A. Novak

Proton transfer and superionic conductivity in solids and gels

Abstract Protonic conduction (σ) is a particular case of ionic conduction and can be characterised by the pre-exponential factor (σ0) and activation energy (Ea) in the expression σT = σ0exp - (Ea/kT). A correlation between σ0 and Ea for different types of protonic conductors such as hydrates, anhydrous compounds, ionic and superionic conductors is given, and various materials are classified according to their principal conductivity mechanism. In order to understand these mechanisms, it appears necessary to determine the structure of the rigid framework as well as thet of potentially mobile protonic species and the dynamic and conducting properties of the latter in the very broad frequency range 105–1013 Hz. In the identification of protonic species and in the determination of their configuration, vibrational (optical and neutron) spectroscopy appears particularly efficient; it is also useful in determining the nature and degree of structural disorder. The complex impedance spectroscopic methods, quasi-elastic neutron scattering and proton magnetic resonance, on the other hand, contribute to a better knowledge of proton dynamics. Such structural and dynamic information has been obtained for various materials representing different types of protonic conduction: the ion jumps mechanism in anhydrous and hydrated lattice is illustrated by oxonium (ammonium) β alumina and a low-temperature phase of hydrated uranyl phosphate (HUP), respectively; the quasi-liquid state, in the lattice and on the surface, by the high-temperature phase of HUP, H+(H2O)nβ (β″) alumina and Zr (HPO4)2·nH2O; and the proton transfer assisted by orientational disorder of the rigid framework by H3OClO4 and Cs-HSO4. The influence of the partial water pressure and electrical field on the electrical properties of protonic conductors via proton transfer between mobile species and rigid framework is also discussed.

Infrared and Raman study of polyaniline Part II: Influence of ortho substituents on hydrogen bonding and UV/Vis—near-IR electron charge transfer

Abstract IR and Raman spectra of chemically (electrochemically) prepared protonated forms of polymers of 2-methyl (PANIME), 2-methoxy (PANIMEO), 2-chloro (PANICL) and 2-fluoroanilines (PANIF) and their corresponding bases have been investigated in the 4000−100(200)cm−1 region. Exciting lines at 457.94, 514.53, 632.81 and 1064 nm have been used for the investigation of Raman resonance spectra in order to characterize the benzenoid and (semi)quinoid (SQ) parts of the repeat unit. Highly conducting poly-2-methyl and poly-2- methoxyaniline on the one hand show spectral features which can be interpreted in terms of a strong NH⋯N hydrogen bond (dNN ≈ 0.25 nm) with a NH stretching broad absorption centred near 1300 cm−1 cut by numerous Evans transmission bands. On the other hand, poly-2-chloro and poly-2-fluoroaniline which have much lower conductivity have much weaker NH⋯X hydrogen bonds characterized by NH stretching bands near 3000 cm−1 (dNN ≈ 0.28/0.285 nm). Conductivity mechanisms are discussed. UV/Vis—IR spectra show that conductivity is essentially associated with interchain coupling. IR spectra suggest that in PANIME and PANIMEO as in PANI the interchain charge carriers constitute bipolaronic species, i.e. −NH+SQNH+−, cross-linked by NH⋯N hydrogen bonds. In weakly conducting PANIF and PANICL, however, disproportionation of the bipolaronic species, no longer stabilized by H-bonding, leads to localized polaronic moieties. Possible mechanisms for proton conduction are discussed.

Equilibrium of the protonic species in hydrates of some heteropolyacids at elevated temperatures

Abstract Hydrates of three heteropolyacids: 12-molybdophosphoric H3PMo12O40·nH2O (MoPA·nH2O), 12-tungstophosphoric H3PW12O40·nH2O (WPA·nH2O) and 12-tungstosilisilic acid H4SiW12O40·nH2O (WSiA·nH2O) were examined by using thermal analysis, infrared spectroscopy and impedance measurements. These compounds are known as superionic proton conductors and their behaviour was examined as a function of temperature. At the TGA and DSC curves, in the 300–500 K temperature range, transformations were observed at several temperatures. These transformations are partially due to the dehydration process and partially to the change of equilibrium of protonic species (H3O+, H2O and OH- and their possible association: H5O+2, H7O+3, ...), as shown by thermal analysis and infrared spectroscopy.

