Vaporization Thermodynamic Studies by High-Temperature Mass Spectrometry on Some Three-Phase Regions over the MnO−TeO₂ Binary Line in the Mn−Te−O Ternary System

Knudsen effusion mass spectrometric measurements have been performed in the temperature range of 850−950 K over four three-phase mixtures, each phase mixture having at least one phase lying on the MnO−TeO₂ binary line of the Mn−Te−O phase diagram, and the rest of the phases lying above this binary line. The three-phase mixtures investigated are Mn₃O₄ + MnO + Mn₆Te₅O₁₆; Mn₃O₄ + Mn₆Te₅O₁₆ + MnTeO₃; Mn₃O₄ + Mn₃TeO₆ + MnTeO₃; and Mn₃TeO₆ + MnTeO₃ + Mn₂Te₃O₈. The vapor pressures of the gaseous species TeO₂, TeO, and Te₂ over these three-phase mixtures were measured, and various heterogeneous solid−gas reactions were evaluated along with the homogeneous gas-phase reaction TeO₂(g) + 0.5Te₂(g) = 2 TeO(g). The enthalpy and Gibbs free energy of formation of the four ternary Mn−Te−O phases were deduced at T = 900 K. These values (in kJ·mol^-1), along with the estimated uncertainties in them are Δ_f = 4150 ± 19, 752 ± 11, 1710 ± 11, 1924 ± 40, and Δ_f = 2835 ± 28, 511 ± 11, 1254 ± 19, 1238 ± 38, for Mn₆Te₅O₁₆, MnTeO₃, Mn₃TeO₆, and Mn₂Te₃O₈, respectively. A thermochemical assessment was made to examine the conditions under which the ternary Mn−Te−O phases could be formed on a stainless steel clad of mixed-oxide-fuelled (MO₂; M = U + Pu) fast breeder nuclear reactors. The phase Mn₃TeO₆ could be formed when the fuel is even slightly hyperstoichiometric (O/M = 2.0002) and the phase Mn₆Te₅O₁₆ could also be formed when O/M = 2.0004. The threshold tellurium potential for the formation of Mn₃TeO₆ is higher than that for MnTe_0.80 and CrTe_1.10, but is comparable to that for MoTe_1.10, and even lower than that for FeTe_0.81 or NiTe_0.63.