Ph. Colomban

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.

Structure of oxide gels and glasses by infrared and raman scattering

Optically clear monolithic gels and fine gel powders have been synthesized using various alkoxide hydrolysis reactions. The gels have been characterized using various methods to determine their structures. (X-ray diffraction, DTA, TGA, DSC, IR and Raman spectroscopies). The spectra and the nature of gels depend on the solvent and the hydrolysis conditions (rate, pH, etc.). The use of acetone as solvent allows reduction of the hydrolysis time, from weeks to hours. If the hydrolysis of aluminium sec-butoxide is too rapid, at high pH, crystalline bayerite Al(OH)3 is formed. Regular hydrolysis leads to amorphous optically clear gel with sometimes boehmite (or diaspore) traces. Formation of the (porous) glass (300 to 600°C) and also of the γ-alumina does not modify the Raman spectra strongly whereas large modifications are observed on IR spectra with the evolution of protonic species. The structure of alumina gel and glass is of the spinel type. The α-alumina phase grows above 1200 to 1250°C (above 1050°C if boehmite traces are present).

Orientational disorder, glass/crystal transition and superionic conductivity in nasicon

Abstract The relations between the orientational disorder of SiO 4 (PO 4 ) tetrahedra and fast sodium diffusion in superionic NASICON have been studied by conductivity (complex impedance method), DSC, X-ray powder difraction and vibrational spectroscopy (IR and Raman). Sol-gel routes allow to obtain pure glassy NASICON (Na 1+ x Zr 2 Si x P 3- x O 12 x ≅2) in the 500−700°C temperature range. Tetragonal zirconia nucleates above 700°C and disappears at about 900°C when the isolated tetrahedra framework is formed: a high orientational static disorder of tetrahedra exists and the symmetry is rhombohedral at all studied temperatures (20−600 K). Thermal treatment above 1100°C induces a drastic decrease of the static orientational disorder and nucleation of monoclinic zirconia. The resulting compound exhibits a monoclinic symmetry at R.T. and three phase transitions, two diffuse at about 60 K and 520 K and the 423 K monoclinic-rhombohedral transition associated with the superionic conducting state. An increase in dynamic disorder (broad Rayleigh wing up to 500 cm -1 is simultaneously observed. The lower the static disorder at low temperature, the higher the dynamic orientational disorder and the phase transitions, and the lower the activation energy of conductivity at high temperature.

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.

Relation structure-fast ion conduction in the NASICON solid solution

Crystal determinations of the rhombohedral phase (space group R3c), for different compositions (2 < x < 2.4) in the true NASICON solid solution Na1+xZr2SixP3−xO12, have been performed at different temperatures by X-ray diffraction. We observe, as a consequence of interionic repulsions, the partial occupation of a mid-Na interstitial site within the conduction path. The composition dependence of the mid-Na occupation factor, maximum at x = 2, explains the maximum of the c hexagonal parameter and of the Na(1)-oxygen average distance observed at about x = 2. Moreover, structural results clearly suggest that the enhanced conductivity at x = 2 arises from sodium interactions instead of geometry changes of the framework.

Domaines d'existence, distorsions structurales et modes de vibration des ions conducteurs dans les reseaux hotes de type nasicon

Abstract The NASICON structure domain of stability has been investigated in the ZrO 2 -P 2 O 5 -M 2 O-SiO 2 (M = Li, Na, K, Ag) system (Hf, Ti and partial Ta substitutions of Zr ions have also been studied) using X-Ray diffraction and vibrational spectroscopy. Metastability phenomena have been pointed out for some compositions. The different compounds have been synthetized in the form of powder using a sol-gel process and in the form of single crystals using various techniques. IR and Raman spectra are interpreted by using a neso-silicate (phosphate) structural description: a framework of XO 4 tetrahedra (X = P, Si) surrounded by octahedral cavities filled with M and M″ cations M″ = Sc, Ti, Zr, Hf). As different kinds of structural distortions are observed as a function of composition, distortions also occur against temperature. Local disorder and long range correlations are simultaneously observed : for some compositions e.g. Na 5 Zr(PO 4 ) 3 , a NASICON single crystal can be described as a “glass” with respect to the tetrahedra sublattice and as quasi a perfect crystal with respect to the cations one. Assignment of internal modes (tetrahedra modes), of M″ cations modes and of conducting cations (M) modes and of tetrahedra external modes is proposed on the basis of chemical substitution shift. Phase transitions are pointed out for the two kinds of sublattice (tetrahedra, cations). The influency of the “history” of samples is evidenced by DTA and vibrational spectroscopy. Correlations between local disorder, phase transitions and variations of conductivity activation energy are discussed.

Fast ion transport in LiZr2(PO4)3: Structure and conductivity

The room temperature modification of LiZr/sub 2/(PO/sub 4/)/sub 3/ is monoclinic. At a temperature (25-60/sup 0/C) which depends on the preparation process, there is a first order transition to a normal conducting rhombohedral form R3-barc. Li(1) sites are fully occupied, whereas Li(2) ones are empty. At 280/sup 0/C, there is a transition to a superionic phase with a long-range lithium motion.

Well densified nasicon type ceramics, elaborated using sol-gel process and sintering at low temperatures

Abstract NASICON (Na 3 Zr 2 Si 2 PO 12 ), TITSICON (Na 3·1 Zr 1·55 Si 2·3 P 0·7 O 11 ) and Li 0·8 Zr 1·8 Ta 0·2 (PO 4 ) 3 powders have been synthesized using various sol-gel processes and sintered in the form of well-densified ceramics (>96%). For a fixed sintering cycle, the structure of NASICON type ceramics depends on the sol-gel process : the more elements introduced by means of metal-organic reagents, the more shifted structural evolution towards monoclinic structure to high temperatures. Sintering behaviour also strongly depends on the sol-gel process. Aluminum doping keeps on the NASICON structure and allows to obtain dense ceramics at temperatures lower than 1100°C. Thus materials sintered below 1100°C are free of monoclinic zirconia traces and the structure is quasi rhombohedral : consequently, phase transition effects are lowered. Comparison with hot-pressed NASICON is given.

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.