Nonuniversal features of the ac conductivity in ion conducting glasses

Recently, Sidebottom [Phys. Rev. Lett. 82, 3653 (1999)] proposed a new scaling approach for the conductivity spectra of ion conducting glasses. This approach is based on the condition that the shape of the spectra is universal. In this Letter, we show that this condition is generally not fulfilled, but that the shape depends on the glass composition. In single alkali glasses, the frequency dependence of the conductivity varies with the alkali oxide content. Furthermore, the mixing of dissimilar alkali ions leads to pronounced changes in the shape of the conductivity spectra.     

A comparative study of lithium and sodium salts in PAN-based ion conducting polymer electrolytes

The conducting polymer electrolyte films consisting of polyacrylonitrile (PAN) as the host polymer, lithium triflate (LiCF₃SO₃) and sodium triflate (NaCF₃SO₃) as inorganic salts were prepared by the solution-cast technique. The pure PAN film was prepared as a reference. The ionic conductivity for the films is characterized using impedance spectroscopy. The room temperature conductivity for the PAN + 26 wt.% LiCF₃SO₃ film and the PAN + 24 wt.% NaCF₃SO₃ film is 3.04 × 10⁻⁴ S cm⁻¹ and 7.13 × 10⁻⁴ S cm⁻¹, respectively. XRD studies show that the complexation that has occurred in the PAN containing salt films and complexes formed are amorphous. The FTIR spectra results confirmed the complexation has taken place between the salt and the polymer. These results correspond with surface morphology images obtained from SEM analysis. The conductivity–temperature dependence of the highest conducting film from PAN + LiCF₃SO₃ and PAN + NaCF₃SO₃ systems follows Arrhenius equation in the temperature range of 303 to 353 K. The PAN containing 24 wt.% LiCF₃SO₃ film has a higher ionic conductivity and lower activation energy compared to the PAN containing 26 wt.%LiCF₃SO₃ film. These results can be explained based on the Lewis acidity of the alkali ions, i.e., the interaction between Li⁺ ion and the nitrogen atom of PAN is stronger than that of Na⁺ ion.     

Aging behavior and ionic conductivity of ceria-based ceramics: a comparative study

An understanding of high-temperature aging effects on the electrical properties of electrolytes is very important in selecting optimum compositions for practical applications. The aging behavior and mechanisms of doped zirconia ceramics have been extensively studied. However, little information is available regarding the aging behavior of ceria-based electrolytes. The present study has demonstrated that a high-temperature aging at 1000 °C has a significant effect on the ionic conductivity of the Y- or Gd-doped ceria (Ce_1−xY_xO_2−δ and Ce_1−xGd_xO_2−δ), especially in the case of the Gd doping. The aging behavior is characterized by a critical dopant concentration, above which the aging has a detrimental effect on the conductivity of the doped ceria ceramics. The aging behavior in the doped ceria cannot be explained using the aging mechanisms applied to the doped zirconia. Instead, the formation of the microdomains in the doped ceria has been acknowledged to be the main contribution to the aging behavior of the Y- or Gd-doped ceria ceramics. The formation ability of microdomains has been estimated to be in the order of La³⁺>Gd³⁺>Y³⁺, based on the degree of size mismatch between the dopant ion and Ce⁴⁺ ion. The critical dopant concentrations at which the microdomains start to form for La³⁺, Gd³⁺ and Y³⁺ in the doped ceria ceramics are x=0.15, 0.2 and 0.25, respectively. This critical dopant concentration is also an important indication: below which the conductivity is governed by only the association enthalpy, and above which the conductivity is dominated mainly by the microdomains rather than the association enthalpy.     

A Raman and dielectric study of ferroelectric ceramics

Dielectric and Raman scattering experiments were performed on various ceramics with composition Ba(Ti_1-xZr_x)O₃. Such lead-free, environmental-friendly materials were shown, from dielectric measurements, to exhibit behaviours extending from conventional to relaxor ferroelectrics on increasing the zirconium concentration. The evolution of the Raman spectra was studied as a function of temperature for various compositions, and the spectroscopic signature of the corresponding phases was determined. In the relaxor state, the variation of the integrated intensity of the Raman lines with temperature showed a plateau at low temperature. This anomaly was also detected as a peak in depolarization current measurements, and attributed to ergodicity breaking which characterizes usual relaxor systems. Raman results hint at locally rhombohedral polar nanoregions resulting from the random fields associated with Zr ions. Received 25 September 1998     

Factors affecting ionic conductivity in the lithium conducting glassy solid electrolytes

The electrical conductivity results of lithium borosilicate glasses with addition of Li₂SO₄ and LiCl have been critically analyzed. In general, it is observed that the factors viz. lithium fraction, f_Li and the number of non-bridging oxygens (NBOs) govern the ionic conductivity in the lithium conducting glasses. For the same f_Li, the presence of mixed formers in the glass gives higher conductivity compared to that of the glass with only one former. Thus the competitive network of glass in mixed former systems provides higher mobilities for lithium ions and hence high ionic conductivity. The addition of Li₂SO₄ and LiCl in the lithium borosilicate glasses gave enhancement in the conductivity. However, the mechanism of enhancement in conductivity is different in the two glass systems. The comparison of the result of binary, ternary and quaternary glass systems suggests that in general, the decrease in activation energy, increase in f_Li and increase in NBOs gives rise to enhancement in conductivity. For the same value of f_Li the higher conductivity is exhibited by glasses with lower value of K (K=SiO₂/B₂O₃). Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Scaling characteristics of ac conductivity and dielectric function in fractal regime of the metal-insulator transition

