Ionic mobility and electrical transport in 45ZrF<Subscript>4</Subscript> · 35BiF<Subscript>3</Subscript> · 20MF (M - Li,Na, K) glasses studied by NMR and impedance spectroscopy

The ionic mobility and electrical transport in 45ZrF₄ · 35BiF₃ · 20MF (M - Li, Na, K) glasses have been studied by ⁷Li, ¹⁹F, and ²³Na NMR and impedance spectroscopy at temperatures from 200 to 500 K. In the range 400–440 K, the main kinds of ionic mobility in these glasses are local motions of fluorine-containing groups and lithium ion diffusion. The temperature range where the dominant kind of ion motion in the glasses is fluoride ion diffusion depends on the nature of the M⁺ cation. Above 470 K, the glasses offer rather high ionic conductivity: σ ≥ 10^?5 S/cm.     

Influence of processing conditions on IR edge absorption in fluorohafnate and fluorozirconate glasses

The influence of processing conditions on the IR absorption edge of fluoride glasses with the composition 62 MF4 1b 33 BaF₂ 1b 5 LaF₃ (M = Hf or Zr) is reported. At higher values of the absorption coefficient (α > 1 cm^?1), the IR edge is nearly independent of processing conditions and appears to be intrinsic. At lower values (α < 1cm^?1) extrinsic features attributed to oxide impurities appear; these are suppressed when glass melting is carried out in a reactive atmosphere of CCl₄. Comparison of our results for HfF₄- and ZrF₄ based glasses shows that substitution of HfF₄ for ZrF₄ shifts the IR edge to longer wavelengths, but the intrinsic absorption curves are nearly parallel over the frequency range investigated.     

Determination du coefficient d'extinction molaire de OH− dans les verres fluores a base de metaux lourds

The extinction coefficient ? at 2.9 μm for OH in fluoride glasses is measured by determining the amount of HF evolved during heating of the glass under steam and the corresponding intensity of the OH absorption band. This coefficient is respectively equal to 31 litre mole^?1 cm^?1 for BTYbZ glass (15 BaF₂ - 29 ThF₄ - 28 YbF₃ - 28 ZnF₂) and 19.5 litre mole^?1 cm^?1 for BALLA glass (34 BaF₂ - 57 ZrF₄ - 4 AlF₃ - 5 LaF₃).     

Etude de la conduction ionique de verres a base de ZrF4

The electrical and dielectric properties of new ZrF₄ based glasses have been studied in the frequency and temperature ranges 5Hz-500kHz and 130–280°C. Transport number measurements following Tubant's method showed that the glasses investigated are F^? conductors. The conductivities and activation energies for conduction are about 10^?6(Ωcm)^?1 at 200°C and 18 kcal.mole^?1 respectively. Preliminary interpretation of the change of conductivity with composition is given.     

Ionic conductivity of Ag+ exchanged α-zirconium phosphate

The conductivity of silver ion in silver exchanged α-zirconium phosphate, Zr(AgPO₄)₂, was measured in the temperature range 80 to 330°C. The data were found to conform to an Arrehnius type behavior in this temperature range with an activation energy of 0.53eV and ⁰300°C = 1.94 × 10^?4 ohm^?1 cm^?1. The conductivity values for Ag⁺ are higher than the corresponding values obtained for Na⁺ and K⁺ conduction in their respective exchanged forms. This enhancement is attributed to the greater polarizibility of the silver ion.     

