The influence of oxygen activity on T, T′, and T∗ phase stabilities in (La,RE)2CuO4 systems

The effect of oxygen activity on the formation of (La,RE)2CuO4 (RE = rare earth) solid-solutions has been investigated. Compositions from the (La,Nd)2CuO4 and (La,Dy,M)2CuO4 (M = Ce, Th) systems, for example, can be prepared in either the T and T′ or T∗ and T′ structure types, respectively, at different oxygen contents but identical cation stoichiometries, thereby considerably extending the stability field of each structure. These results in part explain some of the differences in several published “equilibrium” phase diagrams for the (La,Nd)2CuO4 system.