Chemistry Reference
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Fig. 9.19 Schematic representations of the diffuse lines (
,
n
+ 0.5,
l
)(
n
; integer) on a projection
plane perpendicular to the
b
* axis with the Ewald sphere [
32
]. The sample is oriented as (a) the
a
*
and
b
* directions perpendicular to the incident X-ray beam (same orientation as Fig.
9.17a
), and
(b) the
b
* direction perpendicular to the incident beam and the
a
* parallel to it (same as Fig.
9.17b
)
perpendicular to the
b
*axis(
chain axis
b
) and parallel to the
a
* axis and are indexed
as (-,
n
+0.5,
l
) and shown in Fig.
9.19
. This implies that the periodic ordering with
twofold repetition length of the MMX unit existing in the 1D chain is strongly coupled
with each other along the
c
direction, resulting in 2D order in the
bc
plane.
Wakabayashi et al. have reported that a quantitative analysis of the diffuse
intensity distribution in the metallic state of 2 based on the method comprised of
diffuse scattering and resonant X-ray scattering [
73
]. The observed diffuse scattering
intensity distribution along the
b
* axis at room temperature is shown in Fig.
9.20
.
This figure shows the (0
k
0) intensity within the region of 4.5
¼
k
16.5. Intense
scattering was observed at
k ¼
0.5. The intensity distribution is very similar
to the calculation not for the ACP model but for the CDW model, indicating that
2 contains the CDW type atomic displacement. Incident energy dependence of the
diffuse intensity has also been examined [
73
]. Figure
9.21a
shows the ratio of the
diffuse intensity at (0 6.5 0) to that at (0 7.5 0) around the Pt
L
III
absorption edge as
well as the calculated spectrum for the A-type (CDW-type) structure. Furthermore,
a similar experiment around the I
K
-absorption edge at (0 8.5 0) has also been
carried out in Fig.
9.21b
. These results indicate that the CDW-type structure is
realized in the metallic state. These results strongly support the dynamic valence-
ordering model D2 proposed for in the metallic phase of 2 [
32
].
As shown in Fig.
9.17
, the intensity of the diffuse lines observed in 2 decreases
progressively with decreasing temperature below 252 K accompanied by gradual
changes in shape from lines to continuous sheets around
T
M-S
[
32
]. This indicates
that the periodic ordering of the MMX units changes from 2D to 1D. This transfor-
mation in the dimensionality of valence ordering should be associated with a drastic
change in the valence-ordering state. As shown in Fig.
9.17c
, broad undulation in the
intensity of the diffuse sheets gradually converts to weak but distinct Bragg spots
corresponding to superlattice reflections around temperatures between 161 and
133 K. This fact suggests that new lateral correlation among 1D chains is remark-
ably developed and results in a three-dimensionally ordered array of 1D chains. In
3
n
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