M. C. Schaloske, L. Kienle, Hj. Mattausch, V. Duppel, A. Simon
FULL PAPER
correspond to βЈ- and βЈЈ-Ce6I9C2N, respectively. The pres- and simulated micrographs that are based on the average
ence of structural variants, particularly the superstructure structure models. Even in the case of the image recorded
for βЈ-Ce6I9C2N is not only evident from the high-resolu- with a strongly under-focused beam the simulation corre-
tion imaging data but also from the electron-diffraction lates well with the corresponding experimental micrograph;
data. The selected-area electron diffraction (SAED) pat- however, a perfect match cannot be achieved when applying
terns shown in Figure 4a were recorded on βЈЈ-Ce6I9C2N an average model in the simulation.
(left) and βЈ-Ce6I9C2N (right). A row of reflections in the
In exceptional cases an intergrowth of both structural
pattern of βЈ-Ce6I9C2N shows doubling of the a and b axes variants was observed. In this case, the [100] SAED patterns
compared to the metrics of βЈЈ-Ce6I9C2N (left). The show diffuse streaks along b* between the main reflections.
HRTEM micrographs in Figure 4b exhibit slight differences The intergrowth is clearly observable in the HRTEM micro-
in the phase contrasts of the two structures. Line scans graphs (Figure 6). While in the upper part of the image
within the marked areas of Figure 4b highlight the specific the typical sequence of sheets corresponding to the
periodicities of the structures along Ͻ210Ͼ. In the pattern βЈЈ-Ce6I9C2N structure is present, the lower part shows
of βЈЈ-Ce6I9C2N (Figure 4b, left) all peaks indicated with alternating contrast for the sheets, corresponding to the
arrows have the same intensity, whereas in the pattern for partial ordering of the βЈ-Ce6I9C2N structure. The Fourier
βЈ-Ce6I9C2N alternating intensity maxima occur. These dis- transforms (not shown) calculated for both sides of the
tinct periodicities are also visible in the Fourier transforms phase boundary seen in the image support such an assign-
of the micrographs (not shown) that contain superstructure ment due to the lack of superstructure peaks for hkl reflec-
peaks for βЈ-Ce6I9C2N, i.e. hkl reflections with k = n/2.
tions with k = n/2.
An extension of the real structure model based on two-
dimensional disorder with rods ordered in themselves is
found for samples with the nominal composition Y6I9CN2.
In this case, the [100] SAED patterns show diffuse streaks
between the main reflections along b* (see Figure 7a, left).
Patterns with concentrations of diffuse intensity associated
with hkl reflections with k = n/2 were observed, in-
dicating that these compounds have partial order like in
βЈ-Ce6I9C2N (Figure 7a, centre). Surprisingly, many crystal-
lites exhibit structural disorder along the rod axes. In this
case, the bright-field images (Figure 7b) display broad mod-
ulation waves (highlighted by arrows in Figure 7b) that
propagate along the c axes, and the corresponding SAED
patterns (Figure 7a, right) contain split reflections. The for-
mation of modulation waves strongly suggests that these
disorder phenomena are not restricted to single rods, but
include neighboring rods in adjacent (010) sheets. HRTEM
micrographs clearly show deviations from the sequence of
clusters described for Ce6I9C2N (Figure 7c). For Δf =
+40 nm, the bright crosses correlate with the positions of
the oo-type units, and the sequence along c is consistent
with 1ϱ[oott] of the clusters on the left and right. In the
Figure 4. Comparison of SAED patterns with enlarged sections (a)
and HRTEM micrographs (b), Δf ≈ –30 nm, with line scans (insets)
for βЈЈ-Ce6I9C2N (left) and βЈ-Ce6I9C2N (right). The zone axes are
along [100].
These differences in contrast cannot just be assigned to centre of the pattern shown in Figure 7c, the sequence is
changes in the metrics of the compounds, but the specific disturbed as highlighted by the arrows. Such variations
structural features of βЈЈ- and βЈ-Ce6I9C2N must also be could be due to partially deviating periodic structures with
considered. According to the simulated micrographs shown complex sequences of the clusters, which might be access-
in Figure 5a, the partial ordering of the βЈ-Ce6I9C2N struc- ible in discrete phases.
ture (Figure 5a, right) produces alternating bright and dark
Finally, the question arises as to what causes the different
contrasts for the sheets seen in the micrograph (indicated degrees of disorder in the phases discussed. Possible an-
by horizontal arrows), due to the superposition of twofold- swers include: (i) The distances in the Ce–I–Ce bridges
and threefold disordered rods, respectively. In the case of within the Kagomé-type framework are approximately
βЈЈ-Ce6I9C2N (Figure 5a, left) such contrast differences fade 0.3 Å shorter than those associated with the rods inserted
since all adjacent sheets contain rods with only threefold in the hexagonal channels. Hence, the comparably weaker
disorder. The complete atomic ordering within the rods, bonding in the latter case favours rotational disorder. (ii)
particularly the presence of the cluster sequence 1ϱ[oott], is There exists a triangular arrangement of contoured rods
indicated by the periodicity of the structural motifs along within each structure that have minima and maxima in dia-
the c axis, as indicated by the vertical arrows. The series of meter at identical heights along the c axes. A close packing
images recorded with variable foci that are shown in Fig- of such rods can be optimized by mutually shifting them
ure 5b allows for comparison of experimentally obtained along the c axes to give a positional ratio 1:2 for each tri-
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4049–4056
Schaloske, Manuel C.
