Investigating ionic mobility in solid solutions of Li x Na1 − x Ta y Nb1 − y O3 via Raman spectroscopy

The ionic mobility in Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions (SSes) is investigated by means of Raman spectroscopy. The average lifetime of an Li⁺ ion in the equilibrium position and the magnitude of the potential barrier are estimated from the temperature dependence of the line width corresponding to the vibrations of Li⁺ cations.     

Manifestation of a ferroelectric-antiferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.4</Subscript>Nb<Subscript>0.6</Subscript>O<Subscript>3</Subscript> in its Raman scattering spectra

The ferroelectric-antiferroelectric phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ ceramic solid solution has been studied by the Raman scattering technique. As the temperature approached the transition point from below, we observed an appreciable broadening of the lines associated with the vibrations of the cations occupying octahedral and cubooctahedral cavities of the structure and with the oxygen network vibrations (which implies a substantial increase in disorder on the cation sublattices), as well as a decrease to zero intensity of the 875-cm^?1 line corresponding to stretch vibrations of the bridging oxygen in the BO₆ octahedral anion in the vicinity of the transition. The temperature dependence of the 875-cm_?1 line intensity near the transition was used to study the behavior of the phase transition order parameter η. The behavior of η was found to disagree markedly with the Landau theory of second-order phase transitions. It is shown that discrepancies originate from the increase in disorder in the niobium and tantalum sublattices in the Li_0.12Na_0.88Ta _y Nb_1-y O₃ solid solution system with increasing y. The order of the transition is lowered.     

Concentration-phase transitions in solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 and their manifestations in Raman spectra

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃, Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ (x = 0?0.16, y = 0–1). It has been revealed that, for the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ and Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860–873 and 900–905 cm^?1 upon variation of the concentration of tantalum for solid solutions Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.     

A Raman spectroscopic study on the active site of sodium cations in the structure of Na2Ti3O7 during the adsorption of Sr2+ and Ba2+ cations

A detailed study on the structural deformation of trititanate nanofibers after the adsorption of divalent strontium (Sr) and barium (Ba) cations was conducted by using Raman spectroscopy, X‐ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the Raman bands at 309 cm⁻¹ corresponding to very long Ti O bonds (2.2 Å) and at 883 cm⁻¹ corresponding to the very short Ti O bonds (1.7 Å) decreased in intensity after the adsorption of Ba²⁺ and Sr²⁺ cations. Additionally, the band at 922 cm⁻¹ corresponding to an intermediate length Ti O bond was observed to weaken with the adsorption of divalent cations, indicating that the TiO₆ octahedra in Na₂Ti₃O₇ are more regular. These results suggest that the active sodium (Na⁺) cations in Na₂Ti₃O₇ should be located at the corner of the TiO₆ octahedral slabs, i.e. the plane (003). This was further confirmed by a large decrease of diffraction intensity of the plane (003) observed in the XRD pattern. Copyright © 2010 John Wiley & Sons, Ltd.     

Chloride spinels: A new group of solid lithium electrolytes

The chloride spinels Li₂MCl₄ with M = Mg, Mn, Fe and Cd show very high lithium ionic conductivity in the solid state. The ionic conductivity in the compounds under investigation was established with the help of emf measurements. The specific conductivities measured by both frequency response analysis and the four probe ac method are 1.3 Ω^?1 · cm^?1 for Li₂CdCl₄, and about 0.9Ω^?1 · cm^?1 for Li₂MnCl₄, Li₂MgCl₄, and Li₂FeCl₄ at 773 K. There are several indications that the ternary chlorides become highly disordered at elevated temperatures. Thus the Arrhenius plots, i.e. In σ · T vs 1/T-curves, exhibit significant bends, the slopes below the transition temperature being considerably higher than those above.     

