AC conductivity and dielectric properties of sulphate glasses

AC conductivity and dielectric properties of sulphate glasses have been studied as a function of temperature, frequency and variation in interalkali concentration. AC conductivity at frequencies well beyond the dielectric loss peaks seems to arise from local motion of alkali ions within the neighbouring potential wells. Activation energies for AC conductivity were found to be very much lower than those for DC conductivity. Further, AC conductivity seems to be independent of interalkali variation, whereas ε′ and tan δ show a mild degree of mixed alkali effect. The observations made here have been explained on the basis of a structural model earlier proposed by us for these glasses. Communication No. 167 from Solid State and Structural Chemistry Unit.     

Dynamics of dipolar glasses: elastic and dielectric properties

We have formulated a model for the coupled dynamics of the local polarization and the elastic deformations of dipolar glasses. Randomly interacting dipoles undergo a phase transition to a glassy state, which is signalled above T _c by a divergence of the relaxation time of the dipolar autocorrelation. Dipolar dynamics is directly observable in the dynamic dielectric susceptibility, for which we predict a power law dependence on frequency at and close to T _c. The slowing down of dipolar degrees of freedom also gives rise to dynamic critical anomalies in the elastic and structural properties, like the appearance of a central peak and divergence of the sound damping. All these phenomena are a consequence of dynamic scaling of the dipolar autocorrelation and hence interrelated. We discuss two scaling functions in detail: One is calculated within mean field theory and the other one is taken from numerical simulations of short range Ising spin glasses. The predictions of the model are in accordance with most experimental findings for dipolar glasses.     

Glass forming regions and dielectric properties of new phosphate glasses

The glass forming regions have been determined in the systems P₂O₅-A₂O-MO (A = Li, Na; M = Cu, Cd), P₂O₅-Bi₂O₃-Li₂O and P₂O₅-Bi₂O₃-ZnO. The largest vitreous domain has been found in the ternary diagram P₂O₅-Bi₂O₃-ZnO. The variation of the dielectric constants has been followed inside the glass regions. ε'_r increases with increasing amount of alkaline oxides and substantially diminishes with increasing percentage of either CuO or CdO.     

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⁺.     

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.     

Current transfer mechanism in metal-dielectric-metal systems based on vanadium-phosphorus glasses

Based on an analysis of the volt-ampere characteristics and temperature dependence of conductivity, the mechanism of current transfer is discussed in thin-film systems of metal/vanadium -phosphorus glass/metal. The conclusion is drawn that the magnitude of the current in samples obtained by sputtering of the glass on a hot backing (T_b = 200°C) is determined by the resistance of the volume of the glass film, but for samples sputtered on a cold backing (T_b = 30–90°C), by the resistance of the potential barriers at the metalglass interface. It is supposed that current transfer in a film of glass is accomplished by polarons of small radius.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp. 107–115, February, 1976.     

Ultrasound propagation in dielectric glasses

The susceptibility of a two-level system coupled to a phonon heat bath is calculated by setting up its equation of motion and expanding the memory function (or self energy) and the inhomogeneity to second order in the coupling potential. Averaging over disorder yields attenuation and variation with temperature of sound velocity, which are compared to previous results obtained in the framework of the Bloch equations. The relaxation time approximation is avoided; there are new terms in both relaxation and resonant contributions.     

Effects of vitrification suppression on structure morphology, conductivity and dielectric properties of vanadium phosphate glasses

Vitrification suppression in the (V₂O₅)_1−x (P₂O₅)_x glasses where x=0.10, 0.15, 0.20, and 0.25 was controlled by changing the rate of quenching glasses. The structure variations occurring in the glasses were detected by differential thermal analysis and optical microscope. The results implied the separation and growth of V₂O₅ orthorhombic microcrystal in the samples with x=0.10 and 0.15 whereas other samples did not illustrate remarkable changes in their microstructure. However, in temperature range between 300 and 473 K a semiconducting behavior for all samples appears during the study of electrical conductivity-temperature dependence. A decrease in conductivity values accompanied with some variations in activation energies by reducing quenching rate was observed. The conductivity results suggested that the conduction occurs by the phonon assisted hopping of a small polaron between V⁴⁺ and V⁵⁺ states at relatively higher temperature range above θ_D/2. Whereas at relatively low temperatures the conduction may occur by electron jumping between filled and empty states at Fermi level in the disordered matrix besides polaronic conduction. Reasonable values for the density of localized states, carrier concentration and carrier mobility were estimated and discussed. Also, dielectric constant and dielectric loss were studied as a function of frequency at different temperatures confirming the structure variations in the glass system.     

Superexchange mechanism of energy transfer between neighboring lanthanide ions in dielectric crystals

It is shown that unusually rapid energy transfer between neighboring lanthanide ions in dielectric crystals is caused by the efficient short-range interaction, which is similar to the magnetic superexchange interaction. Both these interactions are related to virtual transition of electrons between lanthanide ions via common bridge ligands. A general method is developed for calculating matrix elements of the effective superexchange Hamiltonian, and the rates of energy transfer in exchange-coupled pairs of lanthanide ions are estimated. The possibility of using the superexchange model of energy transfer for analysis of up-conversion in lanthanide crystals is discussed.     

Low frequency acoustic and dielectric measurements on glasses

The acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T < 0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd₃₀Zr₇₀, T_c = 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd₃₀Zr₇₀ also the internal friction is strain dependent.     

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.     

Order parameters and dielectric relaxation in betaine proton glasses

Mixed crystals of betaine, phosphite and betaine phosphate have been investigated using broadband di-electric spectroscopy at frequencies 10⁻¹Hz≤ν≤10⁹Hz and temperatures 1.5 K≤T≤300 K for several betaine phosphate concentrationsx. For 0.2≤x≤0.65 an orientational glass state is found at low temperatures. The broad susceptibility spectra were analyzed using the concepts of distributions of relaxation times and of distributions of energy barriers. A critical comparison of the different approaches is given. In the mixed crystals that show antiferroelectric order at low temperatures, charge transport phenomena are studied. The static permittivity of the proton glass-forming crystals is analyzed, in terms of effectively one- and three-dimensional Ising models that incorporate random fields and random bonds.     

Order parameters and dielectric relaxation in betaine proton glasses

Mixed crystals of betaine, phosphite and betaine phosphate have been investigated using broadband di-electric spectroscopy at frequencies 10^?1Hz≤ν≤10⁹Hz and temperatures 1.5 K≤T≤300 K for several betaine phosphate concentrationsx. For 0.2≤x≤0.65 an orientational glass state is found at low temperatures. The broad susceptibility spectra were analyzed using the concepts of distributions of relaxation times and of distributions of energy barriers. A critical comparison of the different approaches is given. In the mixed crystals that show antiferroelectric order at low temperatures, charge transport phenomena are studied. The static permittivity of the proton glass-forming crystals is analyzed, in terms of effectively one- and three-dimensional Ising models that incorporate random fields and random bonds.     

Nature of the constant-loss dielectric response of various crystals and glasses

For a wide range of crystalline materials and glasses, it has been shown that, at low temperatures, the ac conductivity () is proportional to frequency , corresponding to a loss per cycle that is independent of frequency. This paper explores the features of this constant-loss behavior, showing that it is a truly bulk phenomenon, that it is only slowly dependent on temperature (i.e., not activated) and, in the case of glasses, that it does not give rise to a mixed alkali effect. The pair approximation model with hopping over an asymmetric barrier is fitted to some of the results, but doubts remain as to whether this model is appropriate for systems with low defect concentrations.