N. S. Murthy

The structure of metallic complexes of polyacetylene with alkali metals

The crystal structures of sodium, potassium, rubidium, and cesium doped polyacetylene have been determined using crystal packing and x‐ray diffraction analyses. Each of these metallic complexes is tetragonal, with the polyacetylene chains forming a host lattice in which the alkali metal ions are present in channels. Lithium appears to be too small to stabilize the channel structure and an amorphous structure is observed. Predicted unit cell parameters and x‐ray diffraction intensities are in agreement with observed values. Similarities with the alkali metal doped graphite suggest that hybridization between carbon pz orbitals and metal s orbitals occurs. Such hybridization is expected to result in a high conductivity component normal to the chain direction. On the other hand, direct overlap between polymer chains appears small, since alkali metal columns separate polymer chains. Compositions calculated for the channel structures (from meridional diffraction spacings, the intensity of equatorial diffraction...

Staging in polyacetylene-iodine conductors

Evidence is presented for the existence of highly conducting polyacetylene complexes with structures related to high‐stage graphite, as well as structures related to first‐stage graphite. X‐ray diffraction measurements on polyacetylene–iodine complexes indicate equatorial lines at 7.7–8.0 and 13.8–14.3 A. The shorter spacing arises in part from a structure in which iodine‐rich planes alternate with planes of polyacetylene chains. The longer spacing, which disappears upon atmospheric exposure, is consistent with a structure analogous to third‐stage graphite in which dopant‐rich planes are separated by three close‐packed planes of polyacetylene chains. The third‐stage complex can be viewed as a perturbation of the structure of undoped polyacetylene, with the region between dopant layers consisting essentially of a one unit cell thickness of the parent polymer structure. Packing calculations for this model, in which a linear column of anions (I3− and/or I5−) displaces either every chain or every other chain ...

Effect of charge transfer on chain dimension in trans‐polyacetylene

X‐ray diffraction measurements on all‐trans‐polyacetylene are consistent with a chain‐axis length elongation upon donor doping (+0.026 A for lithium and +0.04 A for potassium) and a chain‐axis length contraction upon acceptor doping (−0.010 A for iodine), where the changes refer to the length L of a C2H2 unit (2.457 A in the undoped polymer). These new experimentally derived results for heavily doped compositions, which ignore possible corrections for cell nonorthogonality in the lithium and iodine complexes, are similar to experimental results for graphite intercalation complexes and are consistent with theoretical predictions for doped polyacetylene. The meridional diffraction lines observed at L and L/2 for potassium‐doped polyacetylene indicate that there is no lattice symmetry element which includes a translation operation of L/2 in the chain‐axis direction. The observations are consistent with a structural model in which alkali–metal ions with an intracolumn spacing of 4.96 A are commensurate with t...

Structure of polyacetylene–iodine complexes

We confirm the existence of a 15 A period in iodine‐doped polyacetylene and provide a new interpretation for this key feature as part of a general model for structural changes during iodine doping. The observed diffraction intensities for different samples suggest the existence of structures with two different types of dopant‐containing layers: layers obtained by complete replacement of polyacetylene chains by iodine columns (F layers) and layers obtained by replacement of every other polyacetylene chain by an iodine column (P layers). The F layers in the heavily doped complex alternate with dopant‐free layers of polyacetylene chains (U layers), corresponding to a (U F) n stacking sequence. The phase obtained at a lower dopant concentration, which provides the 15 A spacing, is attributed to a (U P U F) n stacking sequence. At still lower dopant concentrations, one obtains a (U P) n stacking sequence. This model, along with published Raman, Mossbauer, and photoelectron spectroscopy data, suggests that the ratio of I− 5 to I− 3 increases in going from P layers to F layers. Intense and monotonically decreasing, diffuse x‐ray scattering suggests that vacancies of size ∼3 A are present, probably in iodine columns. A diffuse reflection at 3.1 A, observed in all iodine‐doped samples, is due to an average iodine–iodine distance in disordered columnar arrays. On the other hand, ordered arrays of iodine columns in oriented samples give rise to sharp meridional reflections. All ten observed reflections (down to 1.17 A) in one sample could be indexed based on a 33.8 A repeat corresponding to (–I− 3–I− 5–I− 3–) n arrays. The observed diffraction pattern was calculated from this model without using any freely adjustable parameters.

