Thermodynamic study of the vaporization of europium monosulfide by Knudsen-cell mass spectrometry Atomization energies of EuS(g), Eu 2S(g), EuS 2(g), Eu 2O(g), Eu 2O 2(g), Eu 2OS(g), and Eu 2S 2(g)

Abstract

The vaporization of EuS(s) is studied in the temperature range 1550 to 2500 K. Mass-spectrometric analysis of the vapour leads to the characterization of the gaseous molecules EuS, EuS 2, Eu 2S, Eu 2S 2, EuOS, Eu 2S, Eu 2O 2, and probably Eu 2S 3 and Eu 3S 4. The main vaporization process is EuS(s) = Eu(g) + αS(g) + { (1 − α) 2 } S 2( g) S 2(g), which leads to congruent effusion. The congruently effusing composition varies slightly with temperature, the mole ratio n (Eu) n (EuS) being displaced from 1830 to 2260 K by (7.6±0.9) × 10 −4 towards the sulphur-rich side. EuS(s) exists within a homogeneity range which extends between EuS 1.027 at 1420 K and at least EuS 0.99 at 1935 K, when the congruently effusing composition at 2000 K is set at exact stoichiometry. Partial pressures determined by the mass-spectrometric Knudsen-effusion method are compared with the literature data. Experimental ionization cross-section ratios are determined for Eu(g), S(g) and S 2(g). The atomization energies are D at o(EuS, s, 0) = (217.4 ± 2.6) kcal th mol −1, D o(EuS, g, 0) = (85.7 ± 3.1) kcal th mol −1, D at o(EuS 2, g, 0) = (171.7 ± 5.0) kcal th mol −1, D at o(Eu 2S, g, 0) = (167.3 ± 5.0) kcal th mol −1, D at o(Eu 2S 2, g, 0) = 283.2 ± 5.0) kcal th mol −1, D at o(EuOS, g, 0) = (315.1 ± 5.0) kcal th mol −1, D at o(Eu 2O, g, 0) = (205.1 ± 5.0) kcal th mol −1, D at o(Eu 2O 2, g, 0) = (345.1 ± 5.0) kcal th mol −1, D at o(Eu 2S 3, g, 0) ⩽ (354.3 ± 10.0) kcal th mol −1, and D at o(Eu 2S 4, g, 0) ⩽ (452.7 ± 10.0) kcal th mol −1. The corresponding standard enthalpies of formation are also listed.