J.P. Pouget

Recent structural investigations of metallic polymers

Abstract Recent structural studies of the undoped and iodine-doped polyacetylenes (prepared by the Tsukamoto and co-workers method), PF 6 − -doped polypyrrole and polyaniline emeraldine base and (hydrochloric acid and camphorsulfonic acid) salts are presented. Some key features of the polymer chain array and its evolution with the conditions of preparation are emphasized. Special attention has been devoted to characterization of the various sources of structural disorder in the relationship with high conductivity and related properties exhibited by these three families of polymers. Variation in the structural order in both crystalline and amorphous regions correlates directly with the electrical and optical properties of the metallic state.

Structural investigation of metallic PAN-CSA and some of its derivatives

Abstract Structural aspects of PAN-CSA, POT-CSA and PAN-MSA prepared with m-cresol are compared. In relationship with the electronic properties measured on the same materials we emphasize that the geometry of the polymer chain packing, the disorder and the interchain separation have a key influence on the charge conduction process.

Inhomogeneous disorder and the modified Drude metallic state of conducting polymers

Abstract The metallic state of the very highly conducting doped polymers, such as polyacetylene, polypyrrole and polyaniline, is shown to have remarkable similarities. Though each of these doped polymers has a different temperature-dependent conductivity, each of these doped polymer systems has a similar metal-like Pauli susceptibility and density of states at the Fermi level. Also, each of these highly conducting systems displays a universal electronic response (dielectric constant and conductivity) as a function of frequency from the microwave regime (10 9 Hz) through to the ‘all conduction electron plasma frequency’ (1 to 3 eV, depending on the polymer). This common behavior despite apparent differences in the temperature-dependent conductivities is proposed to be a consequence of the inhomogeneous crystalline order in these materials, leading to three-dimensional metallic domains or islands (crystallographically coherent regions), separated by poorly conducting weak links (disordered regions and interfiber links).

Hidden Fermi Surface Nesting and Charge Density Wave Instability in Low-Dimensional Metals

The concept of hidden Fermi surface nesting was introduced to explain the general observation that certain low-dimensional metals with several partially filled bands exhibit charge density wave (CDW) instabilities, although their individual Fermi surfaces do not reveal the observed nesting vectors. This concept was explored by considering the Fermi surfaces of the purple bronze AMo6O17 (A = sodium or potassium) and then observing the CDW spatial fluctuations expected from its hidden nesting on the basis of diffuse x-ray scattering experiments. The concept of hidden Fermi surface nesting is essential for understanding the electronic instabilities of low-dimensional metals.

X-Ray Diffuse Scattering Study of Some (TMTSF)2X and (TMTTF)2X Salts

Abstract X-ray diffuse scattering experiments on the superconducting salts (TMTSF)2X with X = PF6, AsF6 and ClO4 reveal no charge density wave instability, in contrast with the poorly conducting (TMTTF)2PF6 salt. New structural phase transitions coinciding with conductivity anomalies are found in (TMTSF)2NO3, (TMTSF)2ReO4, and (TMTTF)2ClO4. It is suggested that they correspond to order-disorder transitions involving the non-centrosymmetric counter ions. Relationships between these structural instabilities and superconductivity are also discussed.

X-ray evidence of charge density wave modulations in the magnetic phases of (TMTSF)2PF6 and (TMTTF)2Br

Abstract We have performed X-ray scattering investigations of (TMTSF)2PF6 and (TMTTF)2 Br in their magnetic ground states. Very weak 2kF and 4kF satellite reflections are observed in the spin density wave (SDW) ground state of (TMTSF)2PF6. They are probably the signature of a purely electronic charge density wave (CDW) modulation of about one tenth of electron per molecule. Our X-ray results together with magnetic measurements show that the ground state of (TMTSF)2PF6 will exhibit an unprecedented 2kF mixed CDW-SDW modulation. 4kF satellite reflections are observed in the antiferromagnetic (AF) ground state of (TMTTF)2Br. In this salt the 4kF CDW modulation is probably set by magnetoelastic coupling of the lattice degrees of freedom with the AF modulation.

