Mixed transition-ion effect in the glass system: Fe2O3-MnO-TeO2

In earlier studies on phosphate and tellurite glasses containing vanadium and iron oxides, non-linear variation of physical properties as functions of the ratios of the transition ions (V/V + Fe) were observed. The most striking effect was observed with electrical conductivity, where a 3 orders of magnitude reduction in conductivity was observed at a V/V + Fe ratio of ~ 0.4. The effect was termed Mixed Transition-ion Effect or MTE. In phosphate glasses, however, MTE was not observed when one of the transition ions was manganese. It was concluded that Mn does not contribute to conduction in these glasses. In the present study, we demonstrate a mixed transition ion effect in tellurite glasses containing MnO and Fe₂O₃ (xFe₂O₃(0.2 ? x) MnO0.8TeO₂ with x varying from 0 to 0.2). A maximum in the property at an intermediate composition (x = 8.5 mol%), was observed in DC resistivity, activation energy, molar volume etc. Mossbauer and optical absorption (UV–VIS–NIR) measurements were performed on these glasses and the transport mechanism has been identified to be hopping of small polarons between Fe^3 + (Mn^3 +) and Fe^2 + (Mn^2 +) sites.     

DC conductivity of rare earth ions doped vanado-tellurite glasses

Two novel sets of vanado-tellurite glasses doped with rare earth ions (La₂O and CeO₂) have been investigated for density and dc electrical conductivity in the temperature range 300–500 K. The conductivity data has been analyzed in the light of Mott’s small polaron hopping (SPH), variable range hopping (VRH) and Greave’s VRH models. The electrical conduction was confirmed to be due to non-adiabatic small polaron hopping in nature. High temperature conductivity behavior was found to be in agreement with Mott’s SPH model. The low temperature conductivity behaved as per Mott’s VRH model. Various physical and polaronic parameters, such as radius, bandwidth and coupling constant, have been determined and discussed.     

Effect of mixed transition-metal ions in glasses. Part III: The P2O5–V2O5–MnO system

Electrical and optical properties of phosphate glasses containing vanadium and manganese ions in the xP₂O₅–[(100 − x)(V₂O₅ + MnO)] (PVM) system have been investigated. This is the last article of a III-part series devoted to the electronic properties of phosphate glasses containing a mixture of transition ions. The first article was devoted to the electrical conductivity of glasses having the general composition: xP₂O₅–[(100 − x)(V₂O₅ + Fe₂O₃)] (PVF). Competitive transport of small polarons on V and Fe ion sites was found to contribute to a mixed transition-ion effect (MTE) in PVF glasses. Several features of MTE were found to be similar to the well known mixed alkali effect, observed in glasses containing two alkali ions. In the second article, optical absorption and electronic conduction of xP₂O₅–[(100 − x)(Fe₂O₃ + MnO)] (PFM) glasses were reported. In the absence of competitive transport between the two transition ions (since Mn ions were determined not to contribute to dc conduction), MTE was not observed. The most important feature of PFM glasses was a sharp increase in resistivity at a critical concentration of iron ions, similar to ‘metal–insulator transition’ (MIT). In the present article, we report a resistivity transition in PVM glasses which is similar to that exhibited by the glasses of the PFM series. While Fe ions contributed the carriers in the PFM glasses, V ions serve the same purpose in the PVM compositions. As the concentration of vanadium ions, n_V, is decreased in the composition range 0.82 > n_V > 0.40, resistivity (ρ) increases marginally. For glasses with 0.2 < n_V < 0.40, resistivity and the activation energy for dc conduction (W) increase sharply with decreasing n_V, marking the incidence of an MIT-type transition. As in the PFM glasses, the observation of MIT coincides with the transformation of small polarons to small bipolarons, which is confirmed by the shifting of the small polaron optical absorption band to higher energies with decreasing V concentration.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Majority charge carrier reversal and its effect on thermal and electron transport in xV2O5–(1 − x) As2O3 glasses

DC resistivity, thermopower and optical absorption of xV₂O₅–(1 ? x) As₂O₃ (0.58 ≤ x ≤ 0.93) glasses have been studied as a function of composition. The transport mechanism in these glasses has been identified to be a combination of hopping of small polarons between V⁴⁺ and V⁵⁺ sites and small bipolarons between As³⁺ and As⁵⁺ sites respectively. Electrical conductivity is found to be more of a function of vanadium content than arsenic concentration in the glasses, indicating that the contribution of bipolarons to the conductivity is negligible. Thermopower has also been found to be sensitive to the composition of the glasses. At low vanadium concentrations, the thermopower is negative, which exhibits a sign reversal as vanadium concentration is increased (at x = 0.7). An important feature of these glasses is that the thermopower is not a function of [V⁵⁺]/[V⁴⁺] ratio, as is normally observed in vanadate glasses, and such a phenomenon suggests that the arsenic ions (bipolarons) in these glasses contribute to the thermal transport phenomena in a significant way.     

