Morphology and growth mechanism of multiply twinned AgBr and AgCl needle crystals

Abstract

In this study the morphology and growth mechanism of AgX (X=Cl, Br) needle crystals will be revealed. The silver halide needles are grown by two different techniques. AgBr needles are grown with an extractive crystallization in a dimethylsulfoxide–water mixture and AgCl needles are grown from the vapor phase. All needles contain nonparallel twin planes. The morphology of the AgBr needles grown from dimethylsulfoxide is the same as the needles grown in the industrial precipitation process in water. The side faces of these solution grown needles are built up of relatively slowly growing {1 1 1} faces. Both needle tops are composed of three relatively fast growing {1 0 0} faces making a ridge structure. The vapor-grown needles have two different morphologies. For both types the side faces consist of four {1 1 1} and two {1 0 0} faces. All {1 1 1} faces are linked via twin planes to a fast growing {1 0 0} face. The preferential unidirectional growth of all needles is caused by cross-twinning. The preferential growth occurs along the intersection line of the twin planes. Between the twin planes a rough growing face appears on the needle top. This face is capable of increasing the growth rate of the other top faces owing to the substep mechanism.