DC conductivity of silver vanadium tellurite glasses

The mixed electronic-ionic conduction in 0.5[xAg₂O-(1−x)V₂O₅]-0.5TeO₂ glasses with x=0.1-0.8 has been investigated over a wide temperature range (70-425 K). The mechanism of dc conductivity changes from predominantly electronic to ionic within the 30?mol% Ag₂O?40 range; it is correlated with the underlying change in glass structure. The temperature dependence of electronic conductivity has been analyzed quantitatively to determine the applicability of various models of conduction in amorphous semiconducting glasses. At high temperature, T>θ_D/2 (where θ_D is the Debye temperature) the electronic dc conductivity is due to non-adiabatic small polaron hopping of electrons for 0.1?x?0.5. The density of states at Fermi level is estimated to be N(E_F)≈10¹⁹-10²⁰ eV⁻¹ cm⁻³. The carrier density is of the order of 10¹⁹ cm⁻³, with mobility ≈2.3×10⁻⁷-8.6×10⁻⁹ cm² V⁻¹ s⁻¹ at 300 K. The electronic dc conductivity within the whole range of temperature is best described in terms of Triberis-Friedman percolation model. For 0.6?x?0.8, the predominantly ionic dc conductivity is described well by the Anderson-Stuart model.     

Electron paramagnetic resonance, optical and electrical properties of vanadyl doped alkali germanoborate glasses

Glasses with composition xGeO₂.(0.30−x)M₂O.0.70B₂O₃ (M=Li, K) containing 2.0 mol% of V₂O₅ have been prepared in the range 0.00≤x≤0.15 by normal melt quenching method. Electron paramagnetic resonance (EPR), optical transmission and absorption spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHPs) of VO²⁺ ions, dipolar hyperfine coupling parameter, P, Fermi contact interaction parameter, K and molecular orbital coefficients (α² and γ²) have been calculated. In GeO₂·Li₂O·B₂O₃ glasses there is no change in the tetragonality of the V⁴⁺O₆ complex and the size of 3d_xy orbit also remains unchanged with increase in GeO₂ content. In GeO₂·K₂O·B₂O₃ glasses, there is an increase in the tetragonality of the V⁴⁺O₆ complex and the 3d_xy orbit expands with increase in GeO₂ content. Values of the theoretical optical basicity, Λ_th, have also been reported. Optical band gap decreases with increase in GeO₂ content. The dc conductivity of these glasses decreases and the activation energy increases with increase in GeO₂:M₂O ratio.     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Characterization and electrical properties of V2O5-CuO-P2O5 glasses

Characterization and electrical properties of vanadium-copper-phosphate glasses of compositions xV₂O₅-(40−x)CuO-60P₂O₅ have been reported. X-ray diffraction (XRD) confirms the amorphous nature of these glasses. It was observed that, the density (d) decreases gradually while the molar volume (V_m) increases with the increase of the vanadium oxide content in such glasses. This may be due to the effect of the polarizing power strength, PPS, which is a measure of ratio of the cation valance to its diameter. The dc conductivity increases while the activation energy decreases with the increase of the V₂O₅ content. The dc conductivity in the present glasses is electronic and depends strongly upon the average distance, R, between the vanadium ions. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron.     

Xanes (X-ray absorption near edge structure) of V in vanadium-iron phosphate glasses

The X-ray Absorption Near-Edge Structure (XANES) of V in vanadium-iron glasses (50P₂O₅ + (50?x)FeO + xV₂O₅) have been measured. The effective charge of V ions in glasses has been determined. At low V₂O₅ concentration (x ～ 5) only V⁴⁺ with 6-fold coordination is present on the contrary a static mixed valence state (V⁴⁺, V⁵⁺) has been found at high concentrations 20?x?50. The results explain the electron hopping conductivity effects at high V₂O₅ concentration (x ～ 50) involving V⁴⁺ ? V⁵⁺ pairs and at low V₂O₅ concentration (x?10) involving V⁴⁺ ? Fe³⁺ pairs.     

Dielectric properties and conductivity in CuO and MoO3 doped borophosphate glasses

A novel set of glasses of the type (B₂O₃)_0.10-(P₂O₅)_0.40-(CuO)_0.50−x-(MoO₃)_x, 0.05≤x≥0.50, have been investigated for dielectric properties in the frequency range 100 Hz-100 kHz and temperature range 300-575 K. From the total conductivity derived from the dielectric spectrum the frequency exponent, s, and dc and ac components of the conductivity were determined. The temperature dependence of dc and ac conductivities at different frequencies was analyzed using Mott's small polaron hopping model, and the high temperature activation energies have been estimated and discussed. The observed initial decrease in conductivity (ac and dc) and increase in activation energy with the addition of MoO₃ have been understood to be due to the hindrance offered by the Mo⁺ ions to the electronic motions. The observed peak-like behavior in conductivity (dip-like behavior in activation energy) in the composition range 0.20-0.50 mol fractions of MoO₃ may be due to mixed transition effect occurring in the present glasses. The temperature dependence of frequency exponent, s, has been analyzed using different theoretical models. It is for the first time that the mixed transition metal ion (TMI) doped borophosphate glasses have been investigated for dielectric properties and conductivity over wide temperature and frequency ranges and the data have been subjected to a thorough analysis.     

