Optical studies of the quasi-one-dimensional charge-density-wave state in [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$ (en=ethylenediamine) under hydrostatic pressure

We have measured optical absorption, photoinduced absorption, and Raman scattering in the quasi-one-dimensional mixed-valence complex [Pt(en${)}_2$][Pt(en${)}_2$ ${\mathrm{Cl}}_2$](${\mathrm{Cl}0\mathrm{}}_4$ ${)}_4$ (where ‘‘en’’ is an abbreviation for ‘‘ethylenediamine’’), under hydrostatic pressure up to 3 GPa at temperatures in the range 110–300 K. Analyzing the results by use of the Morse potential, we have evaluated the Peierls gap $E_g$, the electron-phonon coupling constant β, the Peierls distortion of Cl ions, u, and the supertransfer energy $t_0$: $E_g$=3.0 eV, β=1.94 eV/Å, and $t_0$=0.52 eV at atmospheric pressure; $E_g$, β, and ‖u‖ decrease with increasing pressure because $t_0$ is enhanced significantly by the contraction of the Pt-Pt & electrons of Pt ions are delocalized. It has been established that the intragap absorption which is enhanced by pressure and photoexcitation arises from the soliton-to-band transitions mediated by neutral solitons with a coherence length of only one Pt-Pt distance: The absorption band exhibits two peaks, A and B, at 1.68 and 2.0 eV, corresponding to the van Hove singularities of quasi-one-dimensional conduction and valence bands. Such a small coherence length of the soliton is crucial for an understanding of this novel double-peak spectrum.