GROWTH OF POLYTYPIC CRYSTALS CdI₂, PbI₂ AND STUDY OF LASER INDUCED PHASE TRANSITIONS USING X-RAY DIFFRACTION. S.K. Chaudhary, University College, U.D. University, Rohtak - 124 OO1 India

The single crystals of CdI₂ and PbI₂ are characterised by a marked polytypism i.e. ability of a substance to crystallize in various polytypic modifications. The crystal lattice of these polytypes is formed by structurally identical, yet translationally non-equivalent layer packets. Each layer packet forms a sandwich structure that consists of three monolayers of atoms I-Pb-I, I-Cd-I. The bonding within a sandwich is largely ionic while that between various sandwiches is predominantly Van-der Waals.

For growth of these crystals a zone-refining set up was fabricated by us. The process of purification and growth was carried out in the argon atmosphere. The purity of the crystals was checked using the atomic absorption technique. The XRD oscillation photographs were employed for structure determination. Ruby laser was used to irradiate the crystal at room temperature. The development of cracks and phase transitions in the above crystals when exposed to laser beam have been discussed on the basis of structural consideration of CdI_2.