We have prepared and characterized a series of Co- and Ni-doped $\mathrm{Y}{\mathrm{Ba}}_2({{\mathrm{Cu}}_{1-x}{M}_x)}_3{\mathrm{O}}_y$ compounds and determined the oxygen content for $0.0\le x\le 0.15$ and $0.0\le x\le 0.20$, for $M=\mathrm{Co}$ and Ni, respectively. The oxygen stoichiometry in Co-doped materials does not vary for $x<\mathrm{}0.10q$ but increases to about 7.3 for 15% Co. The oxygen content of Ni-doped materials remains constant at 7.0±0.1 for all $x$. The solubility limits of the solid solutions are $x\approx 0.15$ for Co and $x\approx 0.20$ for Ni. The Co-doped material undergoes a transformation from orthorhombic to tetragonal symmetry at $0.025<x<\mathrm{}0.05\mathrm{}$ with a concomitant drop in $T_c$; the Ni-doped material remains orthorhombic throughout the limit of the solid solution. Based on an empirical model calculation, Co was deduced to occupy metal sites only in the Cu(1)${\mathrm{O}}_3$ chains. The structural transition to tetragonal symmetry is a result of cobalt's preference for octahedral coordination, and is attributed to a disorder of the oxygen atoms (0,½,0) into the sites (½,0,0). The rapid suppression of $T_c$ in the Co-doped samples is ascribed to the structural phase transition. Also presented are magnetic susceptibility and critical-field data.

• Received 22 February 1988

DOI:https://doi.org/10.1103/PhysRevB.38.2432