Anomalous Isomorphism and Mixed Crystals in the Systems NH4Cl:(Cu,Mn)

Anomalous mixed crystals are metastable objects consisting of randomly stacked blocks of several crystalline phases in mutual epitaxial relationships. This article reports the first systematic characterization of crystal growth, phase composition, epitaxial relationships, and imperfections of mixed host/guest NH4Cl:Me crystals forming in the two similar systems NH4Cl-MeCl2-H2O CONH3 (Me = Cu, Mn) with a wide range of NH4Cl/MeCl2 ratios. The normal growth rates of {100} faces of cubic NH4Cl:Me crystals can be described by Bliznakov's kink-blocking model in which lateral interactions between impurity particles is taken into account by using the Fowler-Guggenheim adsorption isotherm. Copper and manganese are incorporated into the crystal structure of ammonium chloride host crystals in significant amounts (up to 7 wt 96) in the form of epitaxially oriented intergrowths of (NH4)(2)MeCl4 center dot 2H(2)O, MeCl2 center dot CONH3, and CuCl2 center dot 2H(2)O guest phases. The presence of guest phases in the host crystal is controlled by epitaxial matching not only in the growth plane {100} but also in the perpendicular direction. The relative amount of different guest phases is primarily dictated by the NH4Cl/MeCl2 ratios in mother liquor. The coherent phase matching in the crystals produces strong mismatch stress that shows up via stress birefringence and relaxes by means of brittle and plastic deformations.