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)(1)Cu2O8-delta were examined by X-ray ponder diffraction, includ ing Rietveld refinement calculations. The aim was to obtain structural prop erties that could be related to the diamagnetic transition onset temperatur e (T-c(onset)), R-w values for nine different samples, with different Ba, S r, and Ca contents, were 0.105 or less. For three of the polycrystalline sa mples, the diamagnetic transitions were confirmed by resistivity measuremen ts, Bond valence sum calculations indicate that T-c(onset) increases (45-10 0 K) with a decreasing formal oxidation state of Cu in the superconducting plane. This tendency might be correlated with the apical distance between t he (Ba, Sr) and Cu layers since the interplanar distance increases proporti onally with the critical temperatures. In turn, this distance increases wit h the Ba/Sr ratio at the larger alkaline-earth metal site. (C) 2000 Academi c Press.