Growth mechanism of vapor-grown silver crystals: Relation between twin formation and morphology

This study deals with the morphology and growth mechanism of multiply twinn ed Ag crystals grown from the vapor phase. Ag crystals were grown under sub limation conditions for small growth rates as a function of the driving for ce and sublimation temperature. At relatively low driving force and low tem perature cubooctahedrons and single twinned crystals appeared. At higher dr iving forces also {111} tabular crystals (containing multiple parallel twin planes) and 5-fold twinned structures resulting in decahedrons, icosahedro ns, and elongated decahedrons bounded by {100} sides faces, were observed t ogether with other nonparallel twinned crystals. Increasing the driving for ce further led to tabular crystals with higher aspect ratios and different side-face structures; the preferential unidirectional growth of the elongat ed decahedrons was also increased. The growth mechanism of {111} tabular Ag crystals and other multiply twinned Ag crystals is for the first time stud ied in detail. The influence of the driving force and the ratio of the grow th rates G{100} and G{111} 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 = C l, Br) grown from solution and from the vapor is discussed. The similaritie s in the growth mechanism give a new insight into the role the ionic charac ter of AgX has in the twin formation.