Graphene‐Encapsulated FeS2 in Carbon Fibers as High Reversible Anodes for Na+/K+ Batteries in a Wide Temperature Range

Developing low cost, long life, and high capacity rechargeable batteries is a critical factor towards developing next‐generation energy storage devices for practical applications. Therefore, a simple method to prepare graphene‐coated FeS₂ embedded in carbon nanofibers is employed; the double protection from graphene coating and carbon fibers ensures high reversibility of FeS₂ during sodiation/desodiation and improved conductivity, resulting in high rate capacity and long‐term life for Na⁺ (305.5 mAh g^?1 at 3 A g^?1 after 2450 cycles) and K⁺ (120 mAh g^?1 at 1 A g^?1 after 680 cycles) storage at room temperature. Benefitting from the enhanced conductivity and protection on graphene‐encapsulated FeS₂ nanoparticles, the composites exhibit excellent electrochemical performance under low temperature (0 and ?20 °C), and temperature tolerance with stable capacity as sodium‐ion half‐cells. The Na‐ion full‐cells based on the above composites and Na₃V₂(PO₄)₃ can afford reversible capacity of 95 mAh g^?1 at room temperature. Furthermore, the full‐cells deliver promising discharge capacity (50 mAh g^?1 at 0 °C, 43 mAh g^?1 at ?20 °C) and high energy density at low temperatures. Density functional theory calculations imply that graphene coating can effectively decrease the Na⁺ diffusion barrier between FeS₂ and graphene heterointerface and promote the reversibility of Na⁺ storage in FeS₂, resulting in advanced Na⁺ storage properties.     

Ionic conductivity in Na5GdSi4O12

Na₅GdSi₄O₁₂ has been prepared by solid state reaction. Na⁺ ion conductivity is 3 × 10^?1 (ohm-cm)^?1 at 300° with an activation energy for conduction of 6.5 kcal/mole. The structure of isotypic Na₅YSi₄O₁₂ is characterized by Si₁₂O₃₆ rings stacked to form columns held apart by MO₆ octahedra. Immobile Na atoms are situated within the rings. The high conductivity arises from the presence of mobile Na atoms between the columns.     

Lithium ion conductivity and mobility in the Li0.12Na0.88Ta0.4Nb0.6O3 solid solution

Lithium ion mobility and the superionic phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ solid solution have been studied using temperature-dependent ionic conductivity measurements and Raman spectroscopy. From the temperature dependences of the conductivity and the width of a Raman line corresponding to Li⁺ and Na⁺ vibrations in the AO _x (A = Na⁺, Li⁺) polyhedra, the average lifetime of the Li⁺ ion in its equilibrium position and the height of the barrier to hopping have been estimated at ?3.9 × 10^?13 s and ?16 kJ/mol, respectively.     

Preparation and properties of fluorozirconate glasses containing divalent europium

The glass forming region of the (ZrF₄BaF₂ThF₄)EuF₂ system has been studied. EuF₂ was found to replace up to about 85% of the BaF₂ in a glass with a composition .615 ZrF₄, .32BaF₂, .07ThF₄. The glass transition temperature is seen to increase with the EuF₂ concentration. The 4f⁷→ 4f⁶5d¹ transition of the Eu⁺² ion in the glass matrix was studied by optical absorption and compared with data taken on crystalline compounds containing Eu⁺² in different environments. It is concluded from these results that the Eu⁺² ion - and also the Ba⁺² ion - is coordinated by 12 fluorides in the glass.     

Synthesis and Li ion conductivity of Li2O-Nb2O5-P2O5 glasses and glass-ceramics

Glasses with the compositions of xLi₂O-(70 − x)Nb₂O₅-30P₂O₅, x = 30-60, and their glass-ceramics are synthesized using a conventional melt-quenching method and heat treatments in an electric furnace, and Li⁺ ion conductivities of glasses and glass-ceramics are examined to clarify whether the glasses and glass-ceramics prepared have a potential as Li⁺ conductive electrolytes or not. The electrical conductivity (σ) of the glasses increases monotonously with increasing Li₂O content, and the glass of 60Li₂O-10Nb₂O₅-30P₂O₅ shows the value of σ = 2.35 × 10⁻⁶ S/cm at room temperature and the activation energy (E_a) of 0.48 eV for Li⁺ ion mobility in the temperature range of 25-200 °C. It is found that two kinds of the crystalline phases of Li₃PO₄ and NbPO₅ are formed in the crystallization of the glasses and the crystallization results in the decrease in Li⁺ ion conductivity in all samples, indicating that any high Li⁺ ion conducting crystalline phases have not been formed in the present glasses. 60Li₂O-10Nb₂O₅-30P₂O₅ glass shows a bulk nanocrystallization (Li₃PO₄ nanocrystals with a diameter of ∼70 nm) and the glass-ceramic obtained by a heat treatment at 544 °C for 3 h in air exhibits the values of σ = 1.23 × 10⁻⁷ S/cm at room temperature and E_a = 0.49 eV.     

