Soliton-to-Band Optical Absorption in a Quasi One-DimensionalPtII-PtIVMixed-Valence Complex under Hydrostatic Pressure

A midgap absorption band is observed in the quasi one-dimensional semiconductor [Pt${\mathrm{}\left(\mathrm{en}\right)\mathrm{}}_2$]-[Pt${\mathrm{}\left(\mathrm{en}\right)\mathrm{}}_2$ ${\mathrm{Cl}}_2$]${\left(\mathrm{C}\mathrm{l}{\mathrm{O}}_4\right)}_4$ under hydrostatic pressures at room temperature, where en is ethylenediamine. The transition is allowed only for the polarization parallel to the -Cl-${\mathrm{Pt}}^{\mathrm{II}}$-Cl-${\mathrm{Pt}}^{\mathrm{IV}}$- chain. The peak position remains near the middle of the Peierls gap at any pressure up to 2.2 GPa. The intensity increases exponentially with the peak shift. The gap states responsible for this band are attributed to soliton excitations corresponding to kinks of the charge-density wave.