Gibbs energy of formation of $CuCrO_4$ and phase relations in the system Cu-Cr-O below 735 K

A solid state galvanic cell incorporating yttria-stabilized zirconia electrolyte and ruthenium(IV) oxide electrodes has been used to measure the equilibrium chemical potential of oxygen corresponding to the decomposition of $CuCrO_4$ in the range 590–760 K. For the reaction CuO(tenorite) + $CuCr_2O_4$(spinel) + $1.5O_2(g)\rightarrow 2CuCrO_4$(orth), $\Delta G^{XXX}$ = -183540 + 249.6T(\pm900) J $mol^{-1}$. The decomposition temperature of $CuCrO_4$ in pure oxygen at a pressure of $1.01 \times 10^5$ Pa is 735(\pm1) K. By combining the results obtained in this study with data on the Gibbs energy of formation of $CuCr_2O_4$ and $CuCrO_2$ reported earlier, the standard Gibbs energy of formation of $CuCrO_4$ and the phase relations in the system Cu-Cr-O at temperatures below 735 K have been deduced. Electron microscopic studies have indicated that the decomposition of $CuCrO_4$ to $CuCr_2O_4$ is topotactic