Phase equilibria of the MgO-Cu2O-CuO system

Phase equilibria of the MgO-Cu2O-CuO system have been determined in air at temperatures ranging from 850 to 1100 °C. The starting compositions ranged from 0 to 100% Mg in (Mg1 − xCux)O. CuO was quite soluble in MgO. The solubility of CuO in MgO at 850 °C was about 9% and at 1050 °C was about 20%. The only known ternary Guggenite phase had a composition of Cu2MgO3. The Guggenite phase exists in three forms, namely, Guggenite A (GA, orthorhombic, space group Pmmn(59)), Guggenite B (GB, orthorhombic, space group I(0)), and Guggenite X (Gx, monoclinic). Guggenite A was obtained as a single phase in air at XCuO = 67% (i.e., 67% CuO and 33% MgO) and 1000 °C. It undergoes a phase transition to the Guggenite X phase when it is heated above 1050 °C. The Guggenite B phase was obtained in a mixture at XCuO = 75% and at temperatures above 1050 °C. MgO was not found to be soluble in CuO. For samples with xcuo ≥ 67%, the Guggenite A phase coexisted with tenorite (CuO, monoclinic) at temperatures ≤ 1000 °C and above 1021 °C, it was in equilibrium with cuprite (Cu2O, cubic). For CuO compositions between 20% and 67%, a mixture of Guggenite A phase and the periclase phase (MgO, cubic) was obtained at temperatures ≤ 1000 °C. The decomposition of Cu2MgO3 (Guggenite A) phase was determined at several oxygen partial pressures and the Gibbs free energy of formation of this phase was also calculated.