Identification of trihalide anions in bis(ethylenedithio)tetrathiafulvalene salts by Raman spectroscopy

Abstract

In order to distinguish individual contributions of trihalide anions in β-like structures of the (BEDT-TTF)2X organic conducting single crystals [where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X stands for I3, I–I–Br, Br–I–Br, Br–I–Cl, Cl–I–Cl or mixed trihalide anions], low frequency Raman spectra have been recorded at 25 K. This technique permitted us to establish a presence of a ternary mixture of I3, I–I–Br, and Br–I–Br anions in one of the nonstoichiometric salts and an existence of Br–I–Br, Br–I–Cl, and Cl–I–Cl anions in another one. It is not possible to detect these anions independently using the x-ray diffraction method. The ab initio calculated interatomic bond lengths and vibrational frequencies of trihalide anions are in good agreement with experimental data, which confirmed the assignment of observed low-frequency Raman bands. At 80 K the Raman spectra were taken for a fixed linear polarization of the incident laser light, and the examined single crystals were rotated in π/8 increments. In this way, an anisotropy of trihalide anion’s polarizability has been investigated to confirm the collinear arrangement of anions and the proposed assignment of Raman bands.