Univariant Three- and Four-Phase Vaporization Equilibria in the Ternary Mn-Te-O System: A High-Temperature Mass Spectrometric Study of Binary xMnO + (1 + x)TeO_2 with x ≥ 0.5

摘要

High-temperature mass spectrometric studies on xMnO + (1-x)TeO_2 samples with x = 0.50, 0.54, 0.57, 0.67, 0.75, and 0.79, were conducted to investigate the vaporization behavior of the MnO-TeO_2 binary system. Along with MnTeO_3, a phase of equimolar composition, another ternary phase Mn_3TeO_6 lying on the oxygenrich side of the MnO-TeO_2 binary line was generated during preparations as well as during isothermal dynamic effusion experiments. Experiments were also conducted by adding a known excess of MnO or Mn to aliquots from different samples to examine whether the resulting condensed phase-vapor phase equilibrium would preclude or at least retard the buildup of Mn_3TeO_6. On the contrary, these experiments many being characterized by high Te_2(g) evolution in the initial stages, yielded residues of MnTeO_3 + Mn_3tEO_6 + Mn_3TeO_6 or Mn_3O_4 + Mn_3TeO_6, the latter when the MnO addition was such that the resulting overall x(MnO) was as high as 0.80. The vapor phase in all cases consisted only of Te- and O-bearing species: TeO_2, TeO, Te_2, and O_2. These observations led to the inference that the MnO-rich region of the MnO + TeO_2 system is not inclined to remain binary during vaporization. Anomalous vaporization behavior was observed for Te_2(g) in some isothermal experiments. While for all other gaseous species monotonic decrease in partial pressures ended with values stabilizing around the respective minimum, that in the p(Te_2) ended with a turn-around in its value; there was a rapid increase to eventually stabilize at a value that was not only about four times higher than the minimum but also even higher than that measured before the onset of monotonic decrease. These features and other aspects associated with different univariant vaporization equilibria in the Mn-Te-O ternary system are discussed.