CRYSTAL-CHEMISTRY AND SUPERCONDUCTIVITY OF THE COPPER OXIDES

This paper highlights the important roles played by crystal chemistry in controlling the superconductive properties of the intergrowth structures of the copper oxides. Bond-length matching across the intergrowth interface stabilizes at least four different structures- T O, T′, T*, and T″-in the simplest system La2-yLnyCuO4 (Ln = lanthanide) depending upon the size of Ln and the value of y. The internal electric field created by the formal charges in the adjacent layers modulates the distribution of holes between the active and inactive layers and the influence of Pr on superconductivity. The coordination geometry preferred by different oxidation states of Cu appears to control the oxygen ordering and the Tc variation in the YBa2Cu3O6+x system. The c-axis CuO distance modulates the width of the conduction band and the electronic properties. Chemical characterization of the thallium cuprates has demonstrated that the oxidation of the Tl2-yBa2Can-1CunO2n+4-x system can be due to either solely an overlap of the Tl:6s band with the conduction band or solely Tl vacancies, depending upon the value of y; as normally prepared, both effects are operative. © 1990.