Thermodynamic properties and phase diagram for the system MoO₂–TiO₂

Abstract

The activity of molybdenum dioxide (MoO₂) in the MoO₂–TiO₂ solid solutions was measured at 1600 K using a solid-state cell incorporating yttria-doped thoria as the electrolyte. For two compositions, the emf was also measured as a function of temperature. The cell was designed such that the emf is directly related to the activity of MoO₂ in the solid solution. The results show monotonic variation of activity with composition, suggesting a complete range of solid solutions between the end members and the occurrence of MoO₂ with a tetragonal structure at 1600 K. A large positive deviation from Raoult's law was found. Excess Gibbs energy of mixing is an asymmetric function of composition and can be represented by the subregular solution model of Hardy as follows ΔG^E(J/mol^-1)=X_TiO2X_MoO2(19 450X_TiO2+28 840X_MoO2) The temperature dependence of the emf for two compositions is reasonably consistent with ideal entropy of mixing. A miscibility gap is indicated at a lower temperature with the critical point characterized by T^c(K)=1560 and X^c_TiO2=0.38. Recent studies indicate that MoO₂ undergoes a transition from a monoclinic to tetragonal structure at 1533 K with a transition entropy of 9.91 J·(mol·K)⁻¹. The solid solubility of TiO₂ with rutile structure in MoO₂ with a monoclinic structure is negligible. These features give rise to a eutectoid reaction at 1412 K. The topology of the computed phase diagram differs significantly from that suggested by Pejryd.