The Ce-Ni-Si System Revisited: More Homologue Compounds?

The nickel-rich region of the system Ce-Ni-Si has been reinvestigated utilizing X-ray single-crystal, powder, and electron diffraction as well as electron microprobe and thermal analyses. Two novel hexagonal compounds, tau-Ce20+xNi36+ySi30-z and tau '-Ce30+xNi50+ySi42-z, were identified. The crystal structure of tau-Ce20+xNi36+ySi30-z was derived from single-crystal X-ray diffraction and found to be isotypic with the Sm10Ni20.8P15-type structure (S.G. P6(3)/m,x = 1.8, y = 3.0, z = 1.8, a = 2.07156(2) nm, c = 0.39990(1) nm, R-F = 0.048). Rietveld refinement of tau '-Ce30+xNi50+ySi42-z revealed isotypism with Tb15Ni28P21 (S.G. P6(3)/m, a = 2.46926(13) nm, c = 0.40019(3) nm, R-F = 0.058). The compound Ce3Ni4Si2 from X-ray single-crystal analysis was found to crystallize in a novel structure type with monoclinic unit cells (S.G. C2/c, a = 1.54708(3) nm, b = 0.58677(1) nm, c = 0.74331(1) nm, beta = 102.985(1)degrees, R-F = 0.017). This compound belongs to a new homologue series in the RE-Ni-Si system (RE = La and Ce) with general formula of (RE)(3x2(n))(Ni)(3x2(n) + 1)(Si)(2(n+1)); n = 0,1, ..., infinity. The crystal structure of this series is characterized by alternating numbers (2(n)) of corner-sharing Si-polyhedral blocks sandwiched between zigzag nickel chains. Higher-order members of this series are produced by the formation of more corner-sharing Si-polyhedral blocks due to removal of nickel chains.