Crystal chemistry and oxygen activity effects on T, T′ and T* phase stabilities in (La,RE)₂CuO₄ systems

La2-xRExCuO4 (RE = Nd-Y) solid solution systems have been investigated to determine the factors stabilizing the T (La2CuO4), T′ (Nd2CuO4) and hybrid T*-type structures. A simple ionic model with a perovskite-like tolerance factor is found to accurately define the existence field of each. A remarkably sharp boundary is found to separate the T and T′ stability fields, while the T* structure occurs in a narrow region adjacent to this boundary. The formation of the T* phase is viewed as resulting from a thermodynamic competition between the T and T′ phases, due to a tendency toward T/T′ site ordering. Oxygen activity is found to have a strong influence on phase selection in (RE)2CuO4 systems to the extent that two different structure-types can be stabilized at a fixed cation composition, but slightly different oxygen stoichiometries. It is found that near the phase boundaries in La2-xRExCuO4, the T and T* structure-types are stable for δ > 0, while the T′ structure is stable for δ < 0.