Pressures of Hg and Selenium over HgSe(c) from Optical Density Measurements

Abstract

The optical density of the vapors at 860°C that are in equilibrium with Hg-saturated, Se-saturated, and congruently subliming HgSe(c) in the range 450° to 816°C has been measured between 2000 and 7000 Å. No indication of the species HgSe(g) is found. Assuming the optical density is the sum of those obtained with pure Se and pure Hg, the partial pressure of Hg(g), pHg, and the total selenium pressure, PΣ, are determined along a substantial portion of the HgSe(c) three-phase lines. Hg-saturated HgSe(c) is in equilibrium with essentially pure Hg(l) up to 659°C and is at least 48.5 at. % Se. The total selenium pressure attains a maximum value of 0.62 atm for Se-saturated HgSe near 722°C. The value of pHg for congruently subliming HgSe(c) is close to that required by the value of PΣ and the published thermodynamics of selenium vapor. Between about 450° and 600°C the standard Gibbs free energies of formation for both Se-saturated and congruently subliming HgSe(c) are obtained and are the same within experimental error. Taking an average, ΔGf°[Hg(g)+½Se2(g) = HgSe(c)] = −41.92+42.40(10−3)T kcal/mole. Extrapolating to 300°K, we obtain ΔHf° = −10.8 kcal/mole and ΔSf° = −4.99 eu/mole for the formation of HgSe(c) from Hg(l) and Se(c).