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 147

system. Interplay between electronic degrees of freedom and molecular dynamics is
also expected to cause an intriguing structural phase transition accompanying an
electronic and/or magnetic transition never observed for [M 2 (MeCS 2 ) 4 I] 1 (M Pt

(1), Ni (7)). With the elongation of alkyl chains in dithiocarboxylato ligands, the
compounds 3-5 undergo two phase transitions at near 210 K and above room
temperature, indicating the existence of the LT, RT, and HT phases. The periodicity
of crystal lattice in the RT phase of 3-5 along 1D chain is threefold of a -Pt-Pt-I-
unit, and the structural disorders have occurred for the dithiocarboxylato group and
the alkyl chain belonging to only the central dinuclear units in the threefold
periodicity. In the HT phase, the dithiocarboxylato groups of all the dinuclear
units in 3-5 are disordered and the lattice periodicities in 3 and 4 change to
onefold of a -Pt-Pt-I- period. Ikeuchi and Saito have revealed from the heat
capacity measurements that the entropy (disorder) reserved in alkyl groups in the
RT phase is transferred to the dithiocarboxylate groups with the RT-HT phase
transition [ 50 - 52 ]. Whereas, the lattice periodicity of 4 in the LT phase changes
to twofold periodicity being assigned to the ACP state similar to the LT phase
of the compound 2 and the dithiocarboxylate groups of all the diplatinum units
are ordered. Furthermore, accompanying to the RT-LT phase transition, the com-
pound 4 exhibits the paramagnetic-nonmagnetic transition originating from the
regular electronic Peierls transition. These facts suggest that the dynamics
(motional degrees of freedom) of the dithiocarboxylato ligands and bridging iodine
atoms affects the electronic and magnetic systems through the electron-lattice
interaction.
On the other hand, unlike the metallic Pt MMX-chain compounds, all the
Ni MMX-chain compounds are Mott-Hubbard semiconductor due to the
strong on-site Columbic repulsion on the nickel atom. The room-temperature
crystal structures of the compounds 7-10 indicate their valence states to be
an averaged-valence (AV)

¼

state or a charge-polarization (CP) state of

-Ni (2.5 d )+ -Ni (2.5+ d )+ -I -Ni (2.5 d )+ -Ni (2.5+ d )+ -Ni (2.5 d )+ -I -( d :

5) close to
an averaged-valence state. With the elongation of the alkyl chains in dithiocar-
boxylato ligands, the periodicity of crystal lattice in 9 and 10 along 1D chain in
the RT phase is threefold of a -Ni-Ni-I- unit by the same origin as the
diplatinum compounds 3-5, and furthermore, the lattice periodicity of 9 changes
to onefold in the LT phase with a first-order phase transition at 205.6 K. The
high temperature magnetic susceptibilities of 8-10 can be described by an
S ¼

0

1/2 1D Heisenberg antiferromagnetic chain model with the very large
exchange coupling constant | J |/ k B ranging from 898(2) to 939(3) K. Furthermore,
the compounds 8 and 9 undergo a spin-Peierls (SP) transition at relatively high
T sp ¼ 47 and 36 K, respectively, which are accompanied by superlattice

reflections corresponding to twofold of a -Ni-Ni-I- period below T sp .The
synchrotron radiation crystal structure analysis of 8 at 26 K revealed that the
valence-ordered state changes from the CP state in the RT phase to the ACP
state in the SP phase. These facts demonstrate that the electronic system of the
Ni MMX-chain compounds in which the on-site Columbic repulsion U plays a
dominant role in determining the electronic system is hardly affected by the
molecular dynamics.