Twinned silver bromide crystals. Some insights into their formation and growth

Growth rate of octahedral silver bromide crystals is controlled by the rate of incorporation of growth species at the 〈111〉 surface when the [Br^-] in the reactor is increased to levels where the tabular AgBr crystals are precipitated. Under these conditions, the supersaturation at the 〈111〉 surface is equivalent to the bulk solution supersaturation. It is suggested that during the nucleation stage of AgBr precipitation, when the bulk and, therefore, the surface supersaturation are high, the 〈111〉 surface becomes unstable because of the high rate of addition of growth species. This instability results in the twinning of octahedral crystals; subsequent formation of crystals with two parallel twin planes and the enhanced growth rate at the reentrant region of these crystals lowers the solution supersaturation and brings the system back to stability. This concept of surface-supersaturation-induced twinning was validated by our experiments in which 〈111〉 surfaces were twinned by exposing them to high supersaturation.