Joseph F. Bringley

An ionic model of the crystal chemistry in the superconducting copper oxides of stoichiometry (RE)2CuO4

Abstract The mixed rare earth systems La 2− x RE x CuO 4 (RE = NdY) have been investigated in an effort to understand the crystal chemistry and phase stability of the related K 2 NiF 4 ( T )-, Nd 2 CuO 4 ( T′ )-, and hybrid T∗ -type structures. The crystal chemistry of these (RE) 2 CuO 4 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) 2 CuO 4 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 REO framework upon the structure and properties of these materials are discussed.

Energetics of La2−xSrxCuO4−y solid solutions (0.0 ≤ x ≤ 1.0)

Compounds in the La{sub 2{minus}x}Sr{sub x}CuO{sub 4{minus}y} series have been synthesized in the single phase range 0 {le} x {le} 1. High temperature solution calorimetry in molten lead borate was used to determine the heats of formation from oxide components. The enthalpy of formation becomes more exothermic with increasing Sr substitution 0 {le} x {le} 0.15 and then remains approximately constant for 0.15 {le} x {le} 1. This trend can be explained by the balance between the (formal) substitution La{sup 3+} + Cu{sup 2+} = Sr{sup 2} + Cu{sup 3+} and the loss of oxygen, keeping copper formally divalent but creating oxygen vacancies. Using a transposed temperature drop method, the heat of oxidation was determined to be {minus}129 kJ/(mole O{sub 2}) by a comparison of samples with the same strontium content but a different oxygen content.

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

Abstract The effect of oxygen activity on the formation of (La,RE) 2 CuO 4 (RE = rare earth) solid-solutions has been investigated. Compositions from the (La,Nd) 2 CuO 4 and (La,Dy, M ) 2 CuO 4 ( 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) 2 CuO 4 system.