Effect of substituting Ce at the Ca site on the high-temperature superconductor Bi2Sr2CaCu2O8-delta

We have investigated the system Bi2Sr2Ca1-xCexCu2O8+delta (0 less than or equal to x less than or equal to 0.5) to study the effect of substituting. Ce for Ca on superconductivity in the Bi2Sr2CaCu2O8+delta 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 similar to 50 at. % substitution is required to suppress superconductivity. Magnetic susceptibility of the Bi2Sr2Ca1-xCexCu2O8+delta 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.