Crystallographic Order and Disorder in Quasi-One-Dimensional Conductors

Citation

Williams, Roger Macauley (1979) Crystallographic Order and Disorder in Quasi-One-Dimensional Conductors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FPJF-PM42. https://resolver.caltech.edu/CaltechTHESIS:12102018-143101993

Abstract

Stacked, conducting isocyanide complexes of rhodium(I) were synthesized and characterized by x-ray diffraction and electrical measurements. Good crystals of orthorhombic [Rh(CNCHCH₂)₄]ClO₄, with a₀ = 8.81 Å, b₀ = 22.85 Å, c₀ = 12.70 Å, were obtained and a structural refinement was carried out in space group Immm. The rhodium chain is nearly uniform (Rh-Rh = 2.94 Å) and ligands and anions show considerable disorder. The room temperature conductivity is -2 Ω^-1 cm^-1 and conductivity is activated. The moderately high conductivity of [Rh(CNCHCH₂)₄]ClO₄ is due to the presence of a low-lying conduction band rather than a non-integral rhodium oxidation state.

The disorder, tetragonal phase of (TTF)Cl_x, a₀ = 11.19 Å, c₀ = 3.60 Å, was studied for compositions x = 0.67, x = 0.70. A room temperature structural refinement in space group P4₂/mnm revealed eclipsed stacking of TTF cations and extremely high disorder of chlorides in channels. Low temperature studies revealed ordering of chloride ions for both compositions. (TTF)Cl_0.67 undergoes an incomplete structural transition to a monoclinic symmetry phase at ~250° K. Ordering of chloride ions occurs at the same temperature. Fast cooling (>1° K/hr) results in peak broadening which is apparently due to the very small size of diffracting domains within the crystal. Both small domain size and the inequivalent environments of TTF cations following chloride ordering may contribute to the drop in conductivity observed at the phase transition.

Structural refinements of both the subcell (space group Cmca, a₀ = 18.47 Å, b₀ = 4.95 Å, c₀ = 18.30 Å) and full cell (space group Pmc2_l, a₀ = 18.47 Å, b₀ = 9.90 Å, c₀ = 18.30 Å) of a low disorder crystal of TTT₂I₃ were carried out. The iodine chain is highly disordered and all sites have less than full occupancy. The presence of I₃^- and I₂ species in the chain is likely. The resulting aperiodic potential due to the iodide chain may be expected to be retained at low temperature and inhibit a metal-to-insulator transition.

Electrochemical crystal-growth experiments involving TTF, TMTSF, and HMTSF gave successful results with the first two donors. Crystals of (TMTSF)Br_0.8 and (TMTSF)(SCN)_0.5 are isostructural, although the latter exhibits satellite reflections (period = 4.6 x c₀) in diffraction patterns. Both structures were refined in space group Cmcm, and the satellite data of (TMTSF)(SCN)_0.5 was modeled in space group Cmc2_l. Unit cell parameters are a a₀ = 9.798 Å, b₀ = 23.837 Å, c₀ = 7.095 Å, for the bromide and a₀ = 9.919 Å, b₀ = 24.124 Å, c₀ = 7.220 Å, for the thiocyanate. The planes of these cations are perpendicular to the z-axis and consecutive cations slip back and forth by ~1.3 Å to reduce methyl group steric repulsion.

All of the systems studied are single carrier conductors with conduction along the cation stack and high disorder of anions. The nature of the disorder and its relationship to phase transitions, as well as interchain coupling and stacking in the cation chain, were evaluated in these compounds. Comparison of (TTF)Cl_0.67 and (TTT0₂I₃ were especially useful, as both exhibit comparably very short range order of halide ions at room temperature, but different cation stacking arrangements (eclipsed and slipped, respectively) and hence different interchain coupling and electronic bandwidth. Structural studies at room temperature and low temperature provided an opportunity to understand the important differences in the electrical properties of these two materials.

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