Mossbauer measurements of YBa2Cu3-xSnxO7-y compounds with high Sn concentrations have revealed that the Sn dopants all exist as Sn4+, with their majority occupying Ba, Y, Cu(2) sites, and only a small part at Cu(l) sites. The amount of Sn at each site varies with Sn content (x). As for superconductivity, the sample systems are all with Tc above 77 K, this indicates that the substitutions for the metal ions on Cu(2) and Y sites have small effect on Tc, which strongly supports the concept that Cu-O plane does not predominate the superconductivity.
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