DISTORTION IN THE CDS1-YSEY LATTICES - AN EXAMPLE OF RELAXATION IN WURTZITE COMPOUNDS

Relaxation in pseudobinary wurtzite lattices is predicted using the va lence force field (VFF) approach. In this hexagonal packing structure, the computer simulation of the quasi-tetrahedral alloy lattice at an arbitrary composition is carried out by searching atomic positions at which the elastic forces acting upon every atom vanish. The VFF model is applied to the II-VI compound CdS1-ySey: the bimodal distribution o f the distances between first neighbouring atoms is split, taking into account the type of chalcogen atom bound to the Cd one and the distor tion of tetrahedra in the non-ideal wurtzite structure. The values of interatomic distances up to the second neighbours agree with the publi shed extended X-ray absorption fine-structure spectroscopy (EXAFS) dat a. The natural distortion (+/- 0.42-degrees for CdS and +/- 0.16-degre es for CdSe) around the regular tetrahedral angle value remains presen t in the alloy. The calculated mixing enthalpy follows a subregular la w: DELTA(M)H = y(1 - y) [2803 - (2y - 1) 125] J mol-1.