Transition Metal Oxide Glasses Research Articles

Effect of sulfur addition on the transport properties of semiconducting iron phosphate glasses

Transport properties and redox state of iron in glasses with compositions × S– (40 Fe 2O 3 – 60 P 2O 5)(mol%), where x = 0, 2,4,6 and 8 (mass%), were studied. The overall features of the XRD curves confirm the amorphous nature of the present glasses. Sulfur acted as a reducing agent for redox reaction during glass synthesis and affected the conductivity. Mössbauer spectral analyses revealed that the Fe 2+ ratio increases with increasing sulfur content. The high temperature above θ/2 ( θ D Debye temperature) dependence of conductivity could be qualitatively explained by the small polaron hopping model. The physical parameters obtained from the best fits of this model are found reasonable and consistent with the glass compositions. The conduction is confirmed to be due to adiabatic small polaron hopping of electrons between iron ions. The electron–phonon interaction coefficient γ p was large (21.42–26.26). The estimated hopping mobility was low, 1.12 × 10 −9–24.83 × 10 −9 cm 2 V −1 s −1 and increased with increasing S(mass%) content. Moreover, the low temperature (below θ/2) conductivity could not be explained either by Mott’s or greaves variable – range hopping model giving rise to unusually large values of the density of states at the Fermi level compared to those of transition metal oxide glasses. The conductivity of the present glasses was primarily determined by hopping carrier mobility.

One- and two-photon absorption in transition metal oxide glasses

Scaling of nonlinear two-photon absorption to optical bandgap (which describes the electronic structure of material) in transition metal glasses is given through a proposed empirical equation. A wide applicability of the relation is proven through the 21 studied glasses. The relationship between the calculated nonlinear absorption coefficient and the measured bandgap energy using Tauc’s parabolic band model is discussed in terms of electronic structure of constituent oxides, ionic polarizability, hyperpolarizability of cations, nonbridging oxygen bonds, and the cationic bond-length approach.

Mixed conductivity in tungstenite–phosphate glasses containing alkali metal ions

The conductivity of glasses in the 50WO3–(50−x)P2O5–xA2O (A=Na, K, Cs) system has been investigated as a function of composition. It is shown that in tungstenite–phosphate glasses containing different alkali metal ions the conductivity decreases with an increase in the alkali metal ion content. A decrease in conductivity is larger for heavier ions and reaches more than seven orders of magnitude in the case of glass containing Cs2O. This behavior remains in contrast to the literature data on conductivity in transition metal oxide glasses containing alkali metal ions where usually strong conductivity anomalies of several orders of magnitude at certain amount of ions are observed. No necessity of ion–polaron interaction has been pointed out.

Current-driven threshold switching of a small polaron semiconductor to a metastable conductor

Steady-state current flow through a nonuniform medium generally alters the local carrier density. In particular, driving conventional high-mobility charge carriers through a region in which they collapse into low-mobility small polarons propels the small-polaron density beyond its equilibrium value. There are two contributions to the local augmentation of the small-polaron density. These contributions are proportional to the ratios of the rate governing intersite motion of a conventional high-mobility carrier ${R}_{f}$ to (i) the (relatively slow) rate governing intersite hopping of a small polaron $R$, and to (ii) the (even slower) rate governing conversion of a carrier between being quasifree and being a small polaron, $r$. As a result of the very large values of these ratios, ${R}_{f}∕R$ and $R∕r⪢1$, large increases in the small-polaron density can be obtained with accessible electric-field strengths, $l{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$. However, upon being driven to a sufficiently high density, small polarons become unstable with respect to conversion into nonpolaronic carriers. As a result, currents beyond a threshold value can convert a small-polaron semiconductor to a high-mobility semiconductor. Reducing the current permits the material's carriers to relax back to being small polarons. This phenomenon may account for the threshold switching that is observed in materials in which equilibrated carriers appear to form small polarons (e.g., amorphous boron, transition-metal oxide glasses, and chalcogenide glasses).

