Thermodynamic properties of the molten Tl-As₂Te₃ and Bi-As₂Te₃ systems

The thermodynamic properties of the molten M-As₂Te₃ (M:Tl,Bi) systems have been measured by means of the electromotive force (EMF) method. The partial molar entropy and enthalpy of Tl in the molten Tl-As₂Te₃ system show a very drastic change at a composition of 54.54 at.% Tl, where the system shows a p-n transition. On the other hand, the activities of Bi show no such drastic change in the molten Bi-As₂Te₃ system, where the p-n transition and subsequent metallization occur at a much lower Bi concentration. However, the EMF data for the Bi-containing system were not accurate enough to separate the entropy and enthalpy contributions from the total free energy change. The present results indicate that strong interactions between Tl and Te in the Tl-As₂Te₃ system are responsible for the preservation of p-type semiconduction in the molten state.