The densities of mixed glass former 0.35 Na2O + 0.65 [xB2O3 + (1 − x)P2O5] glasses related to the atomic fractions and volumes of short range structures

The mixed glass former effect (MGFE) is defined as a non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass forming compositions. In this study, mixed glass former (MGF) sodium borophosphate glasses, 0.35 Na₂O + 0.65 [xB₂O₃ + (1 ? x)P₂O₅], 0 ≤ x ≤ 1, which have been shown to have a strong positive MGFE, have been prepared and their physical properties, density and molar volume, have been examined as predictors of structural change. The density exhibits a strong positive non-linear and non-additive change in the density with x and a corresponding negative non-linear and non-additive change in the molar volume. In order to understand the structural origins of these changes, a model of the molar volume was created and best-fit to the experimentally determined molar volumes in order to determine the volumes of the short range order (SRO) structural units in these glasses, how these volume change from the molar volumes of the binary glasses, and how these volumes change across the range of x in the ternary glasses. The best-fit model was defined as the model that required the smallest changes in the volumes of the ternary phosphate and borate SRO structural groups from their values determined by the densities of the binary sodium phosphate and sodium borate glasses. In this best-fit molar volume model, it was found that the volumes of the various phosphate and borate SRO structural groups decreased by values ranging from a minimum value of ~ 1% for x = 0.1 and 0.9 to a maximum value of ~ 6% for the phosphate and ~ 9% for the borate SRO groups at the minimum in molar volume at x = 0.4. The free volume was found to have a negative deviation from linear which is unexpected given the positive deviation in ionic conductivity.     

Polarons in boron doped iron phosphate glasses

The electrical conductivity and dielectric properties of xB₂O₃–(40 ? x)Fe₂O₃–60P₂O₅ (x = 6–20, mol%) glasses were investigated in the frequency range from 0.01 Hz to 1 MHz and the temperature range from 303 K to 523 K. At temperatures below 523 K an ac conductivity and the dielectric constant follow the universal dielectric response (UDR), being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 523 K this mechanism governs the charge transport in these glasses. The comparison of the values of characteristic coefficients W_∞ and α determined by two different methods confirms the polaronic behavior of boron doped iron phosphate glasses.     

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe₂O₃–60P₂O₅ (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe₂O₃ content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe₂O₃ content from 10 to 30 mol%.     

Studies of lead–iron phosphate glasses by Raman,Mössbauer and impedance spectroscopy

The effect of Fe₂O₃ content on electrical conductivity and glass stability against crystallization in the system PbO–Fe₂O₃–P₂O₅ has been investigated using Raman, XRD, Mössbauer and impedance spectroscopy. Glasses of the molar composition (43.3 − x)PbO–(13.7 + x)Fe₂O₃–43P₂O₅ (0 ⩽ x ⩽ 30), were prepared by quenching melts in the air. With increasing Fe₂O₃ content and molar O/P ratio there is corresponding reduction in the length of phosphate units and an increase in the Fe(II) ion concentration, which causes a higher tendency for crystallization. Raman spectra of the glasses show that the interaction between Fe sites, which is essential for electron hopping, strongly depends on the cross-linking of the glass network. The electronic conduction of these glasses depends not only on the Fe(II)/Fe_tot ratio, but also on easy pathways for electron hopping in a non-disrupted pyrophosphate network. The Raman spectra of crystallized glasses indicate a much lower degree of cross-linking since more non-bridging oxygen atoms are present in the network. Despite the significant increase in the Fe₂O₃ content and Fe(II) ion concentration, there is a considerable weakening in the interactions between Fe sites in crystalline glasses. The impedance spectra reveal a decrease in conductivity, caused by poorly defined conduction pathways, which are result of the disruption and inhomogeneity of the crystalline phases that are formed during melting.     

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.     

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.     

Conductivity percolation transition of Agx(Ge0.25Se0.75)100−x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10⁻¹⁶–10⁻¹⁴ Ω⁻¹ cm⁻¹) to values of fast ionic conductors (10⁻⁷–10⁻¹ Ω⁻¹ cm⁻¹). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100−x glasses was detected at x¹ ≅ 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100−x glasses with x ⩽ 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ⩾ 10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ⩽ x ⩽ 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(−E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.     

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.     

Local study on the MoO4 units in AgI-doped silver molybdate glasses

The short-range order around molybdenum has been investigated in AgI-doped silver molybdate glasses (AgI)_x(Ag₂MoO₄)_1?x (with x = 0.67 and 0.75) by Mo–K edge EXAFS measurements as a function of temperature. The difference from crystalline Ag₂MoO₄ is weak. A softening of the Mo–O nearest-neighbours bond has been detected, but the MoO₄ units still exhibit high rigidity. Above the T_g temperature, no meaningful evidence of local structural and dynamical modifications has been observed around molybdenum.     

Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa₂O · (100 ? x)SiO₂ and (33 ? y)Na₂O · yCs₂O · 67SiO₂ glasses by a transient heating method in the temperature range from about 150 K to room temperature. The conductivity of xNa₂O · (100 ? x)SiO₂ is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33 ? y)Cs₂O · yNa₂O · 67SiO₂ is decreased with the increase in Cs₂O/(Na₂O + Cs₂O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs₂O · 67SiO₂ glass is longer than that of 33Na₂O · 67SiO₂ glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.     

Structure and crystallization behavior of mixed potassium-lithium niobiosilicate glasses

Potassium-lithium niobiosilicate (KLiNS) glasses with a composition of (27 ? x)K₂O · xLi₂O · 27Nb₂O₅ · 46SiO₂ (x = 0, 3, 12 and 20) have been synthesized by a melt-quenching method. The glass structure and devitrification behavior have been studied by Raman spectroscopy, DTA, and XRD. By increasing the lithium content, less distorted niobium octahedra increase, indicating a niobium clustering. This change strongly affects the crystallization behavior. In the glasses x = 0 and x = 3, just above T_g, only nanocrystals of an unidentified phase are formed, while for x = 12 and x = 20 potassium lithium niobate (KLN) solid solutions with tetragonal tungsten–bronze structure crystallize by bulk nucleation. In