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 (a(m)) and free energy of mixing G(m)/RT show good agreement with experiment. The concentration fluctuation in the long wavelength limit S-cc(0) and the Warren-Cowley short range order parameter for the first neighbour shell (alpha(1)) have been studied as a function of composition in the ternary alloys. The results indicate that the TI-As2Te3 ternary alloy is more stable than the Bi-As2Te3 alloys, S-cc(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 C-Tl > 0.5454.