The crystal chemistry of several ${\mathrm{Ba}}_{2}R{\mathrm{Cu}}_3{\mathrm{O}}_7$ high-$T_c$ superconductors ($R=\mathrm{Y},\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m},\mathrm{E}\mathrm{u},\mathrm{G}\mathrm{d},\mathrm{D}\mathrm{y},\mathrm{H}\mathrm{o},\mathrm{E}\mathrm{r},\mathrm{T}\mathrm{m}$) has been studied by x-ray diffraction on twinned crystals. All samples except the Pr compound show superconductivity around 90 K. From the variation of the interatomic distances with different rare-earth ions, we infer the presence of a [Cu${\mathrm{O}}_2$-BaO-CuO□-BaO-Cu${\mathrm{O}}_2$] building unit.

• Received 1 July 1987

DOI:https://doi.org/10.1103/PhysRevB.36.3617