An analysis of the ac conductivity _ac(), and the ac dielectric constant, (), of the metal-insulator percolation systems is presented in the critical regime near the transition threshold. It is argued that the polarization and relaxation of the finite fractal metallic clusters play dominant roles in controlling the dynamic response of the system on both sides of the threshold. The relaxation time constant of a fractal cluster is shown to scale with its size as withd _t = 4 – 2d +d _c + /, whered is tge Euclidean dimension, andd _c , , and are the scaling indices for the charging, the dc conductivity, and the correlation length respectively. The average time dependent response of the system is shown to scale with a new time scale , where is the correlation length and ₀ is a microscopic time constant. It is shown that at frequencies and with /d_t 1, in close agreement with experiments. The effects of the anomalous transport along the infinite cluster and the medium polarizability are also discussed.     

Stretched exponential relaxation and dispersive conductivity behavior in lithium bismuth silicate glasses

Glasses having compositions xLi₂O∙(85 − x)Bi₂O₃∙15SiO₂ (x = 35, 40, and 45 mol%) were prepared by normal melt quenching technique. Electrical relaxation and conductivity in these glasses were studied using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 453 to 603 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency were extracted from the impedance spectra. The dc conductivity increases with increase in Li₂O content providing modified glass structure and large number of mobile lithium ions. Similar values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome the same energy barrier while conducting and relaxing. The non-exponential character of relaxation processes increases with decrease in stretched exponential parameter ‘β’ as the composition parameter ‘x’ increases. The observed conductivity spectra follow a power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Nearly constant losses (NCL) characterize this linearly dependent region of the conductivity spectra. A deviation from the ‘master curve’ for various isotherms of conductivity spectra was also observed in the high-frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.     

Dc-bias-field-induced dielectric relaxation and ac conduction in CaCu3Ti4O12 ceramics

The dielectric relaxation and ac conduction of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated at different temperatures under a dc bias. The dc bias gives rise to space charge accumulation, i.e. an electrode response, resulting in the significant increase of dielectric permittivity and dielectric loss tangent. Two Debye-like relaxations, arising from electrode and grain boundary responses, are present at low frequency with an increase of the dc bias. The electrode and grain boundary relaxations are distinguished according to the impedance spectroscopy and the frequency-dependent ac conductivity. The relaxation times of electrode and grain boundary relaxation are 0.955 ms and 0.026 ms, respectively, with a dc bias of 10 V at 328 K.     

A Raman study of lithium fluoride containing fast ionic conducting glasses

The role of fluorine in the structure of fluoro-borate fast ionic conducting glasses has been studied by Raman spectroscopy. It is shown that addition of LiF results in important structural changes, compared to binary glasses with the same O/B ratio. The Raman results are consistent with the participation of fluorine in the network. Careful control of the O/B ratio is required in order to detect the alkali fluoride-induced structural changes by Raman spectroscopy.     

Effects of uniaxial stress on dielectric properties of lithium modified potassium sodium niobate ceramics

In this work, the solid solution of ((K_0.5Na_0.5)_1−xLi_x)NbO₃ ceramics with x=0.03, 0.04, 0.05, 0.06 and 0.07 was prepared by a conventional mixed-oxide and solid-state sintering method. The structural phase formation and microstructure were characterized by X-ray diffraction technique and scanning electron microscopy. The ceramics were identified by XRD as a single-phase perovskite structure with symmetry gradually changing from orthorhombic to tetragonal. The grain size and the optimum density of the sintered ceramics were noticeably compositional-dependent. The dielectric properties of the ((K_0.5Na_0.5)_1−xLi_x)NbO₃ ceramics under the uniaxial compressive stress were observed at stress up to 180 MPa. The results showed that the dielectric constant and the dielectric loss tangent increased with applied stress. The change in the dielectric properties with stress was seen to depend on the composition and grain size. The observations were interpreted in terms of the intrinsic and extrinsic contributions to the changes in dielectric properties upon the applied compressive stress.     

Structural,dielectric and ac conductivity properties of Ni-doped HoFeO3 before and after gamma irradiation

A series of samples of HoFe_1?x Ni _x O₃ (x = 0.0, 0.1, 0.3) were prepared using the solid-state reaction technique to understand the structural, dielectric and conductivity properties before and after gamma irradiation of accumulated dose of 625 KGy. The X-ray diffraction confirms that all the samples exist in single-phase orthorhombic structure having space group Pbnm. With increasing dopant Ni, the unit cell volume and lattice parameters undergo small change. X-ray analysis show change in the interplanar spacing and full width at half maximum values after gamma irradiation. The Raman spectra of the samples show modifications after gamma irradiation. It can be easily seen that after gamma irradiation intensity, peak width are completely altered by gamma-absorbed dose. Measurement of dielectric loss and dielectric constant at room temperature was performed before and after gamma irradiation in the frequency range of 20 Hz–1 MHz. It is observed that the value of dielectric constant decreases after irradiation. The ac conductivity is estimated from the dielectric constant and loss tangent. Exposure to gamma radiation results in substantial modification in the physical properties of the Ni-doped Ho-based orthoferrites.     

