X-Ray Diffraction Studies of 2212-Type Superconductors in the Tl–Hg–Ba–Sr–Ca–Cu–O System

Superconducting cuprates with the general stoichiometry (Tl, Hg)2(Ba, Sr)2(Sr, Ca)1Cu2O8- were examined by X-ray powder diffraction, including Rietveld refinement calculations. The aim was to obtain structural properties that could be related to the diamagnetic transition onset temperature (Tc(onset)). Rw values for nine different samples, with different Ba, Sr, and Ca contents, were 0.105 or less. For three of the polycrystalline samples, the diamagnetic transitions were confirmed by resistivity measurements. Bond valence sum calculations indicate that Tc(onset) increases (45–100 K) with a decreasing formal oxidation state of Cu in the superconducting plane. This tendency might be correlated with the apical distance between the (Ba, Sr) and Cu layers since the interplanar distance increases proportionally with the critical temperatures. In turn, this distance increases with the Ba/Sr ratio at the larger alkaline-earth metal site.