Structural and transport properties of CdI2 doped silver ion conducting system

The glass system xCdI₂-(100−x)[2Ag₂O-(0.7V₂O₅-0.3B₂O₃)] with different amount of dopant salt have been prepared by melt quenching technique. The sample obtained were pulverized and characterized by XRD, DSC, and FTIR. The electrical conductivity studies of the samples have been carried out at different temperatures and frequencies. Conductivity of the glasses increased with the increase in the CdI₂ contents and attains a value of 7.76×10⁻⁴ S/cm at room temperature for the composition having x=30 mol% of CdI₂. Infrared spectroscopic studies on these glasses indicated that the oxyanion network was not affected by the addition of CdI₂. The transport number of the silver ion determined by emf method is nearly unity. The frequency dependence of electrical conductivity for various glass compositions at different temperature has been analyzed in terms of Jonscher's Universal expression. In the present CdI₂ doped system, the conduction is due to the Ag⁺ ions attached to the AgI which is formed due to the exchange reaction between CdI₂ and Ag₂O.     

DC conductivity of new single and mixed alkali oxyfluorovanadate glasses

New glasses have been prepared according to these formulas (70-x)V₂O₅-30BaF₂-xAF, where AF=LiF or NaF and (60-x)V₂O₅-30BaF₂-10LiF-xAF, where AF=NaF and x=10, 15, 20, 25 and 30 mol%. Density of the glasses was measured and molar volume calculated and they correlated with the AF content. The dc conductivity has been measured in the temperature range from 302 to 453 K. The dc conductivity increases with temperature and V₂O₅ content, while it decreases with the alkali fluoride content. Conductivity has been correlated with the calculated polaron distance, R, and glass transition temperature, T_g. The activation energy, W, increases with the increase in the alkali fluoride, while it decreases with the V₂O₅ content. Some parameters like polaron distance, R, polaron radius, r_p, ion concentrations, n(V), n(Li) or n(Na), hopping energy, W_H, density of localized states at Fermi level, N(E_F), polaron coupling constant, γ_p, polaron band width, J, hopping mobility, μ, and carrier density for electronic conduction, N_c, were calculated to explain the conduction mechanism and behavior of the present glasses.     

Effect of Li2O on the structure,electrical and dielectric properties of xLi2O·(20−x)CaO·30P2O5·30V2O5·20Fe2O3 glasses

Glasses of the general formula xLi₂O·(20?x)CaO·30P₂O₅·30V₂O₅·20Fe₂O₃ with x=0, 5, 10, 15 and 20 mol% were prepared; IR, density, electrical and dielectric properties have been investigated. Lithia-containing glasses revealed more (P₂O₇)^4?, FeO₆, V–O^? and PO^? groups and mostly have lower densities than those of lithia-free ones. The electrical properties showed random behavior by replacing Li₂O for CaO, which has been assigned to the change of the glass structure. The results of activation energy and frequency-dependent conductivity indicate that the conduction proceeds via electronic and ionic mechanisms, the former being dominant. The mechanism responsible for the electronic conduction is mostly thermally activated hopping of electrons from Fe(II) ions to neighboring Fe(III) sites and/or from V⁴⁺ to V⁵⁺. The dielectric constant (ε′) showed values that depend on the structure of glass according to its content of Li₂O. The (ε′) values are ranging between 3 and 41 at room temperature for 1 kHz, yet at high temperatures, glass with 20 mol Li₂O exhibits values of 110 and 3600 when measurement was carried out in the range 0.1–1 kHz, and at 5 MHz, respectively.     

Electrical conduction and dielectric properties of vanadium phosphate glasses doped with lithium

AC conductivity and dielectric studies on vanadium phosphate glasses doped with lithium have been carried out in the frequency range 0.2-100 kHz and temperature range 290-493 K. The frequency dependence of the conductivity at higher frequencies in glasses obeys a power relationship, σ_ac=Aω^s. The obtained values of the power s lie in the range 0.5≤s≤1 for both undoped and doped with low lithium content which confirms the electron hopping between V⁴⁺ and V⁵⁺ ions. For doped glasses with high lithium content, the values of s≤0.5 which confirm the domination of ionic conductivity. The study of frequency dependence of both dielectric constant and dielectric loss showed a decrease with increasing frequency while they increase with increasing temperature. The results have been explained on the basis of frequency assistance of electron hopping besides the ionic polarization of the glasses. The bulk conductivity increases with increasing temperature whereas decreases with increasing lithium content which means a reduction of the V⁵⁺.     

