The structural and electric properties of ${\mathrm{Y}}_{0.6}$${\mathrm{Pr}}_{0.4}$${\mathrm{Ba}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ have been investigated as a function of x. X-ray powder-diffraction patterns indicate that a solid solution forms with x extending to about 1.4. It is found that the substitution of Sr for Ba results in a contraction in the c axis at the same rate as in ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The oxygen content and ${\mathit{T}}_{\mathit{c}}$ are found to remain essentially unchanged with increasing Sr. Our analysis suggests that the contraction in c arises mainly from the shift of the apical oxygen and from a shift of Ba(Sr) towards the ${\mathrm{CuO}}_2$ planes. The apparent shift of the apical oxygen does not seem to influence the hybridization between the Pr 4f electrons and the valence band of the ${\mathrm{CuO}}_2$ planes significantly. The hybridization is more sensitive to the bond length between the Pr and the oxygen located in the ${\mathrm{CuO}}_2$ planes. ${\mathit{T}}_{\mathit{c}}$ in this system appears to be controlled both by the shift of the apical oxygen and by the hybridization.

• Received 16 February 1995

DOI:https://doi.org/10.1103/PhysRevB.51.16444