New Li+ ion conductors in the system Li4SiO4−Li3AsO4

In the system Li₄SiO₄-Li₃AsO₄, Li₄SiO₄ forms a short range of solid solutions containing up to 14 to 20% Li₃AsO ₄, depending on temperature, and γ-Li₃AsO₄ forms a more extensive range of solid solutions containing up to ≈55% Li₄SiO₄. The Li₄SiO₄-Li₃AsO₄ phase diagram has been determined and is of binary eutectic character. The ac conductivity of polycrystalline samples was measured over the range 0 to at least 300°C for nine different compositions. The two solid solution series have much higher conductivity than the pure end-members; maximum conductivity was observed in the γ-Li₃AsO₄ solid solutions containing ≈40 to 55% Li₄SiO₄, with values of $\text{≈2×10}{}^{\mathrm{?6}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 20°C rising to ≈0.02 Ω^?1 cm^?1 at 300°C. These values are comparable to those found in the system Li₄SiO₄-Li₃PO₄. The variation with composition of the Arrhenius prefactor and activation energy has been interpreted in terms of the mechanisms of conduction. Li₃AsO₄ is a poor conductor essentially because the number of mobile Li⁺ ions is very small. This number, and hence the conductivity, increases dramatically on forming solid solutions with Li₄SiO₄, by the creation of interstitial Li⁺ ions. At ≈40 to 55% Li₄SiO₄, the number of mobile Li⁺ ions appears to be optimised. An explanation for the change in activation energy of conduction at ≈290°C in Li₄SiO₄ and at higher temperatures in Li₄SiO₄ solid solutions is given in terms of order-disorder of the Li⁺ ions.     

An inelastic neutron scattering study of C2H2 adsorbed on type 13X zeolites

The inelastic neutron scattering spectra of C₂H₂ and C₂D₂ adsorbed on a Ag⁺ exchanged 13X zeolite (0–800 cm^?1) and of C₂H₂ on the Na⁺ form (0–300 cm^?1) have been obtained. For the Na-13X system no distinct vibrational modes were observed, however for the Ag-13X systems the low frequency intramolecular modes of the adsorbed gas and some of the vibrations of the adsorbed gas relative to the surface have been assigned. From the deuteration shifts it appears that C₂H₂ and C₂D₂, adsorbed on Ag-13X, are non-linear.     

Vibrational spectrum of Li2B4O7 crystals

Polarized IR reflection spectra of Li₂B₄O₇ crystals are studied in a spectral range of 80–1600 cm^?1 and compared with their Raman spectra. Based on the results of the dispersion analysis of the spectra, the frequencies, damping constants, and oscillator strengths of all vibrations are determined. The inversion of frequencies of the longitudinal and transverse vibrations of the A ₁ and E symmetry in a range of 900–1150 cm^?1 is found. Based on the data thus obtained, the effective charges are calculated and the types of chemical bonds are analyzed for structural groups of the Li₂B₄O₇ crystal.     

Raman scattering from 6Li+ and 7Li+ ions in lithium beta-alumina

Raman bands due to translational modes of ⁶Li⁺ and ⁷Li⁺ ions in beta-alumina were found in the frequency range of 340–410 cm^?1, which is much higher than expected from the corresponding values reported for other alkali-metal ions (20–100 cm^?1). Based on a model calculation, it is believed that the large shifts to higher frequencies are due to deviations of the Li⁺ ions from the mirror plane into “pocket sites” formed by oxygen ions above or below the Beevers-Ross sites     

Zur Bildungskinetik der F-Folgezentren in KCl

The process of F-center aggregation under light irradiation, which involves ionic movement at low temperatures (observable down to — 60°C), is not at all understood in its mechanism. It is the aim of this work to evaluate quantitatively the kinetics of this process for different F-aggregate centers. In part I the assoziation of F-centers in KCl crystals with isolated Na⁺ or Li⁺ ions was thoroughly investigated as the clearest model case of F-center-aggregation. The reaction product in these crystals after light irradiation, an F-center associated to a Na⁺ or Li⁺ ion as nearest (100) neighbor (F _A -center), is well established in its model and can be detected by its double peak absorption structure. By optical measurements of the rate of F _A -center formation in dependence on light-intensity, time, Na⁺ or Li⁺-concentration, F→F′ conversion rate and temperature, the kinetics of this reaction could be evaluated in a simple equation of bimolecular type. The analysis leads to the conclusion, that either the anion vacancy or the F′-center must be regarded as a unit of high thermal mobility (activation energy 0·6±0·05 eV, jump frequency about 10² sec^?1 at room temperature) which diffuses randomly in the lattice and can be captured by a Na⁺ or Li⁺ ion.     

