The crystal structure of superconducting LuNi₂B₂C and the related phase LuNiBC

SUPERCONDUCTING intermetallic compounds with relatively high transition temperature (T have been found in the systems Ln-Tr-B-C, where Ln is a lanthanide element (Y, Ho-Lu) and Tr a transition metal (Pd or Ni) ^1,2. In the nickel-bearing system, the superconducting phase has the formula LnNi (ref. 2). Although many structures are known for ternary carbides and borides, only a few have been reported for quaternary boro-carbides ³. Here we describe two new phases that form in the Lu-Ni-B-C system: LuNi, a superconductor with a Tof 16.6 K (ref. 2), and LuNiBC, a closely related non-superconducting phase. The superconductor is a variant on the layered ThCritype structure⁴, with additional carbon in the Lu plane. LuNiBC is derived from LuNi by adding another layer of Lu-C, producing a NaCl-type intergrowth. A new homologous series of inter-metallic phases may be derivable from the simple building principles of these two structures.