Modeling of twinning processes in silver bromide precipitation

A probabilistic model has been developed to predict the various morphologies of silver bromide crystals obtained by twinning octahedral nuclei. The morphological fractions (i.e., untwinned, singly twinned, doubly parallel twinned, etc.) have been calculated as functions of a twinning probability t, a thermodynamic parameter. The model suggests that under commonly used conditions for precipitating twinned AgBr crystals, the time required for a twinning event to occur is of the order of a few msec. The model also predicts a high sensitivity of the various morphological fractions on the twinning probability t and therefore on the 〈111〉 surface supersaturation. The comparable time scales associated with twinning and micromixing and the sensitivity of the precipitated morphological fractions on the twinning probability t highlight the critical role of micromixing in the precipitation of twinned AgBr crystals.