Growth Mechanism of Vapor-Grown Silver Crystals:  Relation between Twin Formation and Morphology

This study deals with the morphology and growth mechanism of multiply twinned Ag crystals grown from the vapor phase. Ag crystals were grown under sublimation conditions for small growth rates as a function of the driving force and sublimation temperature. At relatively low driving force and low temperature cubooctahedrons and single twinned crystals appeared. At higher driving forces also 111 tabular crystals (containing multiple parallel twin planes) and 5-fold twinned structures resulting in decahedrons, icosahedrons, and elongated decahedrons bounded by 100 sides faces, were observed together with other nonparallel twinned crystals. Increasing the driving force further led to tabular crystals with higher aspect ratios and different side-face structures; the preferential unidirectional growth of the elongated decahedrons was also increased. The growth mechanism of 111 tabular Ag crystals and other multiply twinned Ag crystals is for the first time studied in detail. The influence of the driving force and the ratio of the growth rates G100 and G111 on the morphology is examined. The preferential lateral growth of twinned Ag crystals is explained by a substep mechanism. The relation between vapor-grown tabular Ag crystals and tabular AgX (X = Cl, Br) grown from solution and from the vapor is discussed. The similarities in the growth mechanism give a new insight into the role the ionic character of AgX has in the twin formation.