PHASE-EQUILIBRIA OF THE MGO-CU2O-CUO SYSTEM

Phase equilibria of the MgO-Cu2O-CuO system have been determined in ai r at temperatures ranging from 850 to 1100 degrees C. The starting com positions ranged from 0 to 100% Mg in (Mg1-xCux)O. CuO was quite solub le in MgO. The solubility of CuO in MgO at 850 degrees C was about 9% and at 1050 degrees C was about 20%. The only known ternary Guggenite phase had a composition of Cu2MgO3. The Guggenite phase exists in thre e forms, namely, Guggenite A (G(A), orthorhombic, space group Pmmn(59) ), Guggenite B (G(B), orthorhombic, space group I(0)), and Guggenite X (G(X), monoclinic). Guggenite A was obtained as a single phase in air at x(CuO) = 67% (i.e., 67% CuO and 33% MgO) and 1000 degrees C. It un dergoes a phase transition to the Guggenite X phase when it is heated above 1050 degrees C. The Guggenite B phase was obtained in a mixture at x(CuO) = 75% and at temperatures above 1050 degrees C. MgO was not found to be soluble in CuO. For samples with x(CuO) greater than or eq ual to 67%, the Guggenite A phase coexisted with tenorite (CuO, monocl inic) at temperatures less than or equal to 1000 degrees C and above 1 021 degrees C, it was in equilibrium with cuprite (Cu2O, cubic). For C uO compositions between 20% and 67%, a mixture of Guggenite A phase an d the periclase phase (MgO, cubic) was obtained at temperatures less t han or equal to 1000 degrees C. The decomposition of Cu2MgO3 (Guggenit e A) phase was determined at several oxygen partial pressures and the Gibbs free energy of formation of this phase was also calculated. Copy right (C) 1997 Elsevier Science Ltd.