Growth Spirals on NiBr2 Platelets

Abstract
Platelets of NiBr2 have been grown by sublimation and crystallization in an atmosphere of water vapor and helium. Several crystal habits were observed, including dendrites, platelets, and whiskers. Platelets and dendrites grow in the region of substrate temperatures greater than 500°C and although these crystals are anhydrous, water vapor greatly enhances the growth rate. The crystals are deliquescent. In the early stages of water adsorption the surface is not wetted, but water droplets form at surface imperfections. The resultant decoration of surface morphology exhibits an anisotropy related to the surface structure. Photomicrographs are presented of concentric growth spirals on the isolated faces of the same crystal platelet. These structures are demonstrated via direct observation of the opposite sides of the crystal, superposition of the opposite sides in thin regions of growth, and with the aid of Fizeau interferograms.

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