Systematic variation of superconductivity for the quaternary borocarbide systemRNi2B2C (R=Sc, Y, La, Th, U, or a lanthanide)

A superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of 8.0 K is reported for the nonmagnetic (${\mathrm{Th}}^{4+}$) ${\mathrm{ThNi}}_2$ ${\mathrm{B}}_2$C compound with the ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C-type structure and a derived coherence length ξ(0) of 230 Å and a penetration depth λ(0) of 860 Å. No transition down to 2 K was observed for the magnetic, mixed-valence (${\mathrm{U}}^{4+}$/${\mathrm{U}}^{6+}$ or average valence of +5.2) ${\mathrm{UNi}}_2$ ${\mathrm{B}}_2$C compound with the same structure. Systematic variation of ${\mathit{T}}_{\mathit{c}}$ for the quaternary and pseudoquaternary compounds in the R${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$C system (R=Sc, Y, La, Lu, Th, U, or magnetic lanthanides) is studied through the variation of ${\mathit{T}}_{\mathit{c}}$ with the R ionic radius, Ni-Ni nearest-neighbor in-plane distance, and the normal-state Pauli-like temperature-independent paramagnetic susceptibility. Maximum ${\mathit{T}}_{\mathit{c}}$ at 16.6 K for ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C with an optimum Ni-Ni distance of 2.45 Å was observed, along with a large normal-state Pauli paramagnetic susceptibility. As a comparison, the 8.0-K ${\mathrm{ThNi}}_2$ ${\mathrm{B}}_2$C superconductor has a longer Ni-Ni distance of 2.60 Å and a smaller normal-state susceptibility. Variations of the Ni(3d)-dominated conduction band and the density of states at the Fermi level N(${\mathit{E}}_{\mathit{F}}$) are believed to be the driving force for the ${\mathit{T}}_{\mathit{c}}$ variation of nonmagnetic compounds in the present system.