GROWTH OF THE TETRAHEDRAL FACES OF SODIUM-CHLORATE CRYSTALS IN THE PRESENCE OF DITHIONATE IMPURITY

It has been established that trace amounts of S2O62- anions initiate t he appearance of new faces, {111BAR}, on sodium chlorate crystals grow n from aqueous solution. The present study compares the growth mechani sms of the {111} habit faces and {111BAR} type faces using laser inter ferometry techniques. The {111} faces show dislocation layer growth in which the impurity influences the growth by causing the gradual decel eration of step movement. In contrast, the {111BAR} faces incorporate crystal units into an atomically rough surface. Under these circumstan ces the impurity on the surface creates a barrier to continuous growth . The strong impurity interaction with the {111BAR} faces can be expla ined by the close structural similarity of the -SO3 portion of the S2O 62- ions to the ClO3- ions situated on these faces. This allows the di thionate ion to substitute for ClO3-, disturbing the structure and ene rgy of the interface and inhibiting growth.