Electronic and ionic relaxations in oxide glasses

A survey of relaxation processes in glasses exhibiting electronic and electronic–ionic conductivity mechanisms is presented. Electrical conductivity data on a range of transition metal oxide (TMO) glasses shows that a polaronic model of conductivity is generally applicable. It is shown that in TMO glasses both ac and dc conductivity processes are due to the same mechanism, which is hopping of small polarons between transition metal ions in different valence states. By applying scaling using the BNN relation it is possible to obtain universal conductivity curve and to find the relation between ac and dc activation energy. Complex plot of electric modulus gives a simple method of scaling analysis. An attempt of classification of mixed electronic–ionic behaviour in TMO containing alkali ions was performed on the basis of impedance spectroscopy. A deep minimum of conductivity at certain content of alkali ions is observed in glasses where TMO is a glass former. Two different behaviours can be distinguished in glasses where TMO is a glass network modifier. In glasses containing iron oxide electrical properties are only slightly influenced by alkali ions. A mixed electronic–ionic conduction is observed in glasses containing copper oxides. It is shown that the internal friction method is a powerful tool for the study of relaxation processes in glasses. In electronic and ionic conducting glasses internal friction spectra reveal a peak which activation energy is identical as for the dc conductivity. In mixed electronic–ionic conducting glasses the internal friction spectra reveal two peaks, which are attributed to the electronic hopping and the alkali ion migration. A comparison of internal friction and impedance spectroscopy investigations indicates that the internal friction is a more sensitive method for detecting local movement of ions or polarons in the glass network.

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

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.

Electrical transport and small polaron hopping conduction in Mn-doped Bi3PbSr3Ca3Cu4−mMnmOx glasses: precursors for high temperature superconductors

Abstract Multicomponent Bi3PbSr3Ca3Cu4−mMnmOx (m=0, 0.02, 0.03, 0.04, and 0.06) glasses, precursors for high temperature superconductors, have been prepared by fast quenching technique from their melts. These are n-type semiconductors and follow non-adiabatic small polaron hopping conduction mechanism above 1 2 θ D (where θD being the Debye temperature). The lower temperature (below 1 2 θ D ) conductivity data can be fitted with the variable range hopping (VRH) model. The estimated Mott parameters are found to be reasonable for the formation of small polaron and comparable with those of some other similar glasses. However, compared to the usual binary or ternary transition metal oxide glasses, the density of states at the Fermi level N(EF) is found to be one order of magnitude higher which we consider to be due to the formation of nanocrystalline (5–20 nm) clusters in these glasses. All the glasses are found to become superconductors in their glass–ceramic phases and the corresponding superconducting transition temperature decreases continuously with increase of Mn content indicating pair-breaking mechanism.

Multiphonon hopping conduction in nonconventional chromium-doped Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025–0.2) glasses with nanocrystalline particles and clusters

Transport properties of Cr containing multicomponent oxide glasses Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025, 0.05, 0.1, and 0.2) dispersed with nanocrystalline particles (5–20 nm depending on the values of n) have been reported in the temperature range of 250–450 K. Conductivity of this glass-nanocrystal composite system shows little decrease with increasing Cr content. Above θD/2 (θD is the Debye temperature), conductivity data can be analyzed with small polaron hopping models. Interestingly, unlike undoped Bi4Sr3Ca3Cu4Ox (or Bi-4334) glasses [showing nonadiabatic small polaron hopping (SPH) conduction at T>θD/2], the Cr doped glasses supports adiabatic SPH conduction mechanism above θD/2 indicating change of glass network structure due to partial substitution of Cu by Cr. But below this temperature Mott’s or Greaves’ variable range hopping models can be consistently used to fit the experimental conductivity data only with larger (compared to the usual transition metal oxide glasses) values of the density of states at the Fermi level N(EF). The most probable transport mechanism for the entire range of temperature and glass compositions is concluded to be due to multiphonon tunneling of large polarons between the nanoclusters present in the glasses which is also in sharp contrast to the behavior of the undoped (Bi-4334) glass. All the glass samples (except n⩾0.2) are found to become superconductors by annealing at higher temperatures.

