Photoinduced Phase Transitions in MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 235

a b

CDW phase

CP phase

2 2
Pt 2+

3 3 2 3

Pt 3+

2 3

Pt 2+ I Pt 3+

Pt 3+

I

Pt 2+

I I I Pt 2+ Pt 3+ I

t MM , V MM

t MXM , V MXM

CT

CT

2 3
Pt 3+

2 3

3 2
Pt 2+

2 3

Pt 2+

I I Pt 2+ Pt 3+ I Pt 3+ Pt 2+

Pt 3+ I I I

Fig. 11.10 Schematic illustration of the lowest-energy optical transitions in the CDW phase (the
high-pressure phase) (a) and in the CP phase (the low-pressure phase) (b)

shown in Fig. 11.7b (ii). As this process appears to be strongly dependent on the
excitation energy, the PIPT from CP to CDW state is also attributable to an optical
process, although the efficiency of the PIPT from CP to CDW state is much lower
than that from CDW to CP state.
Finally, let us briefly discuss the difference of the photoconversion efficiency
between the CDW to CP and the CP to CDW transitions [ 18 , 19 ]. In Fig. 11.10 ,
the optical transitions of the MMX-chain compound in the CDW and CP
ground states are schematically illustrated. The lowest optical transition in CDW
state is the interdimer CT excitation from [-I-Pt 2+ -Pt 2+ -I-Pt 3+ -Pt 3+ -I-] to
[-I-Pt 2+ -Pt 3+ -I-Pt 2+ -Pt 3+ -I-]. By taking into account the lattice relaxations due
to the halogen displacements, the 2+3+2+3+ valence state will be stabilized in a
photoexcited state as shown in Fig. 11.10a . Namely, in the CDW state, an optical
excitation will produce locally a CP state. This is the reason why the transition from
CDW to CP state is easily induced by lights. In the CP state, it has been establi-
shed that the intradimer CT transition from [-I-Pt 2+ -Pt 3+ -I-Pt 2+ -Pt 3+ -I-] to
[-I-Pt 3+ -Pt 2+ -I-Pt 2+ -Pt 3+ -I-] shown in Fig. 11.10b is the dominant optical excita-
tion corresponding to the optical gap transition. The intensity of the interdimer
transition is smaller than that of the intradimer one. In this intradimer transition
process, the CDW state is never produced, even as a local excited state, as shown in
Fig. 11.10b . As a result, the CP to CDW transition is difficult to be achieved by the
photoirradiation with the energy of 2.41 eV nearly equal to E gap . As observed in the
experiments, the 2.71-eV excitation drives the PIPT from CP to CDW state,
although the transition efficiency is very low. Such a higher-energy excitation
will induce the interdimer CT transition from [-I-Pt 2+ -Pt 3+ -I-Pt 2+ -Pt 3+ -I-] to
[-I-Pt 2+ -Pt 2+ -I-Pt 3+ -Pt 3+ -I-], which may be relevant to the PIPT to CDW state.