Partial Pressures over CdSe (c) from Optical Density Measurements and Optical Density of the 228.7 nm Cd (g) Peak
High Temperatures and Materials Science
The optical density of the 228.7 nm Cd(g) line was measured for cadmium pressures between 10-6 and 10-3 atm, optical path temperatures of 500 and 1100 C, and spectrophotometer band passes of 0.121, 0.235, and 0.334 nm. The optical density at the peak and along the line profile were fit well, assuming a triangular slit function and a Voigt line profile and using an oscillator strength of 1.50 and a self-broadening coefficient 5.1 x 106P/T3/2, where P is the cadmium pressure and T is the temperature of the optical path in K. Then the optical density of the vapor over CdSe(c) was measured with an optical path at 1100 C and found to become almost independent of sample temperature at the highest sample temperatures, indicating complete vaporization. Combining the above with previous measurements of the vapor over Se(l), the partial pressures of Cd and Se2 were obtained. Within experimental error the ratio of the former to the latter is two at the highest temperatures. The Gibbs energy of formation from the gases was calculated and compared to previous measurements. Then the Gibbs energy of formation from the liquid elements was calculated and, along with published values from emf measurements, subjected to a third-law analysis in which the high temperature heat capacity of CdSe(c) was taken from recent measurements. The best overall fit to the Gibbs energies, in the authors` opinion, is given by a standard enthalpy and entropy of formation at 298 K of, respectively, -144.404 kJ/mol and -8.00 J/mol-K. These lead to Gibbs energies 2--4 kJ/mol more negative than the previously selected values.
Brebrick, Robert F., "Partial Pressures over CdSe (c) from Optical Density Measurements and Optical Density of the 228.7 nm Cd (g) Peak" (1997). Mechanical Engineering Faculty Research and Publications. 284.