Twin formation and morphology of vapour grown silver halide crystals

Crystals of pure AgX (X= Cl, Br) were grown from the vapour phase on copper and quartz substrates. For both AgBr and AgCl the same morphologies appear namely, cubic, {100} tabular, {100} tabular with {411} twinning, cubo-octahedral, {111} tabular and needle shaped. These morphologies are the same as obtained for pure silver bromide from precipitation. So, the role of the solvent in the growth process could be examined. The vapour growth morphology turns out to depend strongly on the temperature difference between the furnace and the substrate. At low temperature difference the {100} faces were observed, whereas at higher driving force the {111} faces became more visible. At a critical temperature difference twin planes parallel to the {111} faces were formed resulting in tabular crystals (multi-parallel twinning) and needle shaped crystals (also non-parallel twinning). Due to the occurrence of {111} tabular crystals for both AgBr and AgCl and the constant driving force during the experiments, conclusions can be drawn about the twinning event and the growth mechanism.