I. V. Murin

Transport properties of LaF3 fast ionic conductor studied by field gradient NMR and impedance spectroscopy

The self-diffusion coefficient and the ionic conductivity in LaF3 single crystals are measured over a temperature range from 300 to 1400 K. Three thermally activated processes are found in this range. After heating, both the diffusion coefficient and the conductivity of the as-grown samples undergo strong irreversible changes at T , 1100 K: An activation energy of Ea ¼ 1: 2e V and a Haven ratio of about 0.1 are determined in the intrinsic region. The mechanism of the ion transport and the effect of thermal treatment are discussed. q 2003 Elsevier Science Ltd. All rights reserved.

Ionic motion in the LaF3 superionic conductor studied by 19F NMR with homonuclear decoupling

Abstract We report 19 F NMR studies at 282 MHz on the single crystalline fast ionic conductor LaF 3 from 130 to 490 K. From our high resolution data and from a comparison with computer simulated dipolar spectra we are able to identify the signals from different fluorine sublattices, and to analyse their motional behaviour and their chemical shifts as a function of temperature. The ionic diffusion begins above 290 K, but for the F 1 sublattice only. The F 2 and F 3 sublattices are indistinguishable in the NMR experiments, because their chemical shifts differ by less than 15 ppm. These ions remain static or in relatively slow motion until 490 K.

Glass-forming ability and cationic transport in gallium containing chalcohalide glasses

Abstract The glass formation regions are determined in the Ga2S3–GeS2–MCl (M=Li, Na, K, Cs, Ag, Tl) and GaI3–NaCl systems. It is noted that the formation of complex structural units such as MGaS3/2Cl is favourable for increasing the glass-forming regions. Temperature dependencies of the electric conductivity of 60 samples in the system Ga2S3–GeS2–MCl were measured. All of these glasses have cation conductivity. Activation energy of conductivity for all investigated systems is a linear function on the cubic root of the atomic fraction of cations. These results correspond to the simple model of cation conductivity.

NMR and XAS study on doped LaF3

Abstract Temperature dependent NMR measurements as well as XAS investigations on La 1− x Sr x F 3− x ( x =0.0, 0.01, 0.05, 0.16) have been performed to study the fluorine diffusion in these solid electrolytes and to elucidate the effect that doping with heterovalent SrF 2 has on LaF 3 . A three-stage process of diffusion was observed. Upon increasing temperature, initial ionic exchange begins in the F 1 sublattice. At even higher temperatures, dependent upon dopant concentration, ionic exchange between the F 1 and F 2,3 sublattices takes place. A further increase in temperature eventually leads to exchange over all F − structural positions.

Structural–chemical transformations of α-Fe2O3 upon transport reduction

Abstract Transient regions between iron oxide crystalline phases were found in various stages of α-Fe 2 O 3 structural–chemical transformation into FeO by Mossbauer spectroscopy. Particle sizes in those regions are less than 8 nm. The sequence of phase transformations was established for α-Fe 2 O 3 reduction to iron at temperatures higher and lower than the wustite stability point (∼570°C). Two phases Fe 1− x O and FeO were found to form upon the reduction within the wustite stability region (the stoichiometric phase is formed first). It was shown that reduction of hematite into magnetite occurs within temperature range 570–600°C by the multistage mechanism, while magnetite reduction to wustite occurs by a zonal mechanism, where three phases Fe 3 O 4 , FeO and Fe are present in a sample simultaneously.

Electrical properties and the structure of halogen-containing lead borate glasses: I. The PbF2-2PbO.B2O3 system

The temperature and concentration dependences of the dc electrical conductivity of glasses in the PbF2–PbO · B2O3 system are investigated. It is found that the dependences logσ = f([PbF2]) and Eσ = f([PbF2]) for glasses containing ∼20 and ∼35 mol % PbF2 exhibit kinks, which are interpreted from the standpoint of the microinhomogeneous structure associated with the selective interaction of the components during the synthesis of glasses. Analysis of the transport numbers has revealed that the unipolar fluorine ionic conductivity is observed upon introduction of more than 35 mol % PbF2. It is shown that, in the concentration range from 0 to 35 mol % PbF2, the electric current is provided by both protons and fluorine ions.

Disorder of ionic mobility in crystalline superionic conductors characterized by 19F-NMR

Abstract Fluorine mobility in monocrystalline LaF 3 is analyzed using 19 F NMR spectroscopy. At lower temperatures the motion is restricted to F − ions in the F 1 sublattice and characterized by a strongly heterogeneous behavior. Our approach which is based on line shape analysis shows that the mobility can be well described by a broad distribution of correlation times. The distribution of the correlation times and its variation with the temperature reflects the potential energy landscape in the superionic state.

The distribution of motional correlation times in superionic conductors: nuclear magnetic resonance of tysonite-like

Fluorine mobility at different structural positions in monocrystalline with the tysonite structure is analysed using NMR line-shape analysis. The method is sensitive to ionic exchange with correlation times in the range . For the temperature range between 240 K and 400 K the motion is restricted mainly to the ions in the fluorine layers perpendicular to the main symmetry axis (the sublattice), while ions in the La plane remain immobile. No significant anisotropy of the -ionic diffusion within the layers and along the c-axis is found ( at 400 K). From NMR spectra it is clear that mobility is strongly heterogeneous. The motional disorder can be described well by a broad distribution of correlation times, , which has a shape close to a log - Gaussian function and reflects the potential energy landscape in the superionic state. The variation of the centre position and width of with temperature differs from an Arrhenius law behaviour. Ionic mobility on the microscopical scale, therefore, cannot be considered a process which is activated only thermally. Applying MD techniques shows that the presence of vacancies may lead to pronounced changes of the potential energies, and supports the idea that there is a distribution of activation energies.

Relaxation methods for the study of ion transport in halide systems

Abstract Successful tools for the study of structural and dynamic problems in solid state ionic materials are electric and magnetic resonance and relaxation as well as the ITC (TSD) technique. Some applications of these structure-sensitive methods on fluoro-conducting systems in a wide range of temperature are demonstrated. The advantages, especially of the PMFG-NMR techniques, for a direct study of diffusion coefficients in these substances are shown. Correlations between local structures and ion conductivities are established.

Phase equilibria in fullerene-containing systems as a basis for development of manufacture and application processes for nanocarbon materials

This review is the first attempt to integrate the available data on all types of phase equilibria (solubility, extraction and sorption) in systems containing light fullerenes (C60 and C70). In the case of solubility diagrams, the following types of phase equilibria are considered: individual fullerene (C60 or C70)–solvent under polythermal and polybaric conditions; C60–C70–solvent, individual fullerene–solvent(1)–solvent(2), as well as multicomponent systems comprising a single fullerene or an industrial mixture of fullerenes and vegetable oils, animal fats or essential oils under polythermal conditions. All published experimental data on the extraction equilibria in C60–C70–liquid phase(1)–liquid phase(2) systems are described systematically and the sorption characteristics of various materials towards light fullerenes are estimated. The possibility of application of these experimental data for development of pre-chromatographic and chromatographic methods for separation of fullerene mixtures and application of fullerenes as nanomodifiers are described. The bibliography includes 87 references.