Photoinduced Phase Transitions in MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 232

4

a b

E i // ch ain axis

up

up

0 GPa

0 GPa
0.45

0.45

2

P c

3

0.49
1.15
1.76
2.25

0.49
1.15
1.76
2.25

2

1.84
1.18
0.74
0.43
0.25
0.14

down

down

1

1.84

1.18
0.74
0.43

1

0.25

0.14

P c

0 GPa

0 GPa

0 0

1 2 3 1 2 3

Photon energy (eV)

Fig. 11.6 Pressure dependences of (a) polarized reflectivity spectra and (b) optical conductivity
of [(C 2 H 5 ) 2 NH 2 ] 4 [Pt 2 (pop) 4 I] in the pressure-applying and pressure-decreasing runs

From the photographs of the crystal, the reduction of the sample size along the
chain axis c was observed. Figure 11.8 shows the magnitude of reduction of sample
size along the chain axis as a function of the pressure. As shown in the figure, the
sample size along the chain axis is reduced by about 8 % at 0.85 GPa compared to
ambient pressure. If the Pt-Pt distance d (Pt-Pt) of the Pt-Pt unit is assumed to
remain constant under pressure, d (Pt-I-Pt) at 0.85 GPa is estimated to be 6.2 ˚ .
This assumption is reasonable because d (Pt-Pt) is almost constant in various
MMX-chain compounds. The obtained values, d (Pt-I-Pt) 6.2 ˚

1.6
eV at 0.85 GPa, are very close to those in CDW state (see Fig. 11.4 ). These
observations support the fact that the pressure-induced transition from CP to CDW
state is driven by a decrease in d (Pt-I-Pt) in [(C 2 H 5 ) 2 NH 2 ] 4 [Pt 2 (pop) 4 I].

¼

and E gap ¼

11.3.3 Photoinduced Phase Transition in PtPtI-Chain
Compounds

At the points A and B within the hysteresis loop shown in Fig. 11.7a , the material is
in a meta-stable state as illustrated in the energy potential curves (insets). The PIPT