Conductivity and dielectric studies on 20CdI2–80[xAg2O–y(0.7V2O5−0.3B2O3)] super ion conducting system where 1 ⩽ x/y ⩽ 3

The present paper reports the conductivity and dielectric studies of the quaternary super ionic conducting system 20CdI₂–80[xAg₂O–y(0.7V₂O₅−0.3B₂O₃)], to see the effect of modifier to former ion ratios. The observed frequency dependence of conductivity is found to obey Jonscher’s universal law. The dc conductivity obtained from the low frequency part of the conductivity spectrum is found to be non-linear with change in the x/y ratio. Frequency and temperature dependence of the dielectric constant can be explained by Stevel’s model for dielectric relaxation in glasses. The conductivity and dielectric studies suggest that ionic motion in the system is responsible for the conductivity and relaxation effect. The silver ion transport number (t_Ag+) in the CdI₂ doped system is obtained by emf technique.     

Study of CaO-WO3-P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Dielectric constant ε^′, loss tanδ and ac conductivity σ_ac of 40CaO-xWO₃-(60−x)P₂O₅ (with 0?x?15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., ε^′, tanδ and σ_ac of CaO-P₂O₅ glasses are found to decrease with the introduction of WO₃ up to 3 mol% and increase with further increase in the concentration of WO₃; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tanδ with temperature at different frequencies of CaO-P₂O₅ glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO₃ from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO₃ beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole-Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.     

Dielectric relaxation in iron-containing borate glasses

The ac and dc conductivities, dielectric constant and dielectric loss of a series of calcium borate glasses containing Fe₂O₃ up to 10 mol% were measured as a function of temperature (300–700 K) and frequency (10²–10⁵ c/s). Glasses melted in platinum crucibles were found to show slightly higher dielectric constants and conductivities than glasses melted in zirconia crucibles under the same melting conditions. Relaxation effects with a distribution of relaxation times were observed for all the glasses of this system. The data show that the pre-exponential factor τ₀ is about 10^?11 s. Peaks in tan δ were found to shift to higher temperatures when the frequency of measurements was increased. Activation energy for dc conduction was found to be equal to that of ac conduction. No polarization effects were observed in these glasses, indicating that conduction is predominantly electronic.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Low temperature dielectric dispersion and electrical conductivity studies on Fe2O3 mixed lithium yttrium silicate glasses

Lithium yttrium silicate glasses mixed with different concentrations of Fe₂O₃ of the composition (40 ? x) Li₂O–10Y₂O₃–50SiO₂: x Fe₂O₃, with x = 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5 (all in mol%) were synthesized. Electrical and dielectric properties including dielectric constant, ε′(ω), loss, tan δ, ac conductivity, σ_ac, impedance spectra as well as electric moduli, M(ω), over a wide continuous frequency range of 40 Hz to 10⁶ Hz and in the low temperature range 100 to 360 K were measured as a function of the concentration of Fe₂O₃. The dc conductivity is also evaluated in the temperature range 100 … 360 K. The temperature and frequency dispersions of dielectric constant as well as dielectric loss have been analyzed using space charge polarization model. The ac and dc conductivities have exhibited increasing trend with increasing Fe₂O₃ content beyond 0.5 mol%, whereas the activation energy for the conductivity demonstrated decreasing tendency in this dopant concentration range. Both quantum mechanical tunneling (QMT) and correlated barrier hopping models (CBH) were used for clarification of ac conductivity origin and the corresponding analysis has indicated that CBH model is more appropriate for this glass system. For the better understanding of relaxation dynamics of the electrical properties we have drawn the scaling plots for ac conductivity and also electric moduli. The plots indicated that the relaxation dynamics is independent on temperature but depends on concentration of Fe₂O₃. The dc conductivity is analyzed using small polaron hoping model. The increase of conductivity with the concentration of Fe₂O₃ beyond 0.5 mol% is explained in terms of variations in the redox ratio of iron ions in the glass network. The results were further analyzed quantitatively with the support of experimental data from IR, optical absorption and ESR spectral studies. The overall analysis has indicated that Li₂O–Y₂O₃–SiO₂ glasses containing more than 0.5 mol% of Fe₂O₃ are more suitable for achieving good electrical conductivity in these glasses.     

