LATTICE DISTORTION AND FERROELECTRICITY IN IV-VI AND II-VI SEMICONDUCTOR ALLOYS

Using the x-ray absorption fine structure (EXAFS). we have investigate d the local structure of disordered PbSxTe1-x (x = 0.10, 0.18, and 0.3 0) alloys at temperatures ranging from 10 K to 300 K, and ZnxCd1-xTe ( x = 0.10 and 0.20) alloys at temperatures ranging from 77 K to 250-deg rees-C. In PbSxTe1-x alloys, the sulphur ions are found to be off-cent er in both the high and the low temperature phase and Pb sublattice an d Te sublattice shift relative to one another below the transition tem perature. In ZnxCd1-xTe alloys the zinc ions are found in a slightly a symmetric site in the cages formed by four NN Te ions and no evidence of a relative phase transition is observed. We discuss our results con cerning the ferroelectric phase transitions (FPT) induced by off-cente r ions in these two systems.