Charge ordering with lattice distortions in a conductive MMX-chain complex, Pt-2(dta)(4)I (dta = CH3CS2-)

The charge-ordering states with lattice distortions of a halogen-bridged bi nuclear-metal mixed-valence complex (called MMX chain), Pt-2(dra)(4)I (dta = CH3CS2-), have been investigated by transport, magnetic, and optical meas urements. This complex is a binuclear unit-assembled conductor containing m etal-metal bonds. It exhibits metallic conduction above room temperature, r epresenting the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, wh ich is considered to be of the Mott-Hubbard type due to electron correlatio n. The metal-semiconductor transition at 300 K (= TM-S) is derived from a v alence transition of Pt from an averaged-valence state of 2.5+ to a trapped -valence state of 2+ and 3+. The charge-ordering modes are considered to be -I-Pt2+-Pt3+-I-Pt2+-Pt3+-I-Pt2+-Pt3+-I-Pt2+-Pt3+-I-for the semiconducting phase below TM-S and -I-Pt2.5+-Pt2.5+-I-Pt2.5+-Pt2.5+-I-Pt2.5+-I-Pt2.5+-Pt2 .5+-I-for the metallic phase above TM-S. I-129 Mossbauer spectroscopic stud y is reported for a low-temperature insulating phase below 80 K. The low-te mperature electronic structure is considered to be an alternate charge-orde ring state with lattice distortions of -I-Pt2+-Pt3+-I-Pt3+-Pt2+-I-Pt2+-Pt3-I-Pt3+-Pt2+-I-. The present binuclear platinum complex inherently possesse s valence instability of the intermediate valence 2.5+. X-ray photoelectron Spectroscopy and polarized reflection measurements are also reported.