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I.

INTRODUCTION

CHALCOGENIDES of group IIB elements are important semiconductors. In recent years, they have gained prominence for their applications in various electronic devices, such as photovoltaic, X-ray, and y-ray detectors, electro-optic and acousto-optic modulators, photoresistors, partial reflectors, light emitting diodes, fiber optics, etc. Although these binary chalcogenides possess useful semiconducting properties, they offer limited and fixed values of electronic parameters, viz., energy gap, electrical, thermal, and photoconductivities, dielectric and elastic constants, thermal expansion coefficient, etc. These limitations can be overcome and electronic materials can be designed and tailor-made to suit specific property requirements by alloying. Pseudobinary alloy systems between group liB chalcogenides offering wide selective ranges have thus attracted considerable attention. The electronic properties of CdTe, the most important semiconductor, can be adjusted according to requirements by alloying with other compounds, e.g., CdSe, HgTe, ZnTe, etc. In recent years, CdSe-CdTe mixtures have found challenging applications in thin film transistors tll and high efficient solar cells, t2,31 During the last few years, extensive research has been directed toward the development of device applications by studying electron transport properties of CdTexSet_x solid solutions. However, their thermodynamic properties have not been investigated so far. In order to adjust the semiconducting characteristics by varying the alloy composition, it is important to know the phase diagram and thermodynamic properties of the system throughout the entire range of composition. M. SHAMSUDDIN, Professor of Metallurgical Engineering, and A. NASAR, Research Associate, are with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India. Manuscript submitted April 5, 1991. METALLURGICAL TRANSACTIONS B

The equilibrium phase diagram of CdSe-CdTe system shown in Figure 1 has been adapted from Strauss and Steininger. t~l It consists of two broad single-phase regions, one of solid solutions with wurtzite structure of CdSe and the other of solid solutions with zinc blende structure of CdTe, separated by a very narrow two-phase region (only about 3 mol pct wide) resulting from a eutectic reaction at 1364 --- 1 K near 80 mol pct CdTe. Tai et al. tSJ have reported that the two-phase region is affected by time and temperature of annealing of samples. Thermodynamic data on CdTe [61 and CdSe I71 have recently been published. In view of the lack of thermodynamic data on CdSe-CdTe solid solutions, this system has been investigated. In the present article, the activity of CdTe in Te-saturated solid CdSe-CdTe alloys has been determined by an electrochemical technique using molten salt electrolyte. Various partial integral and excess thermodynamic quantities have been calculated from activity values. The results have been discussed in light of the phase diagram and Darken's stability and