Study of structural variation caused by Ar annealing for (Hg_0.7,Tl_0.3)₂Ba₂(Ca_0.2,Y_0.8)Cu₂O_y

A nearly single phase (Hg,Tl)-bilayer sample, (Hg_0.7,Tl_0.3)₂Ba₂(Ca_0.2,Y_0.8)Cu₂O_y, was synthesized by a high pressure synthesizing technique. The as-sintered sample, which is considered to be underdoped, shows superconductivity at T_c = 65 K. The sample was then annealed in Ar gas at 300 °C for 30 min, 12 h, and 400 °C for 36 h, respectively. The crystal structure of these samples was determined by Rietveld refinement. Both of the a- and c-axes lengths expand with the decrease of hole doping in the crystal. It was found that the interplanar distance between the Ba-(rock salt)-Ba sheets expands, and the interplanar distance between the Ba-(infinite layer)-Ba sheets shrinks with Ar annealing, showing the same variation tendency as found in (Hg,Tl)-2223.