For RBa
2Cu
3O
y(R=Ln and Y), the relationship between superconducting transition temperature T
c, overdoping oxygen content Δy, and R was obtained through studies of oxidation under pressure. T
c of RBa
2Cu
3O
y remains almost unchanged through various levels of overdoping for small inonic radii of R. As the ionic radius of R increases, T
c decreases with the level of overdoping. The decrease changes systematically from Eu to La; the decrease for La is the steepest. This general view is somewhat modified by the temperature of overdoping. Overdoped oxygen seems to settle down on CuO and/or the Y plane depending also on the treating temperature.
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