S. Devautour

Conductivity and Dielectric Relaxation in Various Ni ( OH ) 2 Samples

Conductivity and dielectric relaxation are measured on several uncycled Ni(OH) 2 powders whose composition and degree of crystallization differ. The role played by water molecules intercalated in the a motifs of the interstratified structure is pointed out. Further, it is shown that coprecipitated dopants such as Co(II) and Zn(II) lower conductivity. Finally, a schematic model based on protonic conduction is proposed to account for the electrical and dielectrical properties of these solids.

Dielectric relaxation in ionic solids: experimental evidences

The comparison of different experimental methods of dielectric relaxation measurements applied to diverse ionic solids clearly demonstrates that ac conductivity dispersion observed in these solids is of dielectric nature. Then, by analysing the dielectric signal in terms of distribution of relaxation times, we show that important information on material can be obtained. They are for instance the interaction energy between the ionic charges and their surrounding network or the different types of sites where ions are embedded.

Activated cation motions in zeolites

We apply a Monte Carlo technique specialized for the simulation of rare events to study the activated counterions motions in the aluminosilicate Na+-mordenite. Mean activation barriers are obtained from minimum energy paths calculated on realistic potential energy surfaces by using a Metropolis algorithm. Energy barriers for Na+ hops calculated for lattices with various Si/Al ratio are found in good agreement with the Na+ detrapping energies measured by thermally stimulated current spectroscopy. One shows that the dielectric activated motions of Na+ proceed between degenerated many-body ground states with different dipolar moment by either sequential or collective hopping motions. This provides a first microscopic description of dielectric relaxation measured in zeolites.

Experimental evidence of polarization effects on exchangeable cations trapped in zeolites

The evolution of the activation energy of the conductivity depends on the nature of the exchanged cations, and differs in faujasites X and Y. This surprising phenomenon, reported in many works, is not yet satisfactorily explained. A qualitative explanation is proposed based on well-known results obtained in the study of the interactions between chemical species, by means of density functional theory.

Theoretical prediction of low-frequency vibrations of extra-framework cations in mordenite zeolites

Molecular dynamics simulations, using classical potential models to represent the cation-framework interactions, were performed in order to predict the low frequency region of vibrational spectra for mordenite zeolites. The position and the shape of the bands assigned to the cation vibrations have been studied as a function of the nature of the extra-framework charge balancing cations (alkali, alkaline earth) and of the Si/Al ratio characterizing the zeolite framework. The critical role of the forcefield is also demonstrated by computing the low frequency spectra using two different forcefields which include the flexibility of the host framework.

Analysis of broadband dielectric spectra in a Na+ mordenite

The surface properties of a sodic exchanged mordenite has been studied by means of dielectric measurements. The imaginary part of the permittivity is correctly explained by using a model based on the existence of a distribution function of the interaction energies between sodium ions and the zeolitic lattice. Taking into account the sites involved in the mordenite, four elementary distribution functions have been used. The obtained global distribution function has been compared with that resulting from the application of an “ill-posed and inverse method”. A satisfactory coherence is observed between both approaches.

Computational studies of hydration in Na+-mordenite zeolites

Abstract Computer modeling has been used to explore the effect of water on both the distribution of the extraframework cations and on their interactions with the framework in Na+-Mordenites. Atomistic simulations based on interatomic potentials and minimisation techniques have been used to determine the location of both cations and water molecules as a function of the hydration level. Our calculations showed two different cation behaviours depending on the type of channels that they occupy, the positions of the cations in the main channels being substantially perturbed upon the sorption of water molecules whereas those of the cations located in the small channels being only slightly shifted. Furthermore, Molecular Dynamics simulations allowed us to compare the mobility of the cations in the hydrated and dehydrated states and confirmed the results expected from the previous static calculations. This modelling has been successfully compared with experimental data obtained by dielectric relaxation spectroscopy.

Dielectric Relaxation Spectroscopy for Probing Ion/Network Interactions in Solids

Dielectric Relaxation Spectroscopy by means of Complex Impedance or Thermally Stimulated Currents methods can be a powerful tool for investigating the dynamic of ions in solids. It is one of the few techniques which allows us a quasi-direct quantitative determination of the interaction which bonds the mobile ions to their host network. Furthermore, fits of the dielectric relaxation spectra, both in the frequency or time/temperature domains, can also be a starting point for models whose aims to give a microscopic view of ionic conductivity. Basically, it is admitted that ion displacements can be represented by a succession of hops above a potential barrier. However, this simple image does not account for the rather intricate experimental response observed when ionic solids are submitted to dc or ac electrical fields.

CATIONS MOBILITY AND WATER ADSORPTION IN ZEOLITES

As already pointed out [1], dielectric relaxation spectroscopy can be a convenient tool for probing ion dynamic in solids which depends on i) the structure in which ions are embedded and ii) the nature of the interaction ion/network. Consequently, the results obtained from this technique can be used as a data base for theoretical studies which goal is to calculate the ion binding energy and to simulate ionic displacements. Inversely, theoretical calculations are essential for confirming the experimental data and more particularly the method which is used for analysing the dielectric experimental response.

On the evaluation of intrinsic distributions of relaxation timies using blocking electrodes

We have shown that the use of blocking electrodes results in a spectrum of effective relaxation times, which considerably differs from the intrinsic one characterising the dynamics of bulk material. An effective medium analysis allows us to transform the effective distribution into the intrinsic one and vice versa. The procedure can be applied to data obtained by dielectric relaxation spectroscopy as well as to TSDC data.