Phase Diagram for the System RuO2-TiO2 in Air

Abstract

There are conflicting reports in the literature regarding solid solubility in the system RuO2-TiO2. To resolve this issue a few experiments were conducted in air at 1673, 1723, and 1773 K. The results show limited terminal solid solubility. There is an extended solid-state miscibility gap that intersects the decomposition curve for the RuO2-rich solid solution generating a peritectoid reaction at 1698 K. The measured equilibrium compositions of the solid solutions are used to develop a thermodynamic description of the oxide solid solution with rutile structure. Using the subregular solution model, the enthalpy of mixing can be represented by the expression, \( \Delta H^{\rm M} /{\text{J}} \cdot {\text{mol}}^{{ - 1}} = X_{{{\text{TiO}}_{2} }} X_{{{\text{RuO}}_{2} }} {\left( {34,100X_{{{\text{TiO}}_{2} }} + 30,750X_{{{\text{RuO}}_{2} }} } \right)} \). The binodal and spinodal curves and T-X phase diagram in air are computed using this datum and Gibbs energy of formation of RuO2 available in the literature. The computed results suggest that equilibrium was not attained during solubility measurements at lower temperatures reported in the literature.