Transport processes in heavy metal fluoride glasses

Na self-diffusion, Li self-diffusion, Na⁺–Li⁺ ion exchange, electrical conductivity, and mechanical relaxation have been studied below T_g on glasses of the system ZrF₄–BaF₂–LaF₃–AF (A=Na, Li), with A=10, 20, 30 mol%. Compared to the transport mechanism in alkali-containing silicate glasses, the mechanisms in these non-oxide glasses are anomalous. Thus the self-diffusion coefficient of Na decreases with increasing NaF content, whereas that of Li increases with increasing LiF content. Both the electrical conductivity and the Na⁺–Li⁺ ion exchange reach a minimum at ≈ 20 mol% LiF, and the mechanical relaxation shows one peak for the 20 and 30 mol% LiF-glasses and two peaks for the glass with 10 mol% LiF, evidencing both a contribution of F⁻ and Li⁺ ions to the transport. Moreover, the presence of the three partially interacting mobile species F⁻, Na⁺, Li⁺ obviously leads to an anionic–cationic mixed ion effect. Applying the Nernst–Einstein equation to the Li⁺ transport in LiF-containing glasses shows that its mechanism is dissimilar to that in oxide glasses. Calculated short jump distances possibly can be interpreted as an Li⁺ movement via energetically suitable sites near F⁻ ions. Likewise the Nernst–Planck model, successfully applied to the ionic transport in mixed alkali silicate glasses, obviously does also not hold for the present heavy metal fluoride glasses.     

Infrared absorption of aligned H−Na+H− complexes in Kcl crystals

The localized vibrations of a complex in KCl consisting of a triangular array of two H^? ions and one Na⁺ ion in substitutional, neighboring positions have been observed in infrared absorption. As one would expect for such a molecule, five modes are observed: two longitudinal modes at 365 and 508 cm^-1, two complanar transverse modes along <110> at 459 and 562 cm^?1, and one transverse mode along <100> at 551 cm^?1.     

Enhanced electrical properties of polyethylene oxide (PEO) + polyvinylpyrrolidone (PVP):Li<Superscript>+</Superscript> blended polymer electrolyte films with addition of Ag nanofiller

Polymer blended films of polyethylene oxide (PEO)?+?polyvinyl pyrrolidone (PVP):lithium perchlorate (LiClO₄) embedded with silver (Ag) nanofiller in different concentrations have been synthesized by a solution casting method. The semi-crystalline nature of these polymer films has been confirmed from their X-ray diffraction (XRD) profiles. Fourier transform infrared spectroscopy (FTIR) and Raman analysis confirmed the complex formation of the polymer with dopant ions. Dispersed Ag nanofiller size evaluation study has been done using transmission electron microscopy (TEM) analysis. It was observed that the conductivity increases when increasing the Ag nanofiller concentration. On the addition of Ag nanofiller to the polyethylene oxide (PEO)?+?polyvinyl pyrrolidone (PVP):Li⁺ electrolyte system, it was found to result in the enhancement of ionic conductivity. The maximum ionic conductivity has been set up to be 1.14?×?10^?5 S cm^?1 at the optimized concentration of 4 wt% Ag nanofiller-embedded (45 wt%) polyethylene oxide (PEO)?+?(45 wt%) polyvinyl pyrrolidone (PVP):(10 wt%) Li⁺ polymer electrolyte nanocomposite at room temperature. Polyethylene oxide (PEO)?+?polyvinyl pyrrolidone (PVP):Li⁺ +Ag nanofiller (4 wt%) cell exhibited better performance in terms of cell parameters. This is ascribed to the presence of flexible matrix and high ionic conductivity. The applicability of the present 4 wt% Ag nanofiller-dispersed polyethylene oxide (PEO)?+?polyvinyl pyrrolidone (PVP):Li⁺ polymer electrolyte system could be suggested as a potential candidate for solid-state battery applications. Dielectric constants and dielectric loss behaviours have been studied.     

