Study of (molybdenum + tellurium) by Knudsen-effusion mass spectrometry
Vaporization of (molybdenum + tellurium) alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te 2 (g) and Te(g) over the two-phase field (Mo 3 Te 4 +MoTe 2 ) were determined in the temperature range T = 820 K to 950 K. The boundaries of the Mo 3 Te 4 phase were delineated by a continuous monitoring of the intensity of Te + 2 as a function of time, starting with samples having mass fractions 0.67187 and 0.68997 of Te. The phase-boundary compositions correspond to MoTe 1.10+/-0.03 (Mo-rich boundary) and MoTe 1.30+/-0.04 (Te-rich boundary), respectively. By employing the p(Te 2 ) determined as a function of composition, activities of Te were obtained; and by a Gibbs-Duhem integration, those of Mo were also computed at various compositions within the single-phase Mo 3 Te 4 . Our previously published p (Te 2 ) and p(Te 2 ) over (Mo+Mo 3 Te 4 ) (J. Chem. Thermodynamics1989, 21, 1183) were used to obtain revised enthalpy changes for the reactions: MoTe v (s) = Mo(s)+(v/i)Te i (g), where i = 1 or 2, and v = 1.10, the Mo-rich boundary composition of Mo 3 Te 4 determined in this work. The standard molar enthalpies of formation of MoTe 1.10 and MoTe 1.30 (Mo- and Te-rich boundaries of the Mo 3 Te 4 phase) as well as of MoTe 1.89 (Mo-rich boundary of the α-modification of the MoTe 2 phase) were derived.