Structural and transport properties of the incommensurate organic superconductor (MDT-ST)(I3)0.417

The incommensurate anion sublattice and transport properties of the organic superconductor (MDT- ST) I1+δ [MDT-ST: 5H -2-(1,3-dithiol-2-ylidene)-1,3- diselena-4,6-dithiapentalene] are investigated. The Raman spectra as well as the incommensurate spots observed in the x-ray-oscillation photograph provide clear evidence that the polyiodide chain is composed of I3- units. From these, the composition is determined to be (MDT-ST) (I3) 0.417, and the degree of charge transfer is 0.417. The thermoelectric power shows large negative phonon drag at low temperatures in spite of the clean metallic resistive behavior. That the fundamental Raman line of the I3- chains is split into two lines below 150 K without any extra satellite reflections in the x-ray photographs suggests the possibility of a small structural change in the anion lattice