X-ray Topographic and Polarized Optical Microscopy Studies of Inversion Twinning in Sodium Chlorate Single Crystals Grown in the Presence of Sodium Dithionate Impurities

Sodium chlorate (NaClO3) crystallizes in the enantiomorphic space group P213 which allows two chiral structural configurations (l and d forms). These crystals can form growth twins when the solution contains impurity ions of dithionate with the mole ratio 1:1000 (dithionate/chlorate) or higher. The impurity modifies the crystal habit from a cube-like morphology to one dominated by the minor {111} tetrahedral form. An X-ray topography study and the experimental observation of crystal growth reveal that the twinning phenomenon is consistent either as a result of the surface nucleation of a tetrahedral crystal of the morphologically opposite chirality on an existing tetrahedral habit face or by twinning at the nucleation stage and subsequent growth as two intergrown tetrahedral crystals. Polarized optical microscopy reveals that the optical activities of the twinned crystals observed are inversed with respect to each other. This is consistent with a merohedral mirror twinning system following twin law of m(001) (or identically 1̅). Modeling of the crystal structure with respect to its external morphology is consistent with the dithionate impurity ions substituting for two adjacent chlorate ions at the twin boundary one from each of the contacting twin domains, through this creating a heterochiral twinned interface.