Impact of MoO3 concentration,frequency and temperature on the dielectric properties of zinc phosphate glasses

Phosphate glasses with the chemical composition of 47P₂O₅–24ZnO-(29-x)Na₂O-xMoO₃, x = 0, 2, 4, 6, 8 and 10, have been prepared using the melt quenching technique. Dielectric properties of these phosphate glasses are carried out in the frequency range from 1 to 100 kHz at different temperatures. Dielectric parameters such as dielectric constant ε′, dielectric loss ε′′ and ac conductivity of the investigated glasses have been evaluated. The dependences of these dielectric parameters on frequency, composition and temperature have been discussed. It is found that dielectric constant decreases with increasing frequency due to the reduction of space-charge polarization and dipole polarization. The dependence of ac conductivity on the MoO₃ content indicates a competition between electronic and ionic conduction. The temperature dependence of the dielectric parameters reveals a rising trend of the dielectric parameters with temperature. This rising trend is indicated due to the increase of the amplitude of the thermal vibration of the charge carriers which facilitates the electron hopping and drifting of the mobile ions. The linear trend of the ln(σ_ac)-1000/T plot indicates that ac conductivity of the investigated glasses is thermally-activated transport process and follows the Arrhenius equation. The activation energy and its composition dependence have been reported.     

Investigation of vanadium–sodium silicate glasses using XANES spectroscopy

X-ray absorption near edge structure spectroscopy has been used to investigate the electronic and atomic structure of (V₂O₅)_x(Na₂O)_0.30(SiO₂)_0.70−x (x < 0.1) glasses obtained by melt-quench technique. The results show no sign of metallic clustering of V atoms, but mixed oxidation states (+4 and +5) of V and strong V3d–O2p hybridization in the glasses. Detailed analysis has revealed that the glass samples contain about 15% V⁴⁺ and 85% V⁵⁺ and the ligand-field splitting is about 1.6 eV.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

Conductivity and spectroscopic studies of Li<Subscript>2</Subscript>O-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

Lithium vanadium-borate glasses with the composition of 0.3Li₂O–(0.7-x)B₂O₃–xV₂O₅ (x?=?0.3, 0.325, 0.35, 0.375, 0.4, 0.425, 0.45, and 0.475) were prepared by melt-quenching method. According to differential scanning calorimetry data, vanadium oxide acts as both glass former and glass modifier, since the thermal stability of glasses decreases with an increase in V₂O₅ concentration. Fourier transform infrared spectroscopy data show that the vibrations of [VO₄] structural units occur at V₂O₅ concentration of 45 mol%. It is established that the concentration of V⁴⁺ ions increases exponentially with the growth of vanadium oxide concentration. Direct and alternative current measurements are carried out to estimate the contribution both electronic and ionic conductivities to the value of total conductivity. It is shown that the electronic conductivity is predominant in the total one. The glass having the composition of 0.3Li₂O-0.275B₂O₃-0.475V₂O₅ shows the highest electrical conductivity that has the value of 7.4?×?10^?5 S cm^?1 at room temperature.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

Dielectric,electrical transport and magnetic properties of Er3+substituted nanocrystalline cobalt ferrite

Erbium substituted cobalt ferrite (CoFe_2−xEr_xO₄; x=0.0–0.2, referred to CFEO) materials were synthesized by sol-gel auto-combustion method. The effect of erbium (Er³⁺) substitution on the crystal structure, dielectric, electrical transport and magnetic properties of cobalt ferrite is evaluated. CoFe_2−xEr_xO₄ ceramics exhibit the spinel cubic structure without any impurity phase for x≤0.10 whereas formation of the ErFeO₃ orthoferrite secondary phase was observed for x≥0.15. All the CFEO samples demonstrate the typical hysteresis (M–H) behavior with a decrease in magnetization as a function of Er content due to weak superexchange interaction. The frequency (f) dependent dielectric constant (ε′) revealed the usual dielectric dispersion. The ε′–f dispersion (f=20 Hz to 1 MHz) fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived are ∼10⁻⁴ s and ∼0.61(±0.04), respectively. Electrical and dielectric studies indicate that ε′ increases and the dc electrical resistivity decreases as a function of Er content (x≤0.15). Complex impedance analyses confirm only the grain interior contribution to the conduction process. Temperature dependent electrical transport and room temperature ac conductivity (σ_ac) analyses indicate the semiconducting nature and small polaron hopping.     

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V₂O₅-MnO-TeO₂ system, containing two transition-metal ions, have been measured. The dc conductivity _dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V^4+M and V⁵⁺ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V₂O₅-TeO₂ glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( _ac = Aω^s , with s < 1) is well exhibited by the ac conductivity σ_ac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σ_ac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.