M. Ribes

Electrical properties of lithium conductive silicon sulfide glasses prepared by twin roller quenching

Abstract Glasses with the composition xLi 2 S(1-x)SiS 2 [x ⩽ 0.6] have been prepared by twin roller quenching. Their glass transition temperatures and their electrical conductivities have been measured. The conductivity reaches a maximum value of 5.10 −4 (Ω.cm) −1 at 25°C By dissolving a halide salt (LiI) in the matrix, this value has been improved to 8.2 10 −4 (Ω.cm) −1 which is almost the highest conductivity obtained with Li conductive glasses.

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

Ion dynamics in the argyrodite compound Ag7GeSe5I: non-Arrhenius behavior and complete conductivity spectra

Abstract The conductivity of the superionic conductor argyrodite compound Ag 7 GeSe 5 I has been measured for temperatures between 123 and 473 K and in a broad frequency range from 10 Hz to 60 GHz. The frequency-independent and frequency-dependent conductivity data have respectively been analyzed. A non-Arrhenius behavior of the dc conductivity is clearly observed. Such a behavior appears to be quite common in fast ionic conductors as if the conductivity would reach a limit value. The very complete conductivity versus frequency measurements allowed the observation of distinct regimes and the exploration of various ion-relaxation processes within the material. The conductivity spectrum can be described by superposition of several contributions: the Universal Dynamic Response due to diffusive motions, a linearly frequency-dependent regime attributed to local motions, and finally, a vibrational contribution in the far infrared domain.

Silver sulfide based glasses (I). Glass forming regions, structure and ionic conduction of glasses in GeS2Ag2S and GeS2Ag2SAgI systems

Abstract Ag2S forms with GeS2 stable glasses over a wide range of compositions (0–55% Ag2S mol%). In the same system, more complex glasses obtained by dissolving silver iodide have been synthesized with up to 50 mol% AgI. Raman spectra are presented and a vibrational assignment in terms of bridging and non-bridging sulfur has been made. The electrical conductivity of these glasses has been measured over a temperature range (−50°C− + 50°C) and for various compositions by the complex impedance diagram method. At 25°C, the conductivity reached a maximum value of 6 × 10−3 Ω−1 cm−1. Whatever the glass used, the same limit value of conductivity (σ − 10 su−2 Ω −1 cm −1 ) and activation energy ( E σ ⋍ 0.25 eV ) are obtained for the highest content of silver iodide. A conduction mechanism is proposed.

Sulfide glasses: Glass forming region, structure and ionic conduction of glasses in Na2SXS2 (XSi; Ge), Na2SP2S5 and Li2SGeS2 systems

Abstract Na 2 S forms with GeS 2 , SiS 2 and P 2 S 5 stable glasses with a large range of composition as well as a Li 2 SGeS 2 system. A vibrational assignment in terms of terminal (GeS) and bridge (GeSGe) stretching has been made (Na 2 SGeS 2 system). The ionic transport number obtained by e.m.f. measurements (glasses 0.5 Na 2 S0.5 XS 2 (XGe, Si)) was found to be equal to 1. The electrical conductivity of these glasses has been measured over a range of temperature (−20°C, 150°C) and composition by the complex impedance diagram method. The glass 0.5 Li 2 S0.5 GeS 2 exhibits good conductivity 4 × 10 −5 ω −1 cm −1 (20°C) higher than the best conductive glasses currently known. Replacement of the oxygen atom by a sulphur atom (comparison with oxide glasses at the same composition) is found to noticeably improve the ionic conductivity. This may be due to a great polarisability of the sulphur. For the glasses with the same molar ratio in sodium sulfide, the ionic conductivity increases when the electronegativity of the network forming sulfide decreases.

Effect of rapid quenching on electrical properties of lithium conductive glasses

Abstract A twin roller apparatus has been designed to be used in a controlled environment, so that even hydroscopic and oxidizable glasses may be prepared by rapid quenching. xLi2O(1−x)P2O5 and xLi2S(1−x)GeS2 glasses have been prepared and their electrical conductivity measured as a function of temperature. The electrical characteristics of rapidly quenched and conventional glasses are compared in order to study the influence of the cooling rate. The results are quite different for oxide and sulfide glasses. Rapid quenching does not much affect oxide glasses whereas for sulfide glasses important decreases in activation energies and pre-exponential factors are observed.

