Neutron powder diffraction study of the superconducting quaternary intermetallic compound YNi₂B₂C

The crystal structure of YNi₂B₂C has been refined at room temperature using powder neutron diffraction data obtained from samples prepared by arc-melting. The nominal composition YNi₃B_2.5C yields a phase assemblage of principally 85 volume % YNi₂B₂C relative to Ni₂B, and the YNi_2.5B₂C composition yields 93 volume % relative to 7 volume % Ni₃B. YNi₂B₂C is apparently incongruently melting. The crystal structure is simply viewed as a "stuffed" derivative of the common layered ThCr₂Si₂-type structure, by inserting C atoms in the lanthanide (actinide) layer. Structural data for the most phase-pure title compound include: I4/mmm symmetry, a=3.5257(1) A ̊, c=10.5336(4) A ̊ and x(B)=0.3589(4). The atomic sites appear to be fully occupied, and there is no indication of B↔Ni disorder. The arc-melted YNi₃B_2.5C (nominal) sample has a superconducting T_c of 15.6 K, as defined by the onset of diamagnetism.