Study of (molybdenum + tellurium) by Knudsen-effusion mass spectrometry

Abstract

Vaporization of (molybdenum + tellurium) alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te₂(g) and Te(g) over the two-phase field (Mo₃Te₄+MoTe₂) were determined in the temperature range T = 820 K to 950 K. The boundaries of the Mo₃Te₄ 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(Te₂) 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 Mo₃Te₄. Our previously published p (Te₂) and p(Te₂) over (Mo+Mo₃Te₄) (J. Chem. Thermodynamics1989, 21, 1183) were used to obtain revised enthalpy changes for the reactions: MoTev(s) = Mo(s)+(v/i)Te_i(g), where I = 1 or 2, and v = 1.10, the Mo-rich boundary composition of Mo₃Te₄ 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 Mo₃Te₄ phase) as well as of MoTe_1.89 (Mo-rich boundary of the α-modification of the MoTe₂ phase) were derived.