High temperature superconducting phases in the Tl-Ca-Ba-Cu-O system are ideally represented by the formula TlmCan−1Ba2CunO2(n+1)+m, with m either 1 or 2 and n = 1 to at least 3 (Parkin et at., 1988). Each of these phases contains one or more of the nearly planar CuO2sheets common to the cuprate superconductors. A single Ca atom separates adjacent CuO2sheets (n > 1). Single or double rock salt-like Tl-O layers are separated from the Can−1CunO2nregions by single Ba-O layers. Each of the Ca-containing members of this family crystallizes in a tetgragonal unit cell, with space group 14/mmm for the m = 2 series and P4/mmm for the m = 1 series.
Despite the general interest in this family of superconductors, little has been reported about the m = 1, n = 2 member, TlCaBa2Cu2O7−δ, hereafter called 1122. This lack of work is due at least in part to the difficulty in synthesizing the pure compound (Michel et at., 1991). Additionally, technological interest has focused on members of the family with higher superconducting transition temperatures, particularly Tl2Ca2Ba2Cu3Oywith Tcup to 125 K. The critical temperature of 1122 has been reported from as low as 50 K (Hervieu et al., 1988) to as high as 103 K (Morosin et al., 1988), and at several values in between (Ganguli et al., 1988; Liang et al., 1988). Most of the samples had other superconducting phases in addition to 1122. Because of the nearly identical a axis lengths of the unit cells of the Tl-family of superconductors, syntactic intergrowths may be present in such multiphase samples.