Thermodynamic Properties of Solid Solutions in the System Ag2S Ag2Se

Thermodynamic Properties of Solid Solutions in the System Ag2S – Ag2Se

1. Introduction

This paper is about the calculation of standard thermodynamic properties of the four solid solutions in the phase diagram of Ag2S – Ag2Se. They calculated these properties using the model of regular and subregular solutions. The four solid solutions are: a restricted fcc solid solution (γ- Ag2S-Ag2S1-xSe (x<0.3)), a complete bcc solid solution (β- Ag2S – Ag2Se), monoclinic solid solution (α) from Ag2S to Ag2S0.4Se0.6, and a orthorhombic solid solution (α) from Ag2S0.3Se0.7 to Ag2Se. There is also a miscibility gap between Ag2S0.4Se0.6 and Ag2S0.3Se0.7. In this region Gmix and Smix have been calculated using subregular model for asymmetric solutions.

2. Experimental information:

The experimental methods used by them for the study of solid Fig.1. Phase diagram of Ag2S – Ag2Se

solutions of Ag2S – Ag2Se are diffraction techniques, density

measurements methods, and the temperatures of phase transitions. Fig. 1 is a typical phase diagram of

Ag2S – Ag2Se system based on experimental data from many journals. According to this phase diagram the transition temperature for Ag2S is 176°C, and for Ag2Se it is 128°C. The melting temperature for Ag2S is 837°C and for Ag2Se it is 897°C. Pingitore et al. and Ponce studied this system using optical microscopy, electron- probe microanalysis and X – ray diffraction and they found that there are two solid solutions at ambient conditions. One series is monoclinic which is from Ag2S to Ag2S0.4Se0.6 and the other series is orthorhombic which is from Ag2S0.3Se0.7 to Ag2Se.

3. Model Description:

The models used for the evaluation of standard thermodynamic properties of Ag2S – Ag2Se system are regular and subregular for solid solutions. At the high temperature the system is described by phase diagrams with a common minimum point on solidus and liquidus curves. In this region the standard thermodynamic properties are evaluated by using subregular solution model. While at low temperature region where there are two continuous solid solutions (monoclinic and orthorhombic) with a miscibility gap, the standard thermodynamic properties are evaluated by using both models of regular and subregular solutions.