Rare-earth ionic size effects on T_c in the series RBa₂Cu₃O_7-δ

The rare-earth ionic size effects on T_c are investigated in terms of bond valence sums using the neutron diffraction data for nearly optimal doped RBa₂Cu₃O_7-δ (R=Y, La, Nd, Sm, Dy, Ho, Er, Tm, and Yd) and PrBa₂Cu₃O_7-δ systems. The optimal hole concentration in CuO₂ planes is found to decrease systematically with increasing rare-earth ionic radius except Pr-123 in the R-123 phases, which is confirmed by the two similar inverted parabolic curves of T_c vs hole concentration for Y-123 and Er-123. The anomalous property of Pr-123 is due to the depletion of hole concentration. The increase in T_c as R³⁺ionic radius is interpreted as both the increase of hole distribution on the in-plane O sites relative to the in-plane Cu sites and the interplanar coupling.