Incommensurate and commensurate superstructures in the oxycarbonate superconductor TlSr_4-xBa_xCu₂(CO₃)O_y (x≈2)

Superstructures of oxycarbonate superconductor TlSr_4-xBa_xCu₂(CO₃)O_y (x=2; T_c=73K) were examined by electron diffraction and HRTEM techniques. Part of the crystals show a commensurate superstructure with a 6a₀ 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 7.4a₀ 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 7a₀, which is relatively shorter than the value of 8a₀ previously claimed by Goutenoire et al. [Physica C 210 (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 TlSr_4-xBa_xCu₂(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.