Pressure-induced metal-semiconductor-metal transitions in an MMX-chain complex, Pt₂(C₂H₅CS₂)₄I

The electrical conductivity and X-ray diffraction measurements were performed for a highlyconductive halogen-bridged binuclear-metal mixed-valence complex (the so-called MMX chain), Pt₂(C₂H ₅CS₂)₄I, under high pressure up to 2.5 GPa. The complex exhibited pressure-induced metal-semiconductor-metal transitions at 0.5 and 2.1 GPa. The X-ray diffuse scatterings were observed at k = n+0.5 (n: integer) under ambient pressure, which are derived from the charge-density wave (CDW: ⋯Pt²⁺-Pt²⁺⋯I-Pt³⁺-Pt ³⁺-I⋯) fluctuation in the MMX chain. Above 0.5 GPa, where the pressure-induced metal-semiconductor transition occurred, these scatterings disappeared. The electronic phases under high pressure (P) were found to be attributable to the metallic averaged-valence state (AV: -Pt^2.5+- Pt^2.5+-I-Pt^2.5+-Pt^2.5+-I-) with CDW fluctuation of P < 0.5 GPa, semiconducting charge-polarization state (CP: ⋯Pt²⁺-Pt³⁺-I⋯Pt²⁺-Pt ³⁺-I⋯) of 0.5 < P < 2.1 GPa, and metallic AV state of P > 2.1 GPa. The electronic state of Pt₂(C₂H ₅CS₂)₄I is very sensitive to pressure, which implies that the phase competition among the CP, CDW, and AV phases is present in the MMX chain.