Glass formation and ionic conduction in the M2SGeS2 (M = Li, Na, Ag) systems

Abstract GeS 2 forms stable glasses with Li 2 S, Na 2 S, Ag 2 S in a large range of composition (from 1–0.5 in molar ratio of GeS 2 ). Raman spectra are presented and a vibrational assignment in terms of bridging and non-bridging sulfur has been made. The electrical conductivity of these glasses has been measured over a range of temperature (−20–150°C) and composition by the complex-impedance diagram method. At high alkali or silver sulfide content and at 25°C they present a good ionic conductivity (>10 −5 (ω cm) −1 for Li glasses, >10 −6 (ω cm) −1 for Na glasses, >10 −3 (ω cm) −1 for Ag glasses). This result may be due to the large polarisability of the sulfur atoms. A new arrangement with 3 electrodes on solid electrolytes is used for testing the electrochemical stability range of these glasses. A value of 4 V with a 0.05 mA cm −2 current can be estimated.

Quantum confinement effects of CdS nanocrystals in a sodium borosilicate glass prepared by the sol‐gel process

Experimental evidences of both weak and strong confinement regimes are reported on CdSnanocrystals embedded in a sodiumborosilicate glass matrix. A method, based on the sol‐gel technique, is used for the preparation of CdS‐activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low‐temperature linear‐absorption spectra have been analyzed in terms of excitons and electron‐hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope‐function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments.

Percolation transition in Ag-doped germanium chalcogenide-based glasses: conductivity and silver diffusion results

Abstract Conductivity and silver diffusion measured using a 110m Ag tracer have been investigated in AgGeS and AgGeSbSe glasses with silver concentration ranging from 0.008 to 25 at.% Ag. It has been found that the room-temperature conductivity in both systems increases by 9.0–9.5 orders of magnitude with increasing silver content, and its activation energy decreases from ∼ 1 to 0.4 eV. Accordingly, the silver tracer diffusion coefficient at 298 K increases by 5.0–5.5 orders of magnitude with similar decrease of the diffusion activation energy. A comparison of the conductivity and silver diffusion results clearly shows that the ionic transport is predominant in the two systems, even at lowest Ag concentrations. The Haven ratio, H R , decreases with increasing silver content: extremely diluted glasses (0.008–0.1 at.% Ag) exhibit H R ≈ 1; Ag-rich vitreous alloys are characterized by H R = 0.2–0.4. The composition dependencies of the ionic conductivity, σ i , and silver tracer diffusion coefficient, D Ag , exhibit two drastically different transport regimes at low (≤ 2–5 at.%) and high (> 10 at.%) silver concentrations. A power-law composition dependence of σ i and D Ag over 2.5 orders of magnitude in the Ag concentration and 3.5–5.0 orders of magnitude in the ionic conductivity (2–3 orders of magnitude in the diffusion coefficient) is observed at low silver concentrations. This transport regime is attributed to percolation in the critical region just above the percolation threshold. Recent theoretical considerations (the dynamic structure model and statistical (occupation) effects on percolative ionic conduction) are also in good agreement with experimental findings. After essential structural transformations of the glass network on the short- and intermediate-range scales at higher silver content (> 10 at.%), the ionic transport is not caused any more by percolation, i.e., it becomes network-dependent with a strongly correlated motion of the Ag + ions.

Study of Fe2O3-based thin film electrodes for lithium-ion batteries

Abstract Sputter deposited thin films from an α-Fe 2 O 3 target are studied as an alternative to the carbon-based anode for lithium-ion batteries. The elaboration process of the amorphous thin layers as well as their chemical and physical properties are described. The value of the electrical conductivity of the thin films is astonishingly higher than the one measured on the bulk samples. Depending on the amount of intercalated lithium, a drastic change of this value is observed. The electrochemical characteristics obtained on these amorphous thin layer electrodes regarding their specific capacity and cyclability are presented. An attempt to build up a lithium-ion battery associating a Li 3 Fe 2 O 3 -based thin film anode to a vitreous V 2 O 5 –B 2 O 3 -based thin film cathode is described.