Thermal enhancement of the electrical conductivities of alkali metal-doped polyacetylene complexes

Abstract Alkali metal-doped polyacetylene exhibits higher thermal stability than either the parent polymer or acceptor-doped complexes. For the Na, K, Rb and Cs complexes in which a channel structure is indicated, this stability appears to be related to the packing arrangement, which interferes with interchain reactions. In contrast with the case for acceptor doping, the conductivities of the alkali metal-doped cis -polyacetylenes are lower than for correspondingly doped trans -polyacetylenes. However, thermal annealing of the K-doped or Rb-doped polyacetylene (obtained using the naphthalide in THF) results in dramatic enhancement of conductivity for the doped cis -polymer, but not for the doped trans -polymer. After about a six-fold increase in room-temperature conductivity, the conductivity of K-doped polyacetylene is stable for many hours at 200 °C. Cis -polyacetylene films doped in the same way with other alkali metals do not show this enhancement. Significant changes in unit cell parameters, ESR linewidths, electrical anisotropy and contact resistance also occur during the annealing of K-doped cis -polyacetylene. Results presented suggest that thermal annealing of the K-doped polyacetylene eliminates residual conformational defects in partially chemically-isomerized polymer chains, thereby increasing crystalline perfection, increasing the effective conjugation length and increasing the hybridization of alkali metal and carbon orbitals. These changes probably result in the observed conductivity enhancement, the dramatic increase in ESR linewidth (6 – 12 G to 40 – 60 G) and the decrease in unit cell parameter.

Structural changes during annealing and during acceptor doping of oriented poly(p-phenylene sulfide)

Abstract Thermally-annealed, melt-formed poly( p -phenylene sulfide), PPS, has a folded-chain, lamellar structure with a repeat period varying from 90 A to 160 A, depending upon the annealing temperature. Local crystalline order develops in the polymer before the onset of extensive lamellar ordering. On doping oriented PPS with AsF 5 , the polymer swells and loses both crystallinity and chain orientation. The absence of crystalline and lamellar reflections in doped PPS probably results from dopant-induced structural disorder and chemical reactions (formation of dibenzothiophene linkages and crosslinking). These structural changes may explain the relative insensitivity of the conductivity of the doped polymer to the degree of crystallinity and orientation in the precursor polymer.

X-ray diffraction evidence for the formation of a discotic phase during graphitization

Abstract The low-angle peak with a spacing of 10 A observed in X-ray diffraction scans of partially pyrolyzed aromatic molecules is attributed to the formation of a liquid crystalline pre-graphitic phase that we identify as discotic. This conclusion is substantiated using data obtained for partially pyrolyzed 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). The crystalline reflections of PTCDA are replaced by amorphous scattering during pyrolysis at 530°C. In addition to the commonly observed amorphous carbon peak providing an interlayer spacing of about 3.5 A, a low-angle peak corresponding to a 10 A spacing is observed in partially pyrolyzed PTCDA. Based on simulation of structure and diffraction patterns using molecular dynamic calculations, this peak is attributed to the distance between columns of rotationally disordered, partially pyrolyzed PTCDA molecules in a discotic phase.

On the question of metallic and superconducting poly(carbon diselenide)

Polymerization of carbon diselenide, CSe2, at 5 kbar and ∼100 °C has been reported to give a metallic (CSe2)n ladder polymer that superconducts near 6 K at 220 kbar pressure. Using a variety of techniques we find that the material synthesized (showing essentially the same x‐ray diffraction pattern as previously published) is not (CSe2)n but instead consists of a mixture of free, trigonal Se and an amorphous C–Se polymeric composition consistent with the formula (CSe0.5)n. Interestingly, possibly as a consequence of staged reactions during synthesis, the Se phase consists of a mixture of large, aggregated crystallites (∼600 A in diameter) melting at ∼220 °C and nonaggregated small crystallites (∼150–250 A in diameter) showing a broad melting transition with an endothermic maximum at ∼180 °C. Percolation of the C–Se polymeric composition probably provides the high observed electrical conductivity [σ(300 K)≊10–20 S cm−1 and σ(300 K)/σ (8 K)≊2.5] in the presently available samples. The previously reported hig...