Study of microstructural behavior and nonadiabatic small polaron hopping conduction in BaTiO3 doped lead-vanadate glass and glass-ceramics dispersed with ferroelectric nanocrystals

Multicomponent semiconducting oxide glasses like (80 V2O5–20 PbO)+xBaTiO3 (x=5–30 wt %) dispersed with nanocrystalline BaTiO3 have been prepared by a fast quenching technique. Results of microstructural study and transport properties (between 80 and 450 K) of these glasses and some glass ceramics have been reported in this article. It has been shown from x-ray diffraction, scanning electron microscopic, and other studies that homogeneous glasses are formed with wide concentrations of BaTiO3 (x⩽30 wt %). Transmission electron microscopic study, however, indicates the presence of nanocrystalline phases in the glass matrix. Mott’s variable range (nonadiabatic) hopping conduction mechanism is found to be valid at the low temperature regime (below θD/4, θD being the Debye temperature) while in the high temperature regime (above θD/2), the polaron hopping models of Schnakenberg as well as Emin and co-workers can quantitatively predict the conductivity data for all these special types of multicomponent glass-nanocrystal composites. The model parameters, obtained from best fit of the experimental data with these models, are consistent with the glass compositions. The dielectric constants of these glasses are found to be very high (comparable to that of bulk BaTiO3) and about two orders of magnitude higher than those of BaTiO3 free (80 V2O5–20 PbO) and other transition metal oxide glasses. This novel character is considered to be due to the embedded nanocrystalline BaTiO3 phases. The glass samples annealed at 300 °C for 9 h in air also showed nano and microcrystalline BaTiO3 phase along with traces of PbV2O6, PbTiO3, TiO2, and Pb2V2O7 phases. Broad anomaly (with a maximum around 390 K) is observed from the thermal variation of dielectric constant data of these partially annealed glass-ceramic samples. This anomaly resembles to the anomaly observed around the Curie temperature (∼393 K) in bulk ferroelectric BaTiO3 crystal.

Physical and dielectric properties of Bi4−xRxSr3Ca3Cu2O10 glasses (x = 0.5 and R = Ag, Ni)

The reflectivity spectra of semiconducting Bi4Sr3Ca3Cu2O10 glasses were measured over the frequency range between 100 and 4000 cm−1, and were analysed using Kramer-Kronig routines. It is observed that the static and high frequency dielectric constant varies as the bismuth is partially replaced by silver and nickel. The X-ray diffraction and the dc resisitivity measurements shows a semiconducting behaviour for all the samples. The measured values of the activation energy and other physical parameters based on infrared reflectance measurements are consistent with published work on the similar materials, the activation energy W, hopping energy WH and other physical parameters vary with the nickel and silver substitutions. It is shown that the hopping in these materials occurs in non-adiabatic regime, unlike other transition metal oxide glasses.

Concentration-dependent acoustic and electrical properties of highly conducting Bi3.5Pb0.5Sr3Ca3Cu4Oz + xAg2O-type glassy precursors for high-T c superconductors

Abstract We report the concentration-dependent electrical conductivity and acoustic parameters of the (Bi-Pb)4Sr3Ca3Cu4Oz + xAg2O (x = 0-10 wt%)-type glassy precursors for high T c superconductors. Unlike (Bi, Pb)4Sr3Ca3Cu4OZ, or many other transition-metal oxide glasses, these Ag-containing glasses are highly conducting. Unlike many other transition-metal oxide glasses, the small-polaron hopping mechanism of conduction is found not to be applicable for these glasses, which might be due to a partial contribution of the Ag ions to the conductivity. The velocities of sound, elastic constants, microhardness, acoustic impedance and Debye temperatures are found to decrease monotonically, as does the density, with increasing Ag concentration. On the other hand, the thermal expansion coefficient, internal friction and Poisson's ratio increase with increasing content of Ag2O. The concentration-dependent acoustic parameters and the corresponding derived quantities indicate a softening of the glass structure with a...

