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Fig. 9.17 Portions of X-ray diffraction photographs of [Pt 2 (EtCS 2 ) 4 I] 1 (2) taken at different
temperatures and crystal orientations with a fixed-film and fixed-crystal method [ 32 ]. The b * axis
is vertical in these figures. The sample is oriented as (a) the a * and b * directions perpendicular to
the incident X-ray beam, (b) the b * direction perpendicular to the incident X-ray beam and the a *
direction parallel to it, (c) the b * direction perpendicular to the incident X-ray beam and the c *
direction parallel to it
b is the reciprocal lattice vector, and the reciprocal-lattice vector 0.5 b *
coincides with the 2 k F wave vector. Therefore, the observed diffuse scattering
with twofold repetition length of a -Pt-Pt-I- period can be associated with
-Pt 2+ -Pt 2+ -I- Pt 3+ -Pt 3+ -I- (2 k F -CDW) or -Pt 2+ -Pt 3+ -I-Pt 3+ -Pt 2+ -I- (2 k F ).
We have proposed a possible valence-ordering model in the metallic state based on
the twofold periodical ordering and the average temperature factor U 22 of the platinum
and iodine atoms along the 1D chain direction [ 32 ]. The crystal structure analyses of 2
have revealed that all the Pt-Pt distances are crystallographically identical, that the
two Pt-I distances are almost equivalent within experimental errors, and that U 22 of
the iodine atom becomes exceptionally large above T M-S . The exceptionally large
temperature factor of the iodine atom along the 1D chain in the metallic state suggests
the dynamic valence-ordering models, rather than the static ones, shown in Fig. 9.18 ,
in which the valence fluctuation and phonon vibration are coupled with each other.
If the bridging iodine atom of the MMX-chain compound is shifted from being bound
to Pt 3+ to a position close to the neighboring Pt 2+ , electron transfer from Pt 2+ to Pt 3+
will be induced. This electron transfer is a result of the strong coupling between the
coordination geometries of the Pt complexes and their valence states, the so-called
electron-phonon coupling. The D1 model is the iodine vibration mode but does not
correspond to twofold periodical ordering. The D3 model is the vibration mode of the
diplatinumunit, whereas D2 is themode in which the stretching of Pt-Pt bonds and the
shifts of the bridging iodine atom positions are strongly coupled. Although the D3
model can reproduce the twofold periodical ordering, the large U 22 of Pt rather than
that of I must be expected. From the viewpoint of twofold periodical ordering and the
large U 22 of the iodine atom, the model D2 appears to be the most appropriate model in
representing the valence-ordering state of 2 in the metallic state. On the other hand, the
diffraction pattern in Fig. 9.17b exhibits distinct spots on diffuse lines. Considering
both the diffraction patterns, it can be determined that the diffuse lines are extended
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