Knudsen effusion measurements have been carried out with stoichiometric MnSe over a temperature range from 1414°–1628°K, and a pressure range from 10−6−10− 4 atm. Two differently prepared samples and two different molybdenum effusion cells were used in four independent runs, representing a total of 40 vapor pressure measurements. The results are concordant and demonstrate that MnSe vaporizes congruently in this range. The calculational technique for the evaluation of the Knudsen data is based on the general reaction MnSe(s) = Mn(g) + 12(1 − α)Se2(g) + αSe(g). The calculated mean value for the degree of dissociation α is 0.45 and for the third-law heat of reaction 140.6 ± 2.5 kcal/mole at 298°K, in good agreement with the corresponding second-law value of 143.2 ± 3 kcal/mole. The heat of formation of MnSe(s) is found to be −37.4 ± 2.5 kcal/mole.

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