Dc conductivity of binary and modified transition metal oxide glasses

Abstract Binary and modified transition-metal-oxide (TMO) tellurite (V2O5-TeO2, MoO3-TeO2 and WO3-TeO2) glasses have been prepared by the method of rapid cooling. Dc conductivity measurements, on 20TMO-80TeO2 (TMO=V2O5, MoO3 and WO3) glasses in the temperature range 300K to 450K and on (50 - x)V2O5-xMoO3-50TeO2 (x = 0, 0.5, 1.0, 3.0 and 5.0) glasses in the temperature range 77K to 400K, have been performed. The results are discussed in the light of physical and structural models. The influence of the TMO in binary TMO-tellurite glasses and of the modifier (M0O3) in (50 - x)V2O5-xMoO3-50TeO2 glasses, is discussed in detail.

Electrical properties of CuO doped sodium borate glasses

Electrical conduction in transition metal oxide (TMO) glasses is generally explained on the basis of the small polaron theory. Present results on CuO doped sodium borate glasses seem to be inexplicable on the basis of the above model. Experimental data on dc and ac conductivities of CuO doped sodium borate glasses at different frequencies are presented. It is found that these results can be explained in terms of the concept of small ionic polarons developed for superionic conductors.

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO3-doped (5–40 wt %) 90V2O5-10Bi2O3 (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, σd.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V4+ and V5+ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V2O5), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO3-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(EF), electron wave-function decay constant, α, polaron radius, r p, and its effective mass, m p * , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.

Study of the a.c. conductivity of BaTiO3-doped V2O5-Bi2O3 glasses using Long's overlapping large-polaron tunnelling model

Abstract In the present paper we report the results of a study of the frequency-dependent a.c. conductivity σa.c. of the BaTiO3-doped (5–40 wt%) 90V2O5-10 Bi2O3 (VB) glassy semiconductor system over a wide frequency range (500-10 000Hz) and wide temperature range (80–450 K). The semiconducting behaviour of these glasses is due to the presence of V4+ and V5+ valence states. While the correlated barrier hopping of bipolarons is responsible for the a.c. conductivity in the undoped glasses, Long's overlapping large polaron tunnelling mechanism is found to be valid for all these BaTiO3-doped VB glass compositions. Such a change in conduction mechanism is not usually observed in transition-metal oxide glasses and it is due to the change in network structure of the VB glass arising from the addition of BaTiO3. The doped glasses, however, show phase separation even in the glassy phase in contrast with the pure VB glass. The d.c. conductivity of doped glasses showed a non-adiabatic conduction mechanism in contrast...

Correlation between mechanical and electrical losses in transition metal oxide glasses

The internal friction in iron phosphate glasses containing various glass modifiers has been studied in the temperature range from 120 to 700 K. The low frequency torsion pendulum technique has been employed. The dielectric loss has been investigated by two independent methods: the absorption current method and with an a.c. transformer bridge. The internal friction spectra exhibit a low temperature peak that can be related to the electron transfer between iron atoms in different valence states. It is confirmed that the dielectric loss peak and d.c. conductivity reveal a similar activation energy to the internal friction peak. The obtained data have been interpreted in the framework of a new model of relaxation in glasses recently proposed by Hunt.

