Study of cracks and phase transitions in cadmium iodide crystals using X-Ray diffraction

Polytypism has been observed in a large number of materials where the nearest. Neighbour relationship between identical two-dimensional layers of atoms can be satisfied in more than one way. The phenomenon has posed interesting problem for the Scientists, since the nature of force that causes ordering over the scale ranging from few angstrom to few thousands of angstrom units is not known. The Cadmium iodide has been purified and single crystal grown using a zone-refining system. The frequent appearance of cracks in the grown crystals when exposed to N2 laser at room temperature has been explained on the structural considerations of CdI2. The crystals of cadmium iodide have also been grown from solution in two batches using unpurified and purified material. A comparative study has been made on these batches for polytypism using x-ray diffraction technique. Formation of small period polytype 2H is governed by both temperature and impurities contained in the starting material. 4H is the most stable polytype. Higher occurrence of unidentified polytype in crystals of purified material has been attributed to free movement of edge dislocations during growth. The results have been examined, keeping in view the empirical considerations of earlier investigations.