THERMODYNAMIC INVESTIGATIONS OF TE-SATURATED SOLID CDSE-CDTE ALLOYS

The activity of CdTe in Te-saturated solid CdSe-CdTe alloys in the temperature range 720 to 840 K was measured by an electrochemical technique using LiCl-KCI + 5 wt pct CdCl2 as the molten salt electrolyte. The system exhibits positive deviation from Raoult's law. From the electromotive force (emf) values measured at different temperatures, the partial, integral, and excess molar thermodynamic quantities have been calculated. All the excess partial quantities at infinite dilution have been accurately estimated by Chiotti's method. The composition dependence of the alpha-function confirms that the system does not follow a regular solution model. The ln-gamma(CdTe) vs (1 - x(CdTe))2 plot clearly demonstrates that the system consists of two terminal regions: (a) 0 less-than-or-equal-to x(CdTe) less-than-or-equal-to 0.35 and (b) 0.45 less-than-or-equal-to x(CdTe) less-than-or-equal-to 1.0, in which the thermodynamic behavior is relatively simple. The two terminal regions are connected by a narrow transient or central region (0.35 less-than-or-equal-to x(CdTe) less-than-or-equal-to 0.45), in which thermodynamic behavior appears to be complicated. Pronounced changes in activity coefficient, stability, excess stability, relative stability, and relative excess stability in the central region correspond to the two-phase region in the phase diagram of the system. X-ray diffraction analysis of samples at and near x(CdTe) = 0.4 presents no evidence of formation of an intermetallic phase or intermetallic compound.