MODELING 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 ap plied to the study of Bi-As2Te3 and Tl-As2Te3 ternary systems by assum ing a pseudo binary configuration for the systems. The calculated Valu es of the activity (a(m)) and free energy of mixing G(m)/RT show good agreement with experiment. The concentration fluctuation in the long w avelength limit S-cc(0) and the Warren-Cowley short range order parame ter for the first neighbour shell (alpha(1)) have been studied as a fu nction 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 experiment al values for the systems investigated and for both systems indicate t hat they are compound forming. In addition for the Bi-As2Te3, the conc entration dependence of the concentration-concentration fluctuations s hows a regular solution behaviour while that of Tl-As2Te3 manifests it s highest tendency for compound formation in the range C-Tl > 0.5454.