Synthesis and characterization of superconducting YbBa2Cu3O7−δ and Y1−xLuxBa2Cu3O7−δ (0.0 < x < 0.75): Lower limit of the rare-earth ion radius tolerated in the 123 cuprate system

YbBa2Cu3O7−δ with a Tc (zero-resistance) of 89K has been prepared without contamination of Yb2BaCuO5. Monophasic LuBa2Cu3O7−δ cannot be prepared. Solid solutions of the type Y1 xLuxBa2Cu3O7−δ (0.0 < x ⩽ 0.75) are, however, stable and superconducting (zeroresistance Tc, 89K). It appears that the lower limit of the rare earth ion radius tolerated by the 123 cuprate structure is close to that of Yb3+ (0.985 Å).