Model family of high-temperature superconductors:TlmCan−1Ba2CunO2(n+1)+m(m=1,2;n=1,2,3)

Abstract

We describe the structures and superconducting properties of six compounds in the Tl-Ca-Ba-Cu-O system of the general form, ${\mathrm{Tl}}_m{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Ba}}_2{\mathrm{Cu}}_n{\mathrm{O}}_{2(n+1\mathrm{})+m}$, where $m=1\mathrm{} or 2$ and $n=1,2, or 3$. One of the compounds displays the highest known superconducting transition temperature, $T_c\simeq 125$ K. The structures of these compounds consist of copper perovskitelike blocks containing 1, 2, or 3 Cu${\mathrm{O}}_2$ planes separated by one or two Tl-O layers and thus form a model family of structures in which both the size and separation of the copper oxide blocks can be independently varied. The superconducting transition temperature increases with the number of Cu${\mathrm{O}}_2$ planes in the perovskitelike block for both the Tl-O monolayer and bilayer compounds. For each pair of compounds ($m=1,2\mathrm{}$) with the same number of Cu${\mathrm{O}}_2$ planes (same $n$), the transition temperatures are similar but are consistently 15-20 K lower in the materials with single Tl-O layers. Variations in the transition temperatures in the double and triple Cu${\mathrm{O}}_2$-layer compounds are observed to correlate with increased densities of intergrowths of related structures.