Reaction of La₂CuO₄ with binary metal oxides in the solid state: metathesis, addition, and redox metathesis pathways

Abstract

Investigation of the reaction of La₂Cu_O4 with several binary metal oxides in the solid state at elevated temperatures has revealed three different reaction pathways. Reaction of La₂CuO₄ with strongly acidic oxides such as Re₂O₇, MoO₃, and V₂O₅ follows a metathesis route, yielding a mixture of products: La₃ReO₈/La₂MoO₆/LaVO₄ and CuO. Oxides such as TiO₂, MnO₂, and RuO₂ which are not so acidic yield addition products: La₂CuMO₆ (M = Ti, Mn, Ru). SnO₂ is a special case which appears to follow a metathesis route, giving La₂Sn₂O₇ pyrochlore and CuO, which on prolonged reaction transform to the layered perovskite La₂CuSnO₆. The reaction of La₂CuO₄ with lower valence oxides VO₂ and MoO₂, on the other hand, follows a novel redox metathesis route, yielding a mixture of LaVO₄/LaCuO₂ and La₂MoO₆/Cu, respectively. This result indicates that it is the redox reactivity involving V^IV + Cu^II → V^V + Cu^I and Mo^IV + Cu^II → Mo^VI + Cu⁰, and not the acidity of the binary oxide, that controls the nature of the products formed in these cases. The general significance of these results toward the synthesis of complex metal oxides containing several metal atoms is discussed.