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 the appearance of new faces, {111BAR}, on sodium chlorate crystals grown from aqueous solution. The present study compares the growth mechanisms of the {111} habit faces and {111BAR} type faces using laser interferometry techniques. The {111} faces show dislocation layer growth in which the impurity influences the growth by causing the gradual deceleration of step movement. In contrast, the {111BAR} faces incorporate crystal units into an atomically rough surface. Under these circumstances the impurity on the surface creates a barrier to continuous growth. The strong impurity interaction with the {111BAR} faces can be explained by the close structural similarity of the -SO3 portion of the S2O62- ions to the ClO3- ions situated on these faces. This allows the dithionate ion to substitute for ClO3-, disturbing the structure and energy of the interface and inhibiting growth.