Partial Pressures in Equilibrium with Group IV Tellurides. II. Tin Telluride

Abstract

The partial pressures of SnTe(g) and Te₂(g) in equilibrium with SnTe(c) have been measured as a function of the temperature and composition of the solid phase by an optical absorption method. At 806°C, the maximum melting point of SnTe(c), the partial pressures of SnTe(g) and Te₂(g) are 0.86 and 0.3 Torr, respectively. Between 727° and 806°C, the partial pressure of SnTe(g) is given by $\log \mathrm{\hspace{0.17em}p\hspace{0.17em}(}\mathrm{Torr}{\text{)=−[(10.19±0.26)×10}}^3{\mathrm{]/T}}^{\circ }\mathrm{K}\text{+(9.38±0.26)}$ , independent of the composition of SnTe(c) within our experimental error of about 3%. The difference between the standard free energies of formation of SnTe(c) and SnTe(g) from Sn(g) and Te₂(g) is therefore ${\mathrm{\Delta G}}^{\circ }[\frac{1}{2}\hspace{0.17em}\mathrm{SnTe\; (c)}{\mathrm{]-\Delta G}}^{\circ }[\frac{1}{2}\hspace{0.17em}\mathrm{SnTe\; (g)}{\text{]=−23.31+14.87\hspace{0.17em}(10}}^{\text{−3}}\mathrm{)T\hspace{0.17em}}\mathrm{kcal}/\mathrm{g-atom}$ . Combining this result with the value of ΔG°[½ SnTe(c)] obtained from the electrochemical data of McAteer and Seltz gives ${\mathrm{\Delta G}}^{\circ }[\frac{1}{2}\hspace{0.17em}\mathrm{SnTe\; (g)}{\text{]=−27.34+6.81\hspace{0.17em}(10}}^{\text{−3}}\mathrm{)T\hspace{0.17em}}\mathrm{kcal/g-atom}$ . The partial pressure of Te₂(g), which varies strongly with the composition of SnTe(c), has been determined for samples of seven different compositions within the SnTe(c) solidus field. Solidus temperatures in good agreement with those found by an independent method are obtained from the results. These results have also been used to obtain relative partial molal enthalpies of Sn and Te in SnTe(c) as a function of temperature and composition, and in addition they are discussed in terms of the disorder in SnTe(c).