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-Ce 20+ x Ni 36+ y Si 30- z and τ'-Ce 30+ x Ni 50+ y Si 42- z , were identified. The crystal structure of τ-Ce 20+ x Ni 36+ y Si 30- z was derived from single-crystal X-ray diffraction and found to be isotypic with the Sm 10 Ni 20.8 P 15 -type structure (S.G. P 6 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 τ'-Ce 30+ x Ni 50+ y Si 42- z revealed isotypism with Tb 15 Ni 28 P 21 (S.G. P 6 3 / m , a = 2.46926(13) nm, c = 0.40019(3) nm, R F = 0.058). The compound Ce 3 Ni 4 Si 2 from X-ray single-crystal analysis was found to crystallize in a novel structure type with monoclinic unit cells (S.G. C 2/ 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 (3×2 n ) Ni (3×2 n  + 1) Si (2 n +1 ) ; n = 0,1, ..., ∞. 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.