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, τ-Ce₂₀₊Ni₃₆₊Si_30- and τ'-Ce₃₀₊Ni₅₀₊Si_42- were identified. The crystal structure of τ-Ce₂₀₊Ni₃₆₊Si_30- was derived from single-crystal X-ray diffraction and found to be isotypic with the Sm₁₀Ni_20.8P₁₅-type structure (S.G. P6₃/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 τ'-Ce₃₀₊Ni₅₀₊Si_42- revealed isotypism with Tb₁₅Ni₂₈P₂₁ (S.G. P6₃/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, β = 102.985(1)°, 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_(3×2Ni_(3×2Si₍₂; n = 0,1, ..., ∞. The crystal structure of this series is characterized by alternating numbers (2) 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.