Charged defects in liquid chalcogenides

The conductivity of liquid chalcogenides has been measured at frequencies 2 Hz - 2×10⁵ Hz and 10¹⁰ Hz in the temperature range 500 K to 830 K. While in liquid Se the conductivity increases with increasing frequency no frequency dependence of the conductivity is found in liquid As₂Se₃. A simple dielectric model explains the change of conductivity with frequency and temperature.     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Frequency dependence of conduction and polarization in conductive polymers

We report the results of the measurement and analysis of the complex conductivities of two high polymers over the frequency range 10²–10⁶ Hz, and temperature range 70–300 K. Giant polarization of the nomadic type is observed, with dielectric constants ranging from about 50 to 6000 in these aromatic hydrocarbon polymers. The complex conductivities resemble power law behavior, σ_ac = Aω^s (with s in the range 0.7–1.0) in some temperature ranges, and deviates from this in others. The dc conductivity and the real part of the ac conductivity at various frequencies follow a $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law. The dielectric constant varies as expected for nomadic polarization in long-chain molecules. An attempt is made to develop an understanding of the observed dependences of the complex conduction or polarization on temperature and frequency in terms of interchain and intrachain transport processes.     

Dielectric properties and AC conductivity studies of novel NR/PVA full-interpenetrating polymer networks

Fully interpenetrating polymer networks (IPN) based on natural rubber (NR) and polyvinyl alcohol (PVA) were prepared by using Glutaraldehyde as the common crosslinking agent. Crosslinking reactions were confirmed by Fourier Transform Infra-red Spectroscopy. The electrical properties of full-IPN have been studied in the frequency range of 10²–10⁶ Hz. The dielectric constant, dielectric loss and ac conductivity were analyzed as a function of frequency, temperature and blend composition. It was found that the dielectric constant and dielectric loss increased with the addition of PVA into NR. Interpenetrated system showed a significant reduction in dielectric constant and dielectric loss and almost frequency independent. The dielectric constant and dielectric loss were increased with increase in temperature. The change in these parameters with temperature in the IPN system was found too low as compared to the pure blends. The ac conductivity study revealed that the IPN materials exhibited a considerable reduction in conductivity.     

Elastic and dielectric properties of A<Superscript>II</Superscript>B<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack> (<Emphasis Type="Italic">A</Emphasis> = Cd or Zn, <Emphasis Type="Italic">B</Emphasis> = P or As) single crystals

Elastic and dielectric properties of CdP₂, ZnP₂, and ZnAs₂ single crystals are investigated at frequencies of 10², 10³, 10⁴, 10⁶, and 10⁷ Hz in the [00l], [h00], and [hk0] directions in the temperature range 78–400 K. The elastic constants, the Gruneisen parameters, and the force constants of the crystals are calculated from the measured ultrasonic velocities. The elastic constants C_ij decrease with an increase in temperature and anomalously change in narrow (ΔT = 10–20 K) temperature ranges. The permittivity sharply increases from ε ≈ 7–14 at 78–150 K to ε ≈ 10²–10³ in the temperature range 175–225 K without any signs of a structural phase transition. The behavior of the temperature-frequency dependences of the complex permittivity ε*(f, T) is typical of relaxation processes. The dielectric relaxation in A^IIB ₂ ^V is considered on the basis of the model of isolated defects. The conuctivity σ of the single crystals under study is a sum of the frequency-dependent (hopping) conductivity σ_h and the conductivity σ_s that is typical of semiconductors. The hopping conductivity increases with an increase in frequency according to the law σ _h ～f^α, where α < 1 at low temperatures and α > 1 at high temperatures.     

Growth and study of PbFe1/2Ta1/2O3 single crystals

Single crystals of the composition PbFe_1/2Ta_1/2O₃ are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe_1/2Ta_1/2O₃ ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, T _m, increases by more than 70 K with an increase in the frequency from 10² to 10⁶ Hz. It is assumed that the unusual properties of the PbFe_1/2Ta_1/2O₃ crystals are caused by mesoscopically inhomogeneous compositional ordering and comparatively high conductivity providing favorable conditions for the appearance of the volume-charge and thermal electron polarization.     

