Synthesis and characterization of superconducting $YbBa_2Cu_3O_{7-\delta}$ and $Y_{1-x}Lu_xBa_2Cu_3O_{7-\delta}$ (0.0<x<0.75): lower limit of the rare-earth ion radius tolerated in the 123 cuprate system

$YbBa_2Cu_3O_{7-\delta}$ with a $T_c$ (zero-resistance) of 89K has been prepared without contamination of $Yb_2BaCuO_5$. Monophasic $LuBa_2Cu_3O_{7-\delta}$ cannot be prepared. Solid solutions of the type $Y_1$ $_xLu_xBa_2Cu_3O_{7-\delta}$ $(0.0 < x \leq 0.75)$ are, however, stable and superconducting (zeroresistance $T_c$, 89K). It appears that the lower limit of the rare earth ion radius tolerated by the 123 cuprate structure is close to that of $Yb^{3+}$ $(0.985 \AA)$