X-ray investigation of the mechanism of phase transformation in single crystals of ZnS, Zn_xCd_1-xS and Zn_xMn_1-xS (I). calculation of diffraction effects by a three parameter model

Abstract

Hexagonal close-packed (2H) single crystals of ZnS, Zn_xCd_1-xS are known to undergo solid state transformation to the cubic close-packed (3C) and 6H-structures on annealing at elevated temperatures. The transformations occur by the non-random nucleation of stacking faults on individual close-packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation.It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2-layer separations. The probability corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X-ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations.