium dithionate (Na₂S₂O₆) is used as the habit modifier for crystal growth of sodium chlorate (NaClO₃) as a representative system for such a study.  Ordinary habit of pure sodium chlorate is cubic with dominant habit (100) type of faces. Presence of Na₂S₂O₆ (from 40ppm to 1000ppm) in the growing solution results in a significant decrease in the growth rates of (11), (11), (11) and ( ) faces, leading to tetrahedral morphology. Furthermore, crystal twinning is found to form under relatively high impurity concentration of 1000ppm and above.

X-ray topography has been applied to characterize growth history and defects of the crystals from various doping conditions (e.g. Fig.1). Having been identified to be the only system of impurity-induced crystal twinning, the two twin domains of the crystals are inter-grown each other (e.g. Fig.1 c) and only appear to be a mirror twining system. The twinning origin can be  either on a modified-habit face or at the nucleation stage. By employing X-ray Multiple Diffraction (XRMD) based on synchrotron radiation, 004 reflection Renninger Scans (RS) for the doped crystals show a significant reduction in the secondary diffractions of  (3-43)(3-41) and (343)(341), indicating these lattice planes are structurally disturbed by the impurity incorporation. Nevertheless, in 002 RS the overall lattice parameters on (1-11) and (-111) planes are found increasing as a function of impurity concentration, while that of (111) and (-1-1-1) planes decreasing. Local structural information of impurity obtained by applying X-ray Absorption Fine Structure (XAFS, synchrotron) provides direct proof on the impurity incorporation orientation and structural optimization (relaxation) on specific crystal planes. All experimental evidences are consistent with rational molecular model, in which the incorporation of dithionate impurity on {-1-1-1} faces is orientation-dependent.

1. POSTER: Structural Role of Sodium Dithionate Impurity in Habit Modification of Sodium Chlorate Crystal, Microsoft Office Document, 0.3MB

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