Study of dielectric relaxation behaviour in Li-doped semiconducting BiSrCaCuO glasses

The temperature- (80–450 K) and frequency- (50−10 4 Hz) dependent dielectric constant, ϵ′, and loss, ϵ″, for Li-doped Bi 4Sr 3Ca 3− z Li z Cu 4O x ( z = 0.0, 0.1, 0.5 and 1.0) glasses, which become high T c superconducting oxides in their crystalline phases, have been measured. Unlike many other semiconducting transition metal oxide glasses, the temperature variations of ϵ′ and ϵ″ of the Li-doped glasses do not have a peak within the frequency range of the investigations. However, the imaginary ( M″) parts of the dielectric modulus, M ∗ , have peaks indicating Debye-type dielectric relaxation behaviour with a distribution of relaxation times. The relaxation behaviour is thermally activated.

Debye-type dielectric behavior of Bi-Sr-Ca-Cu-O-based transition-metal oxide glasses: Precursors for oxide superconductors.

Detailed measurements of the frequency- (50 Hz to 10 kHz) and temperature (80 loss (\ensuremath{\epsilon}'') have been performed for the ${\mathrm{Bi}}_{4}$${\mathrm{Sr}}_{3}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{\mathit{y}}$${\mathrm{O}}_{\mathit{x}}$ (y=0 to 5) glasses. Some of these glasses become high-temperature superconducting oxides in their respective glass-ceramic phases. The dielectric properties of these glasses are found to follow a Debye-type relaxation behavior with relaxation frequency ${\mathit{f}}_{\mathit{c}}$ [=${\ensuremath{\nu}}_{\mathit{c}}$ exp(-${\mathit{W}}_{\mathit{c}}$/${\mathit{k}}_{\mathit{B}}$T)]. The relaxation behavior is thermally activated in nature similar to the dc electrical-conduction process of these glasses observed in our earlier investigation.

On the mechanism of d.c. and a.c. transport in transition metal oxide glasses

Abstract Although it is well established that small polarons are formed in transition metal oxide (TMO) glasses, no satisfactory quantitative analysis of d.c. and a.c. transport exists. The temperature dependences of the d.c. and a.c. conductivities for TMO glasses are re-examined. The charge transport cannot be explained quantitatively by the small-polaron model (strong electron-lattice coupling). An alternative possibility—a multiphonon tunnelling process with weak coupling—is also discussed. The a.c. conductivity, which is strongly correlated to the d.c. conductivity and which has never been explained by current theories based on the pair approximation, is interpreted in terms of the continuous-time random-walk approximation.

Anomalous conductivity and other properties of V 2O 5-P 2O 5 glasses with Bi 2O 3 OR Sb 2O 3

To characterize the V 2O 5P 2O 5 glasses with Bi 2O 3 or Sb 2O 3, the DC and AC conductivities, dielectric constants, SEM, X-ray diffraction, and DTA studies have been made. Electrical properties have been measured in the temperature range 80–420 K and in the frequency range 0.1–100 kHz. The DC conductivity of the 50V 2O 5(50− x)P 2O 5− x Bi 2O 3 ( x = 5–40 mol%) glasses is found to be always higher than that of the pure base glass 50V 2O 5P 2O 5, while the DC conductivity of the 50V 2O 5-(50− x)P 2O 5− xSb 2O 3 glasses is always lower than that of the base glass. Above 200 K the activation energy of the V 2O 5P 2O 5Bi 2O 3 glasses depends on the concentration of Bi 2O 3, while that of the V 2O 5P 2O 5Sb 2O 3 glasses is almost independent of Sb 2O 3 concentration. However, both of these glasses show an interesting conductivity minimum for about 25–30 mol% of Bi 2O 3 or Sb 2O 3. This behavior was not found earlier in any such transition metal oxide glasses. The anomalous behavior, also observed in the concentration (Bi 2O 3 or Sb 2O 3) dependence of the density, glass transition temperature T g, AC conductivity, and dialectric constant data, is found to be associated with the lowest value of the ratio V 5+/V 4+ occurring at ∼ 25–30 mol% of Bi 2O 3 or Sb 2O 3. The AC conductivity is found to be proportional to ω s , where ω is the frequency and s lying between 0.85 and 0.98, obtained from the theory based on the hopping-over barrier (HOB) model.