Infrared and Raman study of polyaniline Part I. Hydrogen bonding and electronic mobility in emeraldine salts

Abstract Infrared and Raman spectra of chemically prepared (doped) protonated forms of polyaniline and its deuterated ND and —C 6 D 4 —derivatives have been examined. Broad infrared absorption centered near 1100 cm −1 and cut by numerous Evans holes has been observed and interpreted as an NH stretching band of a strong asymmetric interchain NH + ⋯ N hydrogen bond (d N-N ≈ 2.5 A). Evans holes, which have corresponding Raman bands, have been assigned to various benzenic and semiquinoidal vibrations. A model of highly conducting polyaniline taking into account the proton transfer is proposed and discussed. The inversion of the NH + ⋯ N hydrogen bond leads to interchain conversions which can generate charge carriers.

Phase transitions in superionic protonic conductors CsHSO4 and CsHSeO4

Abstract CsHSO4, CsDSO4 and CsHSeO4 crystals have been investigated by calorimetry, infrared and Raman spectroscopy and inelastic neutron scattering in the 100–500 K temperature range. Three phases have been shown to exist for CsHSO4 (CsDSO4) and four for CsHSeO4 and the corresponding transition temperatures and enthalpies are given. Spectroscopic results show that heating induces a progressive structural disorder in these crystals. The Raman spectra of the high conductivity phase are similar to those of plastic phase implying a free rotation of HSO4- ions. Structural rearrangements of HSO4- ions in various phases and the contribution of cations to the conductivity of these mainly protonic conductors are discussed.

Thermal history and phase transitions in the superionic protonic conductors CsHSO4 and CsHSeO4

Abstract Single crystals and polycrystalline samples of CsHSO4 and CsHSeO4 have been studied by differential scanning calorimetry (DSC) and IR and Raman spectroscopy at various temperatures. The influence of thermal history is evidenced and phase-transition diagrams are proposed. Ionic and protonic conductivity, phase-transition mechanisms, incommensurability and the nature of defects are discussed.

Infrared and Raman study of some heteropolyacid hydrates

Abstract Infrared and Raman spectra of hydrates od 12-molybdophosphonic, 12-tungstophosphonic and 12-silicotungstenic acid have been investigated over a wide temperature range. Three types of protonic entities H 3 O + , H 2 O and OH groups attached to the skeleton have been shown to exist and there is a temperature dependent dynamic equilibrium between various species. The structural disorder and the conductivity mechanism in these superionic protonic conductors are discussed.

Influence of thermal and mechanical treatment and of water on structural phase transitions in CsHSO4

Abstract The influence of mechanical and thermal treatment on crystalline CsHSO 4 superionic protonic conductor has been studied by DSC, TGA, dilatometry, IR and Raman spectroscopies between 100 and 450 K. Grinding or pressing of CsHSO 4 destroys the (HSO 4 ) - chains and induces a transition to a new phase containing cyclic (HSO - 4 ) 2 dimers. The influence of dry and humid atmosphere on phase transitions is discussed. The structure of CsHSO 4 appears to be able to accomodate a large amount of defects such as S 2 O 2- 7 , SO 2- 4 or H 2 O.

Raman study of the high-temperature phase transition in CsH2PO4

Abstract The Raman spectra of the high temperature superionic phase of CsH 2 PO 4 were investigated in the 4000-20 cm −1 range. There was no evidence for an irreversible phase transition at 422 K and the only reversible high temperature transition was observed near 504 K. The high temperature phase corresponds well to CsH 2 PO 4 and not to CsH 2 P 2 O 7 and its Raman spectra are characteristic of a plastic phase, implying high dynamical disorder of H 2 PO − 4 ions but with a relatively ordered Cs + sublattice.

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

Abstract Infrared and Raman spectra on Na 3 H(SO 4 ) 2 , K 3 H(SO 4 ) 2 and (NH 4 ) 3 H(SO 4 ) 2 crystals have been investigated at 300 and 100 K in the 4000 to 30 cm −1 region. An assignment of bands in terms of OH group frequencies and more or less distorted tetrahedra of ammonium and sulphate ions is given. The crystallographic and spectroscopic symmetry and/or dissymetry of OH⋯O hydrogen bonds linking sulphate ions into dimers is discussed using OH group frequencies and the splitting of the v 1 (SO 4 ) Raman bands as criteria. In the particular case of (NH 4 ) 3 H(SO 4 ) 1 compound containing several solid phases it can be shown that the room temperature phase (II) is strongly disordered, principally because of an orientational disorder of ammonium ions, and that a progressive ordering takes place with temperature lowering.