American Chemical Society
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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

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journal contribution
posted on 2006-12-28, 00:00 authored by T. S. Lakshmi Narasimhan, M. Sai Baba, R. Viswanathan
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 Δf = 4150 ± 19, 752 ± 11, 1710 ± 11, 1924 ± 40, and Δf = 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 MnTe0.80 and CrTe1.10, but is comparable to that for MoTe1.10, and even lower than that for FeTe0.81 or NiTe0.63.