Spectroscopic properties, energy transfer and structural analysis of Sr2CeO4:M+ and Sr2CeO4:Eu3+, M+ (M+ = Li+, Na+, K+)

The room-temperature luminescent emission characteristics of Sr₂CeO₄:M⁺ and Sr₂CeO₄:Eu³⁺,M⁺ (M⁺ = Li⁺, Na⁺, K⁺) have been investigated under UV excitation. By introducing appropriate alkali metal cations dopants (Li⁺, Na⁺, K⁺) into the crystalline lattice, not only emission color of the blue-white-emitting Sr₂CeO₄ doped with low Eu³⁺ content can be tuned to green, but also the red emission intensity of Sr₂CeO₄ doped with high Eu³⁺ concentration is strengthened significantly. The relevant mechanisms have been elucidated in detail.     

Temperature investigations of Raman spectra of stoichiometric and congruent lithium niobate crystals

In the temperature range 100–450 K, we have investigated Raman spectra of congruent and stoichiometric LiNbO₃ crystals. We have found that, in this temperature range, frequencies and widths of all the spectral lines depend linearly on temperature. However, the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Li⁺ ions depends on temperature much more weakly than the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Nb⁵⁺ ions. This fact indicates that the anharmonicity of vibrations of Nb⁵⁺ ions along the polar axis is much stronger compared to vibrations of Li⁺ ions. It is likely that this anharmonicity is noticeably contributed by O^2? ions, which are characterized by an anharmonic potential, vibrations of which, according to calculations from first principles, are mixed with vibrations of Nb⁵⁺ ions. The anharmonicity of vibrations of O^2? ions is evidenced by a strong temperature dependence of the width of the line that corresponds to vibrations of the A₁(TO) symmetry of O^2? ions perpendicularly to the polar axis. We have found that the temperature dependence of the intensity of lines that correspond to fundamental vibrations is nonmonotonic. At the same time, the temperature dependence of the intensity of “superfluous lines” is strictly linear. It is likely that this behavior of the intensities of lines of fundamental vibrations is related to the occurrence of clusters and microstructures in the crystal structure.     

F−, Cl−, Li+ and Na+ adsorption on AlN nanotube surface: A DFT study

Adsorption of two anions (F⁻ and Cl⁻) and two cations (Li⁺ and Na⁺) on the surface of aluminum nitride nanotubes (AlNNTs) is investigated by density functional theory. The reactions are site-selective, so that the cations and anions prefer to be adsorbed atop the N and Al atoms of the tube surface, respectively. The adsorption energies of anions (−4.46 eV for F⁻ and −1.12 eV for Cl⁻) are much higher than those of cations (about −0.17 eV for Li⁺ and −0.12 eV for Na⁺) which can be explained using frontier molecular orbital theory. It was found that the adsorption of anions may facilitate the electron emission from the AlNNT surface by reducing the work function due to the charge transfer occurs from the anions to the tube. It has been predicted that in contrast to the cations the adsorption of anions also obviously increases the electrical conductivity of AlNNT.     

Downregulation of calcium-dependent NMDA receptor desensitization by sodium-calcium exchangers: a role of membrane cholesterol

Background

The plasma membrane Na⁺/Ca²⁺-exchanger (NCX) has recently been shown to regulate Ca²⁺-dependent N-methyl-d-aspartate receptor (NMDAR) desensitization, suggesting a tight interaction of NCXs and NMDARs in lipid nanoclasters or “rafts”. To evaluate possible role of this interaction we studied effects of Li⁺ on NMDA-elicited whole-cell currents and Ca²⁺ responses of rat cortical neurons in vitro before and after cholesterol extraction by methyl-β-cyclodextrin (MβCD).

