Nearly single-phase superconducting (Hg0.7,Tl0.3)2Ba2(Ca x,Y1-x)Cu2O8-2δ samples (x = 0.1, 0.2, 0.4 and 0.6) were prepared by a high pressure synthesis technique. The crystal structure of these samples was determined by X-ray Rietveld refinement. This crystal belongs to a tetragonal system and has a space group of I4/mmm with a ≈ 3.86 and c ≈ 29.0 Å. The unit cell parameter a decreases and c increases with increasing x. The expansion of the c-axis is found to mainly result from the increase in the inter-planar distance between the Ba-Hg planes. The present refinement does not show evidence for the substitution of Cu in the (Hg,Tl) site.
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