Effect of substituting Ce at the Ca site on the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+δ

Abstract

We have investigated the system Bi₂Sr₂Ca_1-xCe_xCu₂O_8+δ(0≤x≤0.5) to study the effect of substituting Ce for Ca on superconductivity in the Bi₂Sr₂CaCu₂O_8+δ system (T_c=90 K). It is observed that, on 25 at. % substitution of Ce for Ca, superconductivity is completely suppressed. This is relatively faster suppression of superconductivity compared to that seen on substituting other rare earths for Ca where ~50 at. % substitution is required to suppress superconductivity. Magnetic susceptibility of the Bi₂Sr₂Ca_1-xCe_xCu₂O_8+δ compounds is found to be weakly temperature dependent showing a small rise at low temperatures and a shallow minima at about 200 K. These results suggest the possible existence of Ce in a tetravalent state rather than in a trivalent state in this system. X-ray-diffraction studies on these compounds indicate a decrease in the c lattice parameter with increased substitution of Ce. This is consistent with nearly tetravalent Ce ion replacing the Ca²⁺ ion since the size of Ce³⁺ is comparable to that of Ca²⁺, while that of Ce⁴⁺ is smaller.