L. Murawski

Electrical conductivity in iron-containing oxide glasses

The electrical properties of silicate, borate, phosphate and tellurite glasses containing iron are reviewed. The highest electrical conductivity has been observed in iron-tellurite glasses. The problem of the structure and influence of different glass modifiers on the electrical conduction of iron-containing glasses is discussed.

Dielectric properties of transition metal oxide glasses

Abstract Dielectric properties of iron and vanadium phosphate glasses containing different glass modifiers have been studied in a broad range of frequency and temperature by the absorption current and with an ac transformer bridge. The dielectric spectra exhibit a peak that can be related to the electron transfer between iron atoms in different valency states. This peak and the dc conductivity revealed a similar activation energy. The data obtained were interpreted in the framework of a new model of relaxation in glasses recently proposed by Hunt.

Electronic conductivity in Na2O-FeO-P2O5 glasses

dc and ac conductivity were studied in 50P2O5–(50−x)FeO–xNa2O and 50P2O5–(50−x)FeO–xCaO (7<x<20; in mol%) glasses. It is shown that the replacement of CaO by Na2O caused rather small increase in dc and ac conductivity. The conduction in these glasses is predominantly electronic controlled by electron hopping between Fe(II) and Fe(III) sites. Internal friction has been used to study the relaxation phenomena caused by electronic and ionic migration in sodium–iron phosphate glasses. Two peaks attributed to these processes were observed on internal friction spectra in the temperature range of 125–500 K at a frequency of about 1 Hz.

Dielectric relaxation in semiconducting oxide glasses

Abstract The ac conductivity of iron and vanadium phosphate glasses containing different glass modifiers have been measured in a broad range of frequency and temperature. The results confirm the applicability of Hunts theory in the pair approximation regime. The relation between ac and dc conductivity: σac ∝ σdc1 −s (for ω = const) is observed in all investigated glasses. The discharge current was measured in several iron phosphate glasses. A comparison was made between two widely used empirical methods of ac data analysis, namely, the Kohlrausch-Williams-Watts (KWW) approach, which starts from a stretched exponential current in the time domain, and the Jonscher power law. The parameter, β, which expresses the non-exponential nature of the electric current decay, is not substantially different if compare to the value of 1−s obtained from the frequency dependence of the ac conductivity.

Mixed electronic-ionic conductivity in transition metal oxide glasses containing alkaline ions

Abstract In alkali free transition metal oxide glasses conductivity is described by small polaron hopping. The carrier concentration is related to the concentration of transition metal ions in different valence states. Generally ionic conduction depends on the alkali concentration and ion mobility. Assuming that the motion of alkali ions and polarons is independent one may expect that the electrical conductivity increases with the increase in the alkali content. However, real transition metal oxide glasses containing alkali show various behaviors – from strong conductivity anomalies to conductivity weakly dependent on alkali content. A comparison of the electrical properties of iron phosphate glasses containing different amounts of sodium ions and copper oxide glasses to other TMO glass systems has been performed. The influence of the role of transition metal oxide in the glass forming network on the electrical properties of glass has been outlined.

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.

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Abstract Vanadium EXAFS spectra of 50V2O5-50P2O5 glasses with different C = V+4/Vtot have been measured. The V-O distances increase by ΔR 1 = (0.03 ± 0.02) A to ΔR 2 = (0.07 ± 0.03) A going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V4+ and V5+ ionic states. The density of the glasses increases with C from the value d1 = 2.81 g/cm3 for C = 0.51 to d2 = 2.92 g/cm3 for C = 0.84 indicating a more random packing of glass units.

The electronic conductivity mechanism in Bi-Sr-Ca-Cu-O glass-ceramics

Abstract The results of electrical conductivity measurements in Bi2O3-SrO-CaO-CuO glass-ceramics are reported. It is shown that a sharp increase in the conductivity observed at 460°C is attributed to the crystallization of the Bi2Sr2CuO6 (Bi-2201) superconducting phase in the glass matrix. The mechanism of electrical conductivity in this glass-ceramic can be explained as a result of discontinuous metallic structures of the Bi-2201 phase which appear in the glass matrix during annealing. The low-temperature conductivity obeys the law [sgrave](T) = [sgrave]0exp[(-T 0 /T)n] with n changing from 1/4 to 1/2 with decrease in the temperature. This cross-over is related to a granular structure in which a parabolic Coulomb gap exists in the density of states N(E) near E F. We also show that the superconductivity effect should be taken into account for T−1/2 conductivity behaviour. It is shown that the Bi-Sr-Ca-Cu-O glass after a very short heat treatment at 820°C exhibits the superconducting transition. In the normal state the conductivity is typical for granular metals and the activation energy continuously decreases with decreasing temperature.

Polaronic hopping conduction in vanadium and iron phosphate glasses

A.c. mechanism of polaronic hopping in vanadium and iron phosphate glasses is discussed in the light of polaron tunnelling model. The polaron parameters calculated from this model appear reasonable and are in agreement with values obtained from analyses of d.c. conductivity data.

Mixed ionic-polaron transport and rapid crystallization in (Bi,Pb)-Sr-Ca-Cu-O glass

Abstract The results of a.c. electrical conductivity measurements in (Bi,Pb)–Sr–Ca–Cu–O glasses were reported. Two components of total conductivity were observed. Activation energies of both processes were determined. The small polaron hopping between copper ions in a different valency state Cu I –Cu II is one process. We propose that the second is ionic conductivity caused by Cu I ions.