We report site-selective oxygen isotope effects on $T_c$ and penetration depth in ${\mathrm{Y}}_{0.7}$${\mathrm{Pr}}_{0.3}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.97}$ and in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. In Pr-substituted (underdoped) samples with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ in only the Cu${\mathrm{O}}_2$ plane-sites, the $T_c$ shift was -1.46 K vs -1.7 K for ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ at all sites; the ${\alpha }_{\mathrm{O}}$ values are 0.2 and 0.24. The corresponding $T_c$ shifts in pure $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ were -0.24 K and -0.3 K. The shift is dominated by Cu${\mathrm{O}}_2$ planar oxygen mass in both compounds, and not by the apical sites. Thus, apical site isotope induced charge transfer to Cu${\mathrm{O}}_2$ planes does not seem a viable explanation for the isotope effects in YBCO. Our results indicate that the oxygen related phonons in the Cu${\mathrm{O}}_2$ planes play a significant role in the pairing mechanism.

• Received 11 April 1996

DOI:https://doi.org/10.1103/PhysRevB.54.14982