INCOMMENSURATE AND COMMENSURATE SUPERSTRUCTURES IN THE OXYCARBONATE SUPERCONDUCTOR TLSR4-XBAXCU2(CO3)O-Y(X-APPROXIMATE-TO-2)

Superstructures of oxycarbonate superconductor TlSr4-xBaxCu2(CO3)O-y(x=2; T-c=73 K) were examined by electron diffraction and HRTEM techniques. Part of the crystals show a commensurate superstructure with a 6a(0) periodicity due to crystallographic shears by c/2 to form -(C-C-C)-(Tl-Tl-Tl)-(C-C-C)-(Tl-Tl-Tl)- type of sequences along the (100) direction in the basal planes. Incommensurate superstructures with longer periodicities up to 7.4a(0) due to mixed intergrowths of-(C-C-C-C)- and -(C-C-C)- sequences were also observed. The average superstructure periodicity of the present specimen is around 7a(0), which is relatively shorter than the value of 8a(0) previously claimed by Goutenoire et al. [Physica C210 (1993) 359] for a similar superconductor with T-c=62 K. This suggests the possibility of a close relationship between the modulation periodicity and the transition temperature in the TlSr4-xBaxCu2(CO3)O-y system with x=1-2.5. Observed structures were compared with an another type of superstructure recently discovered in HgBa2Sr2Cu2(CO3)O-y by Uehara et al.