High Tc Bismuth and Thallium Copper Oxide Superconductors

Abstract

The first report of a superconductor in this family came from the laboratory of B. Raveau. The system was Bi/Sr/Cu/O. The Tc's were in the range 10-20 K, and this system therefore did not attract much attention when there was intense effort on 90 K superconductors. Neither the structure nor the composition of the superconductor in the Bi/Sr/Cu/O system was known at the time of the first publication, a common feature with all discoveries of new high Tc superconductors in the past couple of years. We now know that the first discovered member of this new family of superconductors can be represented as Bi2Sr2CuO6. However, the composition and structure of this phase are still not precisely established.Subsequent to the French work, the Bi/Sr/Ca/Cu/O system was studied in Germany, and higher Tc's were obtained. Although patents were filed; this work was not publicly announced. In January 1988, Maeda announced high Tc in the Bi/Sr/Ca/Cu/O system and Hermann announced high Tc in the Tl/Ba/Ca/Cu/O system.It is now known that there exists a very large family of compounds of the type (AO)mM2Can-1CunO2n+2. The A cation can be Tl, Pb, Bi or mixtures of these elements. The value of m may be one or two but is only two when A is Bi. The M cation is Ba or Sr, and substitution of Ca by Sr is frequently observed. The number of consecutively stacked CuO2 layers is indicated by n. The highest temperature at which zero resistivity has been obtained in this family is 122 K, and this has been obtained for several distinctly different compositions (Table I).