Structural properties and anomalous composition dependences of the superconducting transition temperature of RE(Rh,Co)₄B₄ alloys (REY, Er)

Measurements of the superconducting transition temperature T_c of RERh₄B₄ as a function of mole fraction n of RECo₄B₄ for RE(Rh_1-nCo_n)₄B_4+x (REY, Er; x=0, 0.26) alloys are reported. Dilute cobalt substitutions for rhodium are found to depress T_c rapidly with -(dT_c/dn)_n-0≈ 78 K mol^-1 (REEr). Anomalies in T_c (n) and transition width occur for n>0.10 in the Y(Rh_1-nCo_n)₄B_4+x system. Nevertheless, room temperature powder X-ray diffraction data indicate that samples were predominantly a single phase with the CeCo₄B₄ structure, although mild deviations from Vegard's law were observed in both systems. This behavior contrasts strongly with T_c data previously gathered for superconducting boride systems containing iridium and ruthenium substitutions for rhodium and suggests that the cobalt impurities promote magnetic interactions.