Results

Substitution Li⁺ for Na⁺ in the external solution caused a concentration-dependent decrease of steady-state NMDAR currents from 440?±?71 pA to 111?±?29 pA in 140 mM Na⁺ and 140 mM Li⁺, respectively. The Li⁺ inhibition of NMDAR currents disappeared in the absence of Ca²⁺ in the external solution (Ca²⁺-free), suggesting that Li⁺ enhanced Ca²⁺-dependent NMDAR desensitization. Whereas the cholesterol extraction with MβCD induced a decrease of NMDAR currents to 136?±?32 pA in 140 mM Na⁺ and 46?±?15 pA in 140 mM Li⁺, the IC₅₀ values for the Li⁺ inhibition were similar (about 44 mM Li⁺) before and after this procedure. In the Ca²⁺-free Na⁺ solution the steady-state NMDAR currents after the cholesterol extraction were 47?±?6% of control values. Apparently this amplitude decrease was not Ca²⁺-dependent. In the Na⁺ solution containing 1 mM Ca²⁺ the Ca²⁺-dependent NMDAR desensitization was greater when cholesterol was extracted. Obviously, this procedure promoted its development. In agreement, Li⁺ and KB-R7943, an inhibitor of NCX, both considerably reduced NMDA-activated Ca²⁺ responses. The cholesterol extraction itself caused a decrease of NMDA-activated Ca²⁺ responses and, in addition, abolished the effects of Li⁺ and KB-R7943. The cholesterol loading into the plasma membrane caused a recovery of the KB-R7943 effects.

Conclusions

Taken together our data suggest that NCXs downregulate the Ca²⁺-dependent NMDAR desensitization. Most likely, this is determined by a tight functional interaction of NCX and NMDAR molecules because of their co-localization in membrane lipid rafts. The destruction of these rafts is accompanied by an enhancement of NMDAR desensitization and a loss of NCX-selective agent effects on NMDARs.

     

Infrared intensities of ion-water interactions in aqueous electrolyte solutions

The i.r. absorption intensities have been measured of the fundamental vibrations of water molecules in metal halide aqueous solutions. Pseudo-isosbestic points were observed in the regions of both the stretching and bending vibrations and interpreted in terms of two states of water molecules in the solutions. The intensities of the stretching vibrations of water molecules coordinated to the dissolved ions have been estimated. The intensities become stronger in the order, Li⁺ > Na⁺ > K⁺ for the cations and F^- > Cl^- > Br^- > I^- for the anions. These orders are consistent with the strengths of the ion-water interactions, which are related to the dynamical behavior of the water molecules in the primary hydration shell.     

Manifestation of interaction effects in Raman spectra of CdTe quantum-dot layers with ZnTe narrow barriers

The Raman spectra of superlattices consisting of layers of CdTe self-assembled quantum dots separated by ZnTe narrow barriers with thicknesses of 10 and 5 monolayers are investigated. It is found that, apart from the bands previously observed at frequencies of ～120 and ～140 cm^?1 for samples with thicker barriers (25 and 12 monolayers), the Raman spectra exhibit a band at ～147 cm^?1 in the frequency range of CdTe vibrational modes. This band is attributed to a symmetric vibrational mode of a pair of quantum dots with oppositely directed oscillations of the dipole moments. It is this type of vibrational mode in the material surrounding the ZnTe quantum dot that accounts for the shift of the band at ～200 cm^?1 near the LO mode of ZnTe vibrations toward lower frequencies.     

插层化合物Li1+xV3O8的合成及其物理性能的初步研究

本文成功地合成了插层化合物Li_1+xV₃O₈,并用X射线粉末衍射和等离子体发射光谱元素定量分析进行了鉴定。同时进行了NMR研究。在298—400K之间观察到了锂离子的运动致窄效应,并以此求得它的离子运动激活能为0.18eV。还测得该材料的室温电导率为10^-3Ω^-1·cm^-1数量级。 关键词：