SUPERCONDUCTIVITY IN THE <font>Tl-Ca-Ba-Cu-O</font> SYSTEM: SYNTHESIS, CHARACTERIZATION AND MECHANISM

Synthesis and characterization of some of the members of the Tl-Ca-Ba-Cu-O system are presented. T c in both the TlCa n−1 Ba 2 Cu n O 2n+3 and Tl 2 Ca n−1 Ba 2 Cu n O 2n+4 series increase with the number of Cu-O layers, n; T c in the latter series with two Tl-O layers are generally higher than in the former with a single Tl-O layer. Tl in the cuprates is in the 3+ state while Cu is in the 1+ and 2+ states, showing the importance of oxygen holes. The concentration of these holes seems to increase with the number of Tl-O layers.