Flux growth and low temperature dielectric relaxation in piezoelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

Single crystals of Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT 91/9) have been grown by flux method after modifications in temperature profile, flux ratio and addition of excess ZnO/B₂O₃ which resulted in enhanced perovskite yield (more than 95%). Only a few crystals showed the presence of pyrochlore phase/variation in composition. A comparative characterization of these crystals were carried out in respect of piezoelectric charge coefficient d₃₃, dielectric constant, ac conductivity and hysteresis loop after cutting and poling the crystals along [001] direction. The total activation energy for conduction has been found to increase with Ti‐content in the sample. The effect of ZnO on growth behavior has been analyzed. A detailed analysis of PZNT (91:9) has been carried out at low temperature in respect of the various thermodynamic parameters related to the dielectric relaxation mechanism, like optical dielectric constant, static dielectric constant, free energy of activation for dipole relaxation, enthalpy of activation and relaxation time, have been calculated in the vicinity of transition temperature in the lower temperature region. The activation energy for relaxation at ‐10 and ‐49 °C have been found to be 0.09 and 0.02 eV respectively. The results were analyzed and a detailed dielectric analysis and low temperature relaxation behavior of PZNT crystals were interpreted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

AC conductivity of (As2Te3)95Ge5

The elctrical conductivity of amorphous chalcogenide (As₂Te₃)₉₅Ge₅ is investigated at variable frequency, from 1 kHz up to 35 GHz, and a variable temperature, from 77 to 300 K. The low-temperature conductivity is constant with temperature at a fixed frequency. At a fixed temperature, it obeys a ω^0.8 law only at low frequencies. The results are analysed with respect to the polaron problem, but also with a simple model of several dilute localized levels having a wide energy distribution in the forbidden band.     

Investigation of near constant loss contribution to conductivity in lithium bismo-silicate glasses

Glasses having compositions 20Li₂O · (80 − x)Bi₂O₃ · xSiO₂ (x = 55, 60, 65, 70 mol%) were investigated using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 543 to 663 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. The increase in conductivity with increase in SiO₂ content is attributed to the change in the structural units of bismuth. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Similar values of activation energy for dc conduction and for conductivity relaxation time indicates that the ions overcome same energy barrier while conducting and relaxing. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Near constant losses (NCL) characterize this linearly dependent region of conductivity spectra. A deviation from ‘super curve’ for various isotherms of conductivity spectra was also observed in 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.     

Electrical properties of A2.6+xTi1.4−xCd(PO4)3.4−x (A = Li,K; x = 0.0–1.0) phosphate glasses

Electrical properties of A_2.6+xTi_1.4−xCd(PO₄)_3.4−x (A = Li, K; x = 0.0–1.0) phosphate glasses are investigated over a frequency range from 42 Hz to 1 MHz at different temperatures. Impedance spectroscopy is used to separate the bulk conductivity from electrode effect of electrical conductivity data. The bulk dc conductivity is Arrhenius activated, with activation energies and pre-exponential factors following the Meyer–Neldel rule. The real part of ac conductivity shows universal power law feature. The variation of dielectric constant with frequency is attributed to ion diffusion and polarization occurring in the phosphate glasses. The frequency dependent imaginary part of electric modulus M″(ω) plot shows non-Debye feature in conductivity relaxation. The Kohlrausch–Williams–Watts stretched exponential function was used to describe the modulus spectra and the stretching exponent β is found to be temperature independent. Scaling in M″(ω) shows that the electrical relaxation mechanisms are independent of temperature for given composition at different temperatures.     

Ac conductivity and dielectric relaxation in ionically conducting soda-lime-silicate glasses

The frequency dependent conductivity and dielectric relaxation of alkali ions in some soda-lime-silicate (Na₂O-CaO-SiO₂) glasses are investigated over a frequency range from 50 Hz to 1 MHz and in a temperature range from room temperature to 603 K by using alternating current impedance spectroscopy. The conductivity isotherms show a transition from frequency independent dc region to dispersive region where the conductivity continuously increases with increasing frequency. The electric modulus representation has been used to provide comparative analysis of the ion transport properties in these glasses. The scaling behavior of imaginary part of electric modulus indicates that all dynamical processes occurring at different frequencies give the same activation energy.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li_1+xMn_2−xO₄, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn₂O₄ exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn₂O₄, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons.     

Molybdenum doping of semiconducting vanadium phosphate glasses

The DC conductivity and dielectric properties of glasses of composition (70 ? x) V₂O₅ : x MoO₃ : 30 P₂O₅ have been measured as a function of temperature and frequency for O × 5 mol %. An increase in conductivity by two orders of magnitude is observed for 1 mol % MoO₃ and this is correlated with changes in activation energy and dielectric constant. The results can be explained in terms of small polaron theory, with the main interaction being through the local electronic polarizability at any site. The results indicate that percolation considerations have to be taken into account in describing the electrical properties of transition metal glasses.     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.