Raman study on defect structure of high-temperature protonic conducting ceramics

Various acceptor-doped strontium cerates, Sr(Ce,M)O_3−δ (M; dopant ions), were synthesized and treated under dry atmosphere. Each Raman spectrum was measured just after the dry treatment and compared with that of the as-sintered sample, focusing on the defect Raman band at 630 cm^− 1. Relative intensity of the defect band, which increases monotonously with an increase in δ, increased by the dry treatment due to removal of water from lattice and resulting oxygen vacancy formation. Effect of dopant species on water solubility was discussed from relative intensity and peak center of the defect band.     

High and low frequency Jonscher behaviour of an ionically conducting glass

The ac conductivity of an ionically conducting glass, 0.4Ca(NO₃)₂:0.6KNO₃, is reexamined. The data is seen to exhibit a Jonscher, power-law dependence on frequency; however, two power-law expressions are required to describe the ac response σ=G₀+A₁ω^n₁+A₂ω^n₂. An explanation is advanced involving the formation of clusters within the glass on cooling.     

A study of optical absorption and dielectric properties in lithium chromium diphosphate compound

The optical properties of diphosphate LiCrP₂O₇ compound prepared by the classic ceramic method were recorded at room temperature. Absorption spectrum shows the presence of five characteristics bands related to the octahedral transitions of Cr³⁺ from ground term ⁴A_2g to excited terms. Crystal field strength and inter electronic repulsion Racah parameters were deduced. The calculated value of direct \( {E}_g^{\mathrm{direct}} \)=1.62 eV energy gap has been found using Tauc’s procedure. Besides, the dielectric properties were carried out by impedance spectroscopy at different temperatures (460–700 K). The frequency and temperature dependent of the real ε ^′ and imaginary ε ^″ parts of the dielectric constant were discussed. The variation of the frequency power law of the imaginary part of dielectric constant was analyzed in terms of two different conduction mechanisms. Furthermore, the modulus plots can be characterized by the empirical Kohlrausch–Williams–Watts (K.W.W.) function and the obtained values of activation energies deduced from relaxation frequency are in order of E _a (I) = 0.49 eV and E _a (II) = 0.87 eV.

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Graphical abstract Optical and dielectric proprieties of lithium chromium diphosphate

     

A study on ac and dc conductivity characteristics of Y-doped BaCeO3 modified by Ar+ ion beam

The effects of surface modification on electrical characteristics in bulk, grain boundary and interface (electrolyte/electrode) of BaCe_0.9Y_0.1O_3-δ were investigated. The surface modification was performed by means of two processes: specimen was firstly irradiated by 10 keV Ar⁺ ion with dose of 1 × 10¹⁸ ions/cm² and then exposed to air. The modified surface was investigated by elastic recoil detection analysis (ERDA) for quantitative analysis of hydrogen concentration on the surface and alternating current (AC) and direct current (DC) conductivity measurements, respectively. The ERDA results showed that hydrogen concentration and reaction rate on the modified surface increased. The increase of hydrogen concentration was explained in terms of the increase of proton due to interaction between oxygen vacancy formed by modification and H₂O. In AC and DC electrical conductivity measurements, it concluded that the proton and electronic carrier generated on the surface by modification attributed to the increase of bulk, grain boundary and interface conductivity.     

Novel conducting lithium ferrite/chitosan nanocomposite: Synthesis,characterization, magnetic and dielectric properties

A study on Lithium ferrite/chitosan nanocomposite (LFCN), easily moldable into arbitrary shapes, as the conducting polymer and ferromagnetic characteristics is presented. The composite material is produced in the presence of Li_0.5Cr_0.1Fe_2.4O₄ and Li_0.5Co_0.1Fe_2.4O₄ nanoparticle by ex-situ polymerizations process. Various characterizations techniques have been used to explore the characteristic of the synthesized products. The frequency dependent dielectric properties and electrical conductivity of all the samples have been measured through complex impedance plot in the frequency range of 1 kHz–6 MHz at room temperature. It was observed that in case of (LFCN), fluctuation in value of (ε′) and (ε″) is ceased over the frequency range of 4 Mz which can be attributed to the steady storage and dissipation of energy in the nanocomposite system. Moreover, it is also observed that electrical conductivity of (LFCN) increases with frequency and its value was found to be (0.032–0.048) (ohm-cm)⁻¹ in frequency range of 1 kHz–6 MHz. Due to its low cost, a simple synthesis process and high flexibility, the proposed LFCN may find applications in various types of electronic components.