Theory of MMX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 257

12.13 Comparison with Experimental Results

In summary, the transition from the metastable CDW phase to the stable CP phase,
which is experimentally realized by photoirradiation under pressure, is reproduced
by exciting a sufficient density of electrons. The threshold increases as the energy
difference between the two phases becomes small. This is explained by drawing
their diabatic potentials. On the other hand, a reverse transition from the CP phase
to the CDW phase is hardly realized. Here are two key factors (1) the characters of
the low-energy charge-transfer excitations in the respective phases and (2) how
easily the respective order becomes long ranged. The difficulty of the CP-to-CDW
transition is due to the fact that the low-energy charge transfer occurs within the
binuclear unit in the CP phase and the fact that the coherence length of the CDW
order is very short.
In iodine-bridged binuclear platinum complexes, R 4 [Pt 2 (pop) 4 I] n H 2 O with cat-
ion R, phase transitions are indeed observed after the photoirradiation for a long
time, 8 ms (for CDW-to-CP) and 30 s (for CP-to-CDW) [ 7 ]. To clarify whether the
transitions are induced by an optical process or a laser heating, the dependence of
the converted fraction on the excitation photon density is investigated for two
excitation energies E ex of 1.96 eV and 2.41 eV [ 43 ].
At 0.25 GPa where the CDW phase is metastable, the CDW-to-CP transition
efficiency exhibits a clear threshold in the photon density per pulse N th ,whichstrongly
depends on E ex : N th ~1.4 10 24 cm 3 at

2.41 eV. The magnitude of N th is large, but the irradiated power of the lights is very
small and the pulse duration is very long. The threshold behavior demonstrates that
the observed transitions are driven not by a laser heating but by an optical process.
At 0.45 GPa where the CP phase is metastable, the CP-to-CDW transition could not
be driven by the irradiation of 1.96-eV or 2.41-eV (nearly equal to the optical gap
energy) light, even if the intensity and duration of the light were changed. However,
irradiation with 2.71-eV light for 30 s did result in a CP-to-CDW transition.
Therefore, the efficiency of the photoinduced CP-to-CDW transition is much
lower than that of the CDW-to-CP transition. The former is observed near the
edge of the hysteresis loop [ 43 ]. Thus, all these experimental facts are consistent
with the present theoretical findings and arguments.

10 25 cm 3 at E ex ¼

1.96 eV and ~ 3

Acknowledgments This work was supported by Grants-in-Aid from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.

References

1. Kurmoo M, Clark RJH (1985) Inorg Chem 24:4420
2. Butler LG, ZietlowMH, Che C-M, Schaefer WP, Sridhar S, Grunthaner PJ, Swanson BI, Clark
RJH, Gray HB (1988) J Am Chem Soc 110:1155
3. Yamashita M, Toriumi K (1990) Inorg Chim Acta 178:143