Phase equilibria in the system CoO–MnO–TiO2–Co

The phase diagram of the oxide system TiO2–MnO–CoO in equilibrium with metallic cobalt at variable temperature and oxygen pressure has been constructed using the static method of investigation of heterogeneous equilibria and X-ray diffraction analysis. Three continuous solid–solution series: and with the structures of NaCl, ilmenite and spinel, respectively, have been found. The concentration dependencies of equilibrium partial oxygen pressure on the low-oxygen boundaries of homogeneity regions of solid solutions and have been measured at 1073, 1173 and 1273 K. The equilibrium compositions of coexisting phases have been defined in each equilibrium study. These complex data permit us to present P–T–X diagrams of the system CoO–MnO–TiO2–Co either as three isothermal sections (1073, 1173 and 1273 K) or as series of isobar–isothermal sections. Using an example of the solid solution we showed, that our data can be used for the calculation of concentration dependencies for component activities of solid solutions at various temperatures. We have applied the method based on solving the Gibbs–Duhem equations for each solid phase, that participates in equilibrium, and equations of laws of mass action for all possible independent chemical reactions between system components at equilibrium conditions.