Crystallographic Studies of Iodide-Containing Quasi-One-Dimensional Conductors

Abstract

The crystal structures of three iodide-containing quasi-one-dimensional conductors, (tetrathiotetracene)2(iodide)3 (high disorder), tetrathiotetracene-iodide, and (hexamethylenetetraselenofulvalene)-(iodide)x were solved by single crystal X-ray diffraction methods. These three iodides are single charge-carrier conductors and allow a comparison of such competing effects as disorder, interchain coupling, and overlap. The crystal structure of metallic (tetrathiotetracene)2(iodide)3 (high disorder), TTT2I3 (h.d.), was solved at room temperature (~294° K), 164° K, 74° K, and at 19° K. At all four temperatures the lattice symmetry remained orthorhombic and the structures were successfully refined in the space group Cmca. During slow cooling the diffuse layer lines were also carefully monitored. In contrast to TTFC10.67, even with slow cooling the iodide chains do not three-dimensionally order, and there are no distortions in the TTT lattice to 19° K. A model of the iodide chains is presented which explains the positions and intensities of the diffuse layer lines and also explains why three-dimensional ordering at low temperatures is not observed. The structure of semiconducting tetrathiotetracene-iodide (TTTI) was studied at room temperature. The structure consists of two ordered lattices which are incommensurate along →c, the stacking axis. The unit cell dimensions for Lattice 1 (triclinic, C-l) are a = 13.028(2), b = 16.445(2), c = 3.643(1) A and α = 90.81(1)°, β = 96.11(1)°, and γ = 91 .11(1)°. For Lattice 2, c ≃ 4.78 A. The positions of all of the layer lines, including the two “sixth” layer lines, which are observed on X-ray oscillation photographs of crystals of TTTI rotated about →c, can be explained by the presence of two lattices. The measured density dm ≃ 2.09 g/cm3 and refinement of the [001] projection (hk0 reflections) confirmed that the overall stoichiometry is TTTI (1:1). For a complete data set collected with copper Kα radiation, the refinement of Lattice 1 converged to R = 0.102. For the 1132 reflections with Fo2 > 3σ(Fo2), R = 0.081. The overlap between adjacent TTT cations in the same stack in TTTI is significantly different from that observed in TTT2I3 (h.d.). There is also very little interchain coupling in TTTI. Hexamethylenetetraselenofulvalene-iodide, HMTSF-Ix, is triclinic, P-l, with the unit cell parameters a = 8.056(4), b = 12.740(4), c = 8.016(3) A and α = 81.72(4)°, β = 67.73(5)°, and γ = 102.64(4)°. For a complete data set of 4213 reflections collected with monochromatized molybdenum Kα radiation to 2θ = 60° the structure refined to R = 0.097. For 2042 reflections with Fo2 > 3σ(Fo2), R = 0.051. The hydrogen atoms were not located. There is disordered iodide and solvent at 1/2,1/2,1/2. The HMTSF cations stack along →a. A new type of alternating overlap between adjacent HMTSF molecules was observed. The magnitude of the d.c. electrical conductivity at room temperature suggests that this phase of HMTSF-Ix is semiconducting. These iodide-containing structures show three different types of iodide behavior in quasi-one-dimensional conductors. In TTT2I3 (h.d.) the slip-stacking and large interchain coupling favor formation of a metallic state at high temperatures. At low temperatures the disordered iodide chains have a major effect on the transport properties by allowing states to exist in the semiconductor band gap. In TTTI the iodides are no longer disordered but still dominate the physical properties by causing a modulation of the TTT lattice. There is very little interchain coupling in TTTI. In HMTSF-Ix the iodide is probably of minor importance.