We report a strong rare-earth size dependence of ${\mathit{T}}_{\mathit{c}}$ in R${\mathrm{Sr}}_2$${\mathrm{Cu}}_{2.85}$${\mathrm{Re}}_{0.15}$${\mathrm{O}}_{\mathit{z}}$ (R=rare earth, z≊7.2) compounds, unlike in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ compounds. A strong correlation is observed between the superconducting transition temperature and the orthorhombicity of the structure, which itself is R-size dependent. Orthorhombicity is highest for ${\mathrm{HoSr}}_2$${\mathrm{Cu}}_{2.85}$${\mathrm{Re}}_{0.15}$${\mathrm{O}}_{\mathit{z}}$ and ${\mathrm{ErSr}}_2$${\mathrm{Cu}}_{2.85}$${\mathrm{Re}}_{0.15}$${\mathrm{O}}_{\mathit{z}}$ compounds, which also have yielded the highest ${\mathit{T}}_{\mathit{c}}$ (≊42 K). Significantly, ${\mathrm{TbSr}}_2$${\mathrm{Cu}}_{2.85}$${\mathrm{Re}}_{0.15}$${\mathrm{O}}_{\mathit{z}}$ stabilizes in a 1-2-3 structure and also superconducts (${\mathit{T}}_{\mathit{c}}$≊22 K), whereas ${\mathrm{EuSr}}_2$${\mathrm{Cu}}_{2.85}$${\mathrm{Re}}_{0.15}$${\mathrm{O}}_{\mathit{z}}$ does not superconduct, although it forms a single phase. © 1996 The American Physical Society.

• Received 10 October 1995

DOI:https://doi.org/10.1103/PhysRevB.53.8604