Charge Ordering with Lattice Distortions in a Conductive MMX-Chain Complex, Pt₂(<i>d</i><i>ta</i>)₄I (<i>d</i><i>ta</i> = CH₃CS₂^-)

Abstract

The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2-), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= TM-S) is derived from a valence 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 −IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−I for the semiconducting phase below TM-S and −I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I− for the metallic phase above TM-S. 129I Mössbauer spectroscopic study is reported for a low-temperature insulating phase below 80 K. The low-temperature electronic structure is considered to be an alternate charge-ordering state with lattice distortions of IPt2+−Pt3+−I−Pt3+−Pt2+IPt2+−Pt3+−I−Pt3+−Pt2+I. The present binuclear platinum complex inherently possesses valence instability of the intermediate valence 2.5+. X-ray photoelectron spectroscopy and polarized reflection measurements are also reported.