SYSTEMATIC VARIATION OF SUPERCONDUCTIVITY FOR THE QUATERNARY BOROCARBIDE SYSTEM RNI(2)B(2)C (R=SC, Y, LA, TH, U, OR A LANTHANIDE)

A superconducting transition temperature Tc of 8.0 K is reported for the nonmagnetic (Th4+) ThNi2B2C compound with the LuNi2B2C-type structure and a derived coherence length (0) of 230 and a penetration depth (0) of 860. No transition down to 2 K was observed for the magnetic, mixed-valence (U4+/U6+ or average valence of +5.2) UNi2B2C compound with the same structure. Systematic variation of Tc for the quaternary and pseudoquaternary compounds in the RNi2B2C system (R=Sc, Y, La, Lu, Th, U, or magnetic lanthanides) is studied through the variation of Tc with the R ionic radius, Ni-Ni nearest-neighbor in-plane distance, and the normal-state Pauli-like temperature-independent paramagnetic susceptibility. Maximum Tc at 16.6 K for LuNi2B2C with an optimum Ni-Ni distance of 2.45 was observed, along with a large normal-state Pauli paramagnetic susceptibility. As a comparison, the 8.0-K ThNi2B2C superconductor has a longer Ni-Ni distance of 2.60 and a smaller normal-state susceptibility. Variations of the Ni(3d)-dominated conduction band and the density of states at the Fermi level N(EF) are believed to be the driving force for the Tc variation of nonmagnetic compounds in the present system. © 1995 The American Physical Society.