Abstract
We have observed that if cesium halides are growing from small, isolated, aqueous solution droplets, often well faceted, cubic shape crystals are encountered. These cubes, bounded by {1 0 0} faces, are not stable and after a short period they undergo an instantaneous phase transition and turn rough and grainy. The metastable cubes are a polymorph of cesium halide, most likely with the fcc sodium chloride structure. Upon crystallization of cesium halides from saturated formamide solutions, apart from a number of the above-mentioned metastable cubes, all the other crystals also exhibit a cubic shape. Again they are bounded by the {1 0 0} faces, but they did not undergo the instantaneous phase transition. X-ray diffraction showed the stable cubes to be the stable polymorph of cesium halide having the primitive cubic structure. Surface topography of the {1 0 0} faces using atomic force microscopy showed that the stable cubes grow by steps of d100 in height and that the surface is not reconstructed. The stabilization of the polar {1 0 0} faces is explained, by assuming that the crystal terminates in a Cs+ layer, by a strong interaction of the polar formamide solvent molecules with the Cs+ ions at the top of the crystal surface.