Phase stability in the La---Ba---Cu---O system at 980° C and 200 bar oxygen pressure

Abstract

The LaO1.5---BaO---CuO pseudo-ternary phase diagram at 200 bar oxygen pressure was explored by a solid-state reaction at 980° C of numerous compositions. The phase which formed have been identified by X-ray diffraction. Significant differences from phases formed in air are noticed. BaCuO₂ and Ba₂CuO₃ arc absent, Ba₂Cu₃O_5+δ is stabilized. La₂CuO₄ remains stable, in addition LaCuO₃ is formed in high pressure. Both La₂CuO₄ and LaCuO₃ show an extended solubility with Ba²⁺ replacing La³⁺. The following phases do not form in a high P(O₂): LaBa₂Cu₃O₇, LaBa₂Cu₄O₈, La₄BaCu₅O₁₃, La₃Ba₃Cu₆O₁₀, La₂BaCu₂O₆ and La₂BaCuO₅. This is possibly because LaCuO₃ is a highly oxidized phase (FCV ≈ 3.0), which is therefore strongly favored at elevated P(O₂) of 200 bar. Tie-lines have been worked out and a tentative phase diagram is presented.