Thermodynamic Analysis of the HgTe ‐ CdTe ‐ Te System Using the Simplified RAS Model

Abstract

A quantitatively good overall fit is obtained to the phase diagram and thermodynamic data in the HgTe-CdTe-Te system. In the binaries the partial pressures along the three-phase curves are fit in addition to the liquidus points. Moreover, the liquid phase parameters are constrained so that the enthalpy and entropy of mixing of the stoichiometric binary melts are properly related to the enthalpy and entropy of formation of the binary compounds. Also included in the fit are the HgTe-CdTe pseudobinary liquidus and solidus points, the partial pressure of Hg in the pseudobinary melt, and the partial pressures of Hg, Cd, and Te2 for the Te-saturated pseudobinary solid, Hgl-uCduTe. Finally, a choice is made for the most reliable ternary liquidus points and these are included. Portions of the phase diagram, as yet unmeasured, are then calculated and presented. The liquid phase is described by a simple extension of the regular associated solution model of Jordan and Szapiro in which the interaction coefficients and dissociation constants are allowed to depend upon temperature. The partial molar quantities for the liquid phase components are derived and used in liquidus equations that are more exact than the commonly used version of the Vieland equation. Errors in some previous studies are discussed in the appendix. It is concluded that the model cannot provide a satisfactory fit to the entire Hg-Cd-Te system. Neither can it fit analogous systems in which the liquidus lines of the binary compounds are asymmetric.In this paper we gather the expe~mental data and report quantitatively good overall fits to all the phase diagram and thermodynamic data for the HgTe-CdTeTe system. Recently derived liquidus equations (1), more generally valid than the commonly used Vieland equation (2), are used and the liquid phase is described by the simplified version of the associated solution model (3,4). In the process this model is tested more extensively than previously. The liquid phase parameters leading to an overall optimum fit are used to generate calculated curves that are hopefully useful in smoothing, interpolating, and extrapolating the experimental data and guiding further experimentation. The associated solution model offers a relatively simple way of obtaining a strong composition dependence in the properties of a liquid phase. Such a dependence has been observed in a number of cases, various properties showing an extremum at or near compositions at which solid phases appear at lower temperatures (5, 6). The model has become very popular recently, and has been applied to a number of systems including HgTe-CdTe-Te (7), Zn-Cd-Te (4), a number of III-V binaries (8,9), and to A1-Ga-Sb (10), Unfortunately, the analyses cited are flawed to an unknown extent in that they take as their starting point liquidus equations arrived at by assuming the partial molar enthalpies and excess entropies of the liquid components are independent of T, an assumption that is generally invalid for associated solution models (1). Moreover, they ignore th...

Thermodynamic Analysis and Phase Equilibria Calculations for the Cd‐Te, Cd‐Se, and Cd‐S Systems

“…A comparison between our calculation and the experiment is shown in a later section of this paper. Tung et aL (40) and Brebrick et al (42) have obtained good agreements for the phase diagram and the thermodynamic data; however, in the present study we have used fewer parameters for describing the thermodynamic properties of the liquid phase.…”

“…Liquid phase.--An associated solution model is used to describe the thermodynamic properties of the liquid phases. The associated solution model has been widely used to describe the thermodynamic properties of the liq-uid phase in a number of metal-sulfur, II-VI, IV-VI, and III-V systems (35)(36)(37)(38)(39)(40)(41)(42). Schmid and Chang (43) have reviewed the main features of this model.…”