Structural disorder and solid state transformations in single crystals of Zn_xCd_1-xS and Zn_xMn_1-xS

Abstract

Single crystals of Zn_xCd_1-xS and Zn_xMn_1-x S were grown from the vapour phase at 1100°C in the range x=0.9 to 1. X-ray characterization shows that polytypes and disordered structures occur in Zn_xCd_1-xS for x ≥ 0·94, whereas Zn_xMn_1-xS displays disordered and polytype structures in the entire rangex=0·9 to 1. It is observed that Zn_x Cd_1-xS and Zn_xMn_1-x S undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β » α approximates the actual mechanism of transformation in these crystals.