Modelling of the thermodynamic properties of Bi-As2Te3 and Tl-As2Te3 ternary alloys

The quasi chemical approximation (QCA) which hitherto is used for the calculations of thermodynamic properties of binary alloys have been applied to the study of Bi-As2Te3 and Tl-As2Te3 ternary systems by assuming a pseudo binary configuration for the systems. The calculated values of the activity (am) and free energy of mixing Gm/RT show good agreement with experiment. The concentration fluctuation in the long wavelength limit Scc(0) and the Warren-Cowley short range order parameter for the first neighbour shell (α1) have been studied as a function of composition in the ternary alloys. The results indicate that the Tl-As2Te3 ternary alloy is more stable than the Bi-As2Te3 alloys. Scc(0) computed from the model are in good agreement with experimental values for the systems investigated and for both systems indicate that they are compound forming. In addition for the Bi-As2Te3, the concentration dependence of the concentration-concentration fluctuations shows a regular solution behaviour while that of Tl-As2Te3 manifests its highest tendency for compound formation in the range CTl > 0.5454. (orig.)