Phase relations in the system Cu-Gd-O

Abstract

The phase relations in the system Cu-Gd-O have been determined at 1273 K by X-ray diffraction, optical microscopy, and electron microprobe analysis of samples equilibrated in quartz ampules and in pure oxygen. Only one ternary compound, CuGd₂O₄, was found to be stable. The Gibbs free energy of formation of this compound has been measured using the solid-state cell Pt,Cu₂O+ CuGd₂O₄ +Gd₂O₃//(Y₂O₃) ZrO₂//CuO + Cu₂O, Pt in the temperature range of 900 to 1350 K. For the formation of CuGd₂O₄ from its binary component oxides, CuO (s) + Gd₂O₃ (s) → CuGd₂0₄ (s) ΔG° = 8230 - 11.2T(±50) J mol^-1. Since the formation is endothermic, CuGd₂O₄ becomes thermodynamically unstable with respect to CuO and Gd₂O₃ below 735 K. When the oxygen partial pressure over CuGd₂O₄ is lowered, it decomposes according to the reaction 4CuGd₂O₄ (s) ~ 4Gd₂O₃ (s) + 2Cu₂O (s) + O₂ (g) for which the equilibrium oxygen potential is given by Δμ _O2 = -227,970 + 143.2T (±500) J mol^-1. An oxygen potential diagram for the system Cu-Gd-O at 1273 K is presented.