First-principles calculations on the geometric and electronic structures of undoped and doped polypyrrole chains indicate the appearance upon high doping of bipolaron states in the gap. A picture of the band-structure evolution upon doping is presented, showing the presence of bipolaron bands in the gap for experimentally achieved doping levels. A conductivity mechanism based on motion of spinless bipolarons is consistent with the absence of ESR signal in electrochemically cycled highly conducting polypyrrole.Doped organic polymers' constitute a new fascinating area of condensed matter physics, where nonlinear phenomena play an important role and which has several aspects in common with relativistic field theories. 'It has been demonstrated that many peculiar phenomena that occur in trans-polyacetylene, by far the most experimentally and theoretically studied polymer up to now, ' can be explained by the presence of soliton defects on the chain. ' Solitons are possible in trans polyacetyl-ene (PA) as a result of the existence of a degenerate ground state, a consequence of a Peierls dimerization. The observation between l-at. % and -6-at.% doping levels of high conductivity without significant Pauli susceptibility has been interpreted as due to a conductivity mechanism involving the motion of charged solitons carrying no spin.The other polymers usually do not possess a degenerate ground state; they have, so far, received less theoretical attention. ' ' It must be stressed that these polymers nevertheless display transport properties which are very similar to those of trans-PA. A remarkable example has been recently reported in the case of polypyrrole (PPy); it has been observed that in the electrochemically cycled (highly doped) material, high conductivity is obtained, whereas no ESR signal is present. 'In order, on one hand, to interpret that observation and, on the other hand, to propose a coherent theoretical description for a11 doped organic polymers, we report in this paper an ab initio restricted Hartree-Fock (RHF) study of the electronic and geometric structures of undoped and highly sodiumdoped quaterpyrrole (QPy), a model chain for polypyrroie.The key feature of our results is to demonstrate for the first time in the framework of first-principles calculations that high doping induces strong geometric modifications on the QPy chain and that these modifications very naturally lead to the appearance of two states in the gap. These states can be referred to as bipolaron states and, interestingly, are not symmetrically located with respect to the gap center. A picture of the PPy band-structure evolution upon doping is worked out, showing the presence of bipolaron bands in the gap at doping levels comparable to those experimentally achieved.Calculations are performed in the framework of the RHF self-consistent-fieldlinear combination of atomic orbitalsmolecular orbitals (SCF-LCAO-MO) ab initio technique, at minimal Slater-type orbitalthree-Gaussians (STO-36) basis-set level. For computational reasons, sodium has...
The relation between some electrical properties and thermodynamic properties of adsorbed water has been studied for two montmorillonites, a silica gel, and a window glass powder at room temperature. A relationship has been found between electrical conduction and the mobility of water molecules in the monolayer. These molecules are immobile on the hydroxylic surface of silica gel and mobile on the oxygen internal surface of montmorillonite. The electrical conduction in the monolayer is believed to be mainly protonic. The dissociation degree of water molecules in this situation seems to be of the order of magnitude of 1%.
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