Gibbs energy of formation of $CaCu_3Ti_4O_{12}$ and phase relations in the system $CaO-CuO/Cu_2O-TiO_2$

The standard Gibbs energy of formation of CaCu3Ti4O12 (CCTO) from $CaTiO_3$, CuO and $TiO_2$ has been determined as a function of temperature from 925 to 1350 K using a solid-state electrochemical cell with yttria-stabilized zirconia as the solid electrolyte. Combining this result with information in the literature on $CaTiO_3$, the standard Gibbs energy of formation of CCTO from its component binary oxides, CaO, CuO and $TiO_2$ has been obtained: Delta G(f,ox)(o) $(CaCu_3Ti_4O_{12})/J mol^{-1}$ (+/- 600) = -125231 + 6.57 (T/K). The oxygen chemical potential corresponding to the reduction of CCTO to $CaTiO_3$, $TiO_2$ and $Cu_2O$ has been calculated from the electrochemical measurements as a function of temperature and compared on an Ellingham diagram with those for the reduction of CuO to $Cu_2O$ and $Cu_2O$ to Cu. The oxygen partial pressures corresponding to the reduction reactions at any chosen temperature can be read using the nomograms provided on either side of the diagram. The effect of the oxygen partial pressure on phase relations in the pseudo-ternary system $CaO-CuO/Cu_2O-TiO_2$ at 1273 K has been evaluated. The phase diagrams allow identification of secondary phases that may form during the synthesis of the CCTO under equilibrium conditions. The secondary phases may have a significant effect on the extrinsic component of the colossal dielectric response of CCTO