AN IONIC MODEL OF THE CRYSTAL-CHEMISTRY IN THE SUPERCONDUCTING COPPER OXIDES OF STOICHIOMETRY (RE)2CUO4

The mixed rare earth systems La2-xRExCuO4 (RE = NdY) have been investigated in an effort to understand the crystal chemistry and phase stability of the related K2NiF4 (T)-, Nd2CuO4 (T′)-, and hybrid T*-type structures. The crystal chemistry of these (RE)2CuO4 phases is then discussed on the basis of a simple ionic model. A definitive correlation between the size of the RE cation and structure is found, and the stability limits of the T, T′, and T* phases are defined in terms of a perovskite-like tolerance factor (t). The T structure is found to exist for 0.87 ≤ t ≤ 0.99, while the T′ structure occurs for 0.83 ≤ t ≤ 0.86. The smallest rare earths, or t ≤ 0.83, do not form stable (RE)2CuO4 compounds. The T* structure occurs in a very narrow region adjacent to the boundary of the T T′ stability field and is seen as resulting from a thermodynamic competition between the T and T′ structures, due to a tendency toward T T′-site ordering. Metastable T* phases are observed for the larger rare earths Nd, Eu, and Gd, but the structure becomes quite stable for RE = Tb, Dy. Where both T′ and T* are of comparable thermodynamic stability, high oxygen activity is found to stabilize T*, whereas low oxygen activity favors T′. The role of RE coordination preferences and the influence of the REO framework upon the structure and properties of these materials are discussed. © 1990.