Crystal Structures and Properties of MMX-Chain Compounds Based on Dithiocarboxylato-Bridged Dinuclear Complexes - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 167

transition. This behavior is consistent with the tendency observed in the heat
capacity measurement and indicates that the phase transition is monotropic [ 52 ].
As shown in Fig. 9.16b , the thermoelectric power S in the first cooling process
increases with lowering the temperature and exhibits a maximum at 235 K before
rapidly increasing with the first-order RT-LT phase transition near 205 K. On first
heating, S changes through the same pathway as the first cooling process, and
then exhibits an increase at the RT-HT phase transition in the temperature range
of 330-344 K. Upon second cooling from 400 K, S does not show the rapid decrease
associated with the RT-HT phase transition and exhibits a round maximum near
260 K. The observed irreversibility is consistent with the tendency observed in the
heat capacity and resistivity measurements. On the other hand, Guijarro et al. have
reported the electrical transport property of the micrometer-length fibers of 5 with
the typical height of ca. 1.5-2.5 nm formed by casting deposition on mica from
sonicated diluted tetrahydrofuran (THF) solution [ 67 ].

9.2.6 X-Ray Diffuse Scattering

As already described in Sect. 9.2.2 , the bridging iodine atom in [Pt 2 (EtCS 2 ) 4 I] 1 (2)
is located near the midpoint between two diplatinum units. This indicates valence
states of the platinum atoms of 2 are considered to be an averaged-valence state of
+2.5. However, it is well known that X-ray crystal structure analysis gives only a
time and spatially averaged structure. On the other hand, the XPS spectrum of 2
revealed that the diplatinum complex adopts the Pt 2+ -Pt 3+ mixed-valence state [ 32 ].
Furthermore, the compound 2 is expected to exhibit structural distortion even in the
metallic state, since the 1D halogen-bridged mixed-valence platinum complex is a
typical system having strong electron-lattice interactions. X-ray diffuse scattering
techniques are used to examine subtle periodic lattice distortion [ 68 - 72 ]. To obtain
the information for the crystal structure, including the periodical arrangement of
platinum valences on the 1D chain system, X-ray diffuse scattering has been
studied for 2-4 [ 32 , 34 , 35 , 54 ].

9.2.6.1 [Pt 2 (EtCS 2 ) 4 I] 1 (2)

The X-ray diffraction photographs taken on the different reciprocal planes between
206 and 297 K are shown in Fig. 9.17 [ 32 ]. As shown in Fig. 9.17a , the X-ray
diffraction pattern exhibits weak but sharp diffuse lines at the midpoint between
layers of Bragg reflections, i.e., at the reciprocal lattice positions in k ¼ n + 0.5 ( n ;
integer). The intensity of diffuse scattering increases with increasing scattering
angles. This strongly suggests that diffuse scattering arises from a displacive
modulation of heavy atoms, i.e., static or dynamic distortion of the platinum and/
or iodine positions. When the electron density per unit cell with lattice constant b

is 1, the Fermi wave number of the electron is k F ¼ rb */4

r

¼ b */4, where b * ¼ 2 p /