X-ray study of the order-disorder phase transition in the triiodide chain compound tetraphenyldithiapyranylidene iodide

Abstract

Tetraphenyldithiapyranylidene iodide [DIPS${\ensuremath{\Phi}}_{4}$(${\mathrm{I}}_{3}$${)}_{0.76}$] is a one-dimensional (1D) conductor in which an organic matrix separates parallel channels filled with triiodide ${\mathrm{I}}_{3}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$ anions. At room temperature, the triiodide chains present an important structural disorder characterized by the absence of long-range order along the channels and of lateral correlations between the different chains. A model of a one-dimensional harmonic liquid has been successfully used to account for the observed x-ray scattered intensity. As the temperature is lowered, coupling between the chains develops, leading to a 3D ordered state below ${T}_{c}$=182 K. The ordering process has been studied with respect to temperature by x-ray techniques and the low-temperature structure of the iodine sublattice determined. The order which establishes below ${T}_{c}$ is described by the Fourier components of the ionic density along a channel. They form the order parameters of the system. The temperature dependence of the first five components has been measured. They exhibit different temperature behaviors, showing the unusual character of this structural phase transition. The existing theory accounts only qualitatively for these results. The structural evolution has been correlated with the electrical properties of the material which has a Peierls-like phase transition around ${T}_{c}$. Starting from the fact that the first Fourier component of the iodine density corresponds to the 2${k}_{F}$ wave vector of the organic stack, a new explanation of the phase transition is proposed.