Effects of rare earth doping on the superconducting properties of MgB2

Doping effects of rare earth oxides (REOy: RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) on the superconducting properties of MgB2 were investigated. Large fraction of the added RE turned into impurity phases: REB6 for RE = La–Gd and Yb, and REB4 for RE = Tb–Tm and Lu. We observed an increase in c-axis length from 3.523 to 3.533 Å when doped with Lu. Clear decreases in transition temperatures were observed in samples doped with Sm, Eu, Gd, Tm, Yb and Lu, indicating that these elements were successfully substituted for Mg. The decreases in Tc were most drastic in Yb-doped samples (Tc ∼ 32 K).