Investigations of the thermodynamic properties of zinc chalcogenides

Abstract

The standard free energies of formation of zinc chalcogenides (ZnTe, ZnSe and α-ZnS) have been determined by an electrochemical technique using LiCl-KCl + 5 wt.% ZnCl2 electrolyte. The standard enthalpies of formation of ZnTe and ZnSe, and the enthalpy of fusion of ZnTe have been measured directly in a DTA calorimeter. Good agreement has been observed between the enthalpies of formation obtained by these two independent techniques. The thermodynamic behaviour of these compounds at their melting point has been discussed by using the enthalpy of fusion of ZnSe and ZnS reported in the literature and that of ZnTe determined in this investigation. The lattice constants of these compounds have been calculated from the tetrahedral covalent, ionic and rationalised (after Phillips) radii of the component elements, and compared with those determined by X-ray diffraction technique. The estimated values of the enthalpy of formation (based on Pauling's and Phillips' methods) and the standard entropy (based on Latimer's method) have been compared with the experimental values and discussed in the light of the structure and bonding of the compounds. Detailed analyses of the thermodynamic and structural data reveal that the bonding in zinc chalcogenides is neither predominantly covalent nor ionic. © 1992.