Concerning the band structure of D(M(dmit)2)2 (D=TTF,Cs,NMe4); M=Ni,Pd) molecular conductors and superconductors: Role of the M(dmit)2 Homo and Lumo

Abstract We report tight-binding band structure calculations showing that the partially filled band of D[M(dmit) 2 ] 2 molecular conductors containing dimeric building blocks can originate either from the LUMO (lowest unoccupied molecular orbital) or the HOMO (highest occupied molecular orbital) of the M(dmit) 2 acceptor, depending upon the strength of the intra-dimer transfer integrals (t HOMO and t LUMO ) with respect to the HOMO-LUMO splitting. The first situation is found for D=Me 4 N and M=Ni whereas the second one is found for D=Cs or TTF ( δ phase) and M=Pd. Both the HOMO and LUMO bands are partially filled in the TTF[Ni(dmit) 2 ] 2 and α′-TTF[Pd(dmit) 2 ] 2 salts which contain M(dmit) 2 stacks with uniform chains. The structural reasons behind these surprising results are explained.

Structure of highly conducting PF6−-doped polypyrrole

Abstract The structure of highly conducting PF 6 − -doped polypyrrole, the room-temperature conductivity of which is ≈ 1500 S/cm, is studied using X-ray ‘monochromatic’ Laue photographic methods. The ‘crystalline’ domain size along the polymer chain is estimated to be less than 20 A, while adjacent chains are rather well ordered with spacing of 3.4 A, suggesting that the system can be of two-dimensional nature. The influence of the structural properties on the high electrical conductivity is discussed.

The [(DT‐TTF)2M(mnt)2] Family of Radical Ion Salts: From a Spin Ladder to Delocalised Conduction Electrons That Interact with Localised Magnetic Moments

Electrocrystallisation of the p-electron donor dithiopheno-tetrathia- fulvalene (DT-TTF) with maleonitrile dithiolate (mnt) - metal (M) complexes gives rise to the new family of radical ion salts ((DT-TTF)2M(mnt)2 )( MaAu, Pt, Ni), which are isostructural and crystal- lise in the monoclinic space group P21/n forming regular segregated stacks of donor and acceptor molecules along the b axis. The DT-TTF stacks are paired and interact strongly through S ··· S contacts in a ladder-like motif. The three salts have quite high room-temperature electrical conductivities (9, 40 and 40 S cm ˇ1 for Ma Au, Pt and Ni respec- tively) but their conductivity - tempera- ture dependencies differ. The Au salt has an activated conductivity at room temperature whereas the Ni and Pt salts are metal-like at room temperature and both exhibit a metal - insulator transi- tion around 120 K. These contrasting transport properties are accounted for by the differences in the transfer inte- grals along the DT-TTF stacks. The magnetic susceptibility of the salt with Ma Au, in which the (Au(mnt)2 ˇ ) anion is diamagnetic, can be fitted to a two- legged spin-ladder model. From diffuse X-ray scattering studies it is established that below 220 K the donors dimerise along the b stacking direction, and the spin carrier units in the ladder are identified as those formed by dimerised donors ((DT-TTF)2) a . . Observation in their EPR spectra of a single line which increases dramatically in width as the conductivity increases is evidence for the presence of two magnetic subsys- tems which interact in the salts (Ma Ni, Pt) with paramagnetic (M(mnt)2) ˇ ions.

X-ray structure of polyanilines

Abstract We describe two classes of the emeraldine form of polyaniline, Class I exhibits ES-I crystal structure in the HCl salt form and in the base form it is amorphous EB-I. Class II materials form partially crystalline structures EB-II and ES-II in base and HCl salt forms, respectively. We compare structural results on emeraldine with data for partially crystalline leucoemeraldine base (LEB) and pernigraniline base (PNB) — the fully reduced and oxidized forms of polyaniline, respectively. We contrast our studies of the EB-I/ES-I system with results for poly( o -toluidine) base, POT-EB and hydrochloride, POT-ES.