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+xNi36+ySi30–z and τ′-Ce30+xNi50+ySi42–z, were identified. The crystal structure of τ-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. P63/m,x = 1.8, y = 3.0, z = 1.8, a = 2.07156(2) nm, c = 0.39990(1) nm, RF = 0.048). Rietveld refinement of τ′-Ce30+xNi50+ySi42–z revealed isotypism with Tb15Ni28P21 (S.G. P63/m, a = 2.46926(13) nm, c = 0.40019(3) nm, RF = 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)°, RF = 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(3×2n)Ni(3×2n + 1)Si(2n+1); n = 0,1, ..., ∞. The crystal structure of this series is characterized by alternating numbers (2n) 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.

Figure S2 shows the result of a DTA measurement on a nearly single-phase annealed alloy with a composition of Ce32.6Ni45.0Si22.4revealing two endothermic peaks at 816 C and 851 C.A closer inspection on the latter (see the inset in Figure S2) shows an additional shoulder with onset at 863 C.
The signal at 816 C is stronger than the one at 851 C, indicating that 816 °C is the decomposition temperature of Ce 3 Ni 4 Si 2 .Analysis of the diffraction symmetry revealed the Laue class 6/m as the highest possible symmetry with a reasonable R int = 0.044.Systematic extinctions with 00ℓ reflections only observed for ℓ = 2n suggest the presence of a screw axis 6 3, and consequently the space group P6 3 /m (176) was chosen.
Note that several 00ℓ reflections (ℓ = 2n+1) violate the extinction rule, however, the intensity of those reflections is rather small (<7) compared to the highest intensity (~10000), and thus can be disregarded.Selected area electron diffraction (SAED) was performed to confirm the symmetry and unit cell parameters obtained from single crystal diffraction.
Structural solution employing program SIR92 1 was rather straightforward, revealing a similar set of atomic coordinates as in Sm 10 Ni 20.8 P 15 .Structural refinement with program SHELXL-97 2 quickly converged to an R F value of ~12 %.The refined composition at this stage (Ce22.5Ni47.2Si30.3, in at.%) deviates from the EPMA value (Ce24.6Ni43.5Si31.9, in at.%).However, at this point several sets of residual electron density close to each other were observed near the screw axis 6 3 suggesting the existence of severe disorder around 6 3 , typical for compounds in this family.Figure S3 shows the difference Fourier map at z = 0.25 around the origin.Following triangular arrangements of Ni and Si atoms in the lattice, it is reasonable to assign the peak labelled as Q1 as a partially filled Si site.
Moreover, distances around this electron density comply well with Ni-Si distances, ranging from 0.219 nm to 0.246 nm.Further refinement resulted in an occupancy value of ~0.6 Si in Q1.Since the overall composition lacks some cerium atoms, the second peak (Q2) located next to Q1 was assigned as a partially filled Ce site with occupancy of ~0.3.At this stage, the refined composition became closer to the EPMA composition, however, several small residual electron densities with magnitude ranging from 5 to 10×10 3 e -/nm 3 near the 6 3 -axis remained.Those residual densities were assigned as additional partially filled Ni and Si sites with small occupancies of less than 0.2.Atoms are displayed with their anisotropic thermal displacement ellipsoids as derived from X-ray single crystal refinement at 90% probability level (see Table S1).Ce-atoms in green, Ni-atoms in red and Si-atoms in blue colour.probability level (see Table S3).Ce-atoms in green, Ni-atoms in red and Si-atoms in blue colour.

Figure S4 .
Figure S4.Coordination polyhedra of atoms in -Ce 20+x Ni 36+y Si 30-z outside the disordered area.Atoms are displayed with their anisotropic thermal displacement ellipsoids as derived from X-ray single crystal refinement at 90% probability level (see TableS1).Ce-atoms in green, Ni-atoms in red and Si-atoms in blue colour.
Figure S5shows some small residual electron densities around the 6-fold axis, which were assigned

Figure S5 .
Figure S5.Difference Fourier map at z = 0.25 for alloy Ce 27.0 Ni 41.3 Si 31.7 , showing some residual density around the 6 3 -screw axis.

Table S2 .
Refined atomic position of Ce 30+x Ni 50+y Si 42-z .The atomic positions are standardized according to Tb 15 Ni 28 P 21 .

structure of monoclinic Ce 3 Ni 4 Si 2 (formerly CeNi 1.3 Si 0.7 ) -a new homologue series
Analysis of systematic extinctions suggested two possible space groups Cc (No. 9) and C2/c (No. 15).Despite the E-test giving a hint for centrosymmetry, structure solution employing direct methods was performed first in the non-centrosymmetric space group Cc, and quickly converged to an R F value of ~4 %.Analysis of missing symmetry with program PLATON 3 suggested the centrosymmetric space group of C2/c, and the structure was furthermore refined in this space group.