Near-IR luminescence from subvalent bismuth species in fluoride glass

The broadband NIR luminescence of subvalent bismuth species was demonstrated in partially reduced ZrF₄-BiF₃-NaF and ZrF₄-BiF₃-BaF₂ fluoride glasses. The parameters of luminescence were reported and compared with luminescence from other bismuth-doped materials. Since fluoride glass compositions are based on strong Lewis acids (ZrF₄ in present case) they can stabilize NIR photoluminescent subvalent bismuth species.     

Verres fluores dans le systeme ZrF4ThF4MF3 (M = Y,Lu, Sc,Al)

The glass forming area has been precised in the ZrF₄ThF₄YF₃ ternary system. It is smaller than in the ZrF₄ThF₃LaF₃ system and corresponds to the limits: 45–70 % ZrF₄, 30–50 % ThF₄, 0–12 % YF₃. A series of glass samples with molar composition 0.5 ZrF₄, 0.43 ThF₄, 0.07 MF₃ (M : La, Y, Lu, Sc, Al) has been prepared and the linear evolution of T_G, molar refractivity and anionic compactness versus the ionic radius of M³⁺ is shown. The thermomechanical properties are better than for previous fluorozirconate glasses including barium. The average values are 480° C for T_G, 580° C for T_C, 800° C for T_F, 1.54 for n_D, 90.10^?7 K^?1 for thermal expansion coefficient. The U.V. absorption edge is improved by comparison with standard ZrF₄ -and AlF₃- based glasses. The potential application to I.R. optical fibres is discussed.     

Fluoride bridging modes in fluorozirconate glasses by x-ray and computer simulation studies

A second characteristic ZrZr distance due to edge-sharing (ZrF_n) polyhedra which is observed in crystalline α-ZrF₄ (high temperature form) and is also suggested by recent X-ray studies of BaF₂ZrF₄ glasses, has been observed in ion dynamics computer simulation studies. Experimental X-ray and computer simulation results both imply that about one out of each four bridges between Zr pairs involve two fluoride ions, and that this proportion is almost independent of composition in the binary system.     

Thermally stimulated polarization and depolarization current (TSPC/TSDC) techniques for studying ion motion in glass

The feasibility of studying ionic motion in glass using thermally stimulated polarization (TSPC)/depolarization current (TSDC) techniques was investigated with 4Na₂O-96SiO₂ and 30PbO-70SiO₂ (mol %) glasses. The TSPC peaks in these glasses were dependent on glass composition and attributed to bulk polarization. The high temperature background TSPC is shown to be due to the d.c. conductivity, whereas the TSPC/TSDC peaks in the 4Na₂O glass are attributed to shorter range Na⁺ motion.     

Construction of Bimetallic Selenides Encapsulated in Nitrogen/Sulfur Co‐Doped Hollow Carbon Nanospheres for High‐Performance Sodium/Potassium‐Ion Half/Full Batteries

Metallic selenides have been widely investigated as promising electrode materials for metal‐ion batteries based on their relatively high theoretical capacity. However, rapid capacity decay and structural collapse resulting from the larger‐sized Na⁺/K⁺ greatly hamper their application. Herein, a bimetallic selenide (MoSe₂/CoSe₂) encapsulated in nitrogen, sulfur‐codoped hollow carbon nanospheres interconnected reduced graphene oxide nanosheets (rGO@MCSe) are successfully designed as advanced anode materials for Na/K‐ion batteries. As expected, the significant pseudocapacitive charge storage behavior substantially contributes to superior rate capability. Specifically, it achieves a high reversible specific capacity of 311 mAh g^?1 at 10 A g^?1 in NIBs and 310 mAh g^?1 at 5 A g^?1 in KIBs. A combination of ex situ X‐ray diffraction, Raman spectroscopy, and transmission electron microscopy tests reveals the phase transition of rGO@MCSe in NIBs/KIBs. Unexpectedly, they show quite different Na⁺/K⁺ insertion/extraction reaction mechanisms for both cells, maybe due to more sluggish K⁺ diffusion kinetics than that of Na⁺. More significantly, it shows excellent energy storage properties in Na/K‐ion full cells when coupled with Na₃V₂(PO₄)₂O₂F and PTCDA@450 °C cathodes. This work offers an advanced electrode construction guidance for the development of high‐performance energy storage devices.     

Synthesis and high-temperature electrical conductivity of Ln<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and LnYTi<Subscript>2</Subscript>O<Subscript>7</Subscript> (Ln = Dy,Ho)

Ho₂Ti₂O₇ and LnYTi₂O₇ (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho₂Ti₂O₇ is 4 × 10^?4 S/cm, and that of HoYTi₂O₇ is 1 × 10^?3 S/cm, with activation energies E _a = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi₂O₇ (3 × 10^?4 S/cm at 740°C, E _a = 1.09 eV) is almost one order of magnitude lower than that of HoYTi₂O₇ (1 × 10^?3 S/cm at 740°C, E _a = 1.17 eV).     

X-ray diffraction studies of phase transformations in heavy-metal fluoride glasses

The crystallization of five ZrF₄-based glasses has been investigated using powder X-ray diffraction and differential scanning calorimetry. The crystalline phase in Zr-Ba-La-Pb fluoride glass was found to be-BaZrF₆. In other glasses the crystal phases could not be identified. Reversible polymorphic phase transformations occur in Zr-Ba-La-Li and Zr-Ba-La-Na fluoride glasses when heated to higher temperatures.     

Ion mobility and transport in β-PbF<Subscript>2</Subscript> doped with alkaline-earth fluorides

We have studied ion mobility and conduction in (1 ? x)PbF₂ · xMF₂ (M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺; 0.05≤x≤0.1) solid solutions by ¹⁹F and ²⁰⁷Pb NMR and impedance spectroscopy. The results are used to analyze the factors governing the structure of the fluoride sublattice and the nature and energetics of ionic motion in the temperature range 170–550 K. The solid solutions are shown to have high ionic conductivity (～10^?4 to 10^?3 S/cm at temperatures above 470 K, with activation energies E _a ≤ 0.3?0.6 eV) and are, therefore, potentially attractive for engineering materials with tailored electrical properties.     

Properties of glasses in the ZrF4-AlF3-BaF2-RF (R = Li,Na or K) system

Densities, molar volumes, refractive indices, molar refractivities, glass transition temperatures and elastic constants of glasses with compositions (100–x) (0.6ZrF₄ · 0.1 AlF₃ · 0.3BaF₂) · xRF, where R = Li, Na or K, were studied. All the properties vary continuously with increasing RF concentration for all three alkali atoms. The calculated molar refractivities of LiF, NaF and KF are constants which are very close to the values of the crystalline compounds LiF, NaF and KF, respectively, throughout the composition range studied, suggesting that no change occurs in the coordination state of the alkali ions by fluorine atoms. The comparison with crystalline compounds suggests that Li⁺, Na⁺ and K⁺ ions are all coordinated by six fluorine atoms.     

Rapid H+ conductivity in hydrogen uranyl phosphate-A solid H+ electrolyte

We have found that the layered hydrate HUO₂PO₄. 4H₂O is a rapid proton conductor. The room temperature conductivity of 4 × 10^?3ohm^?1cm^?1 is higher than that of Na⁺ in β alumina. The activation energy is 30± 3 kJ mol^?1. The material is insoluble, and presses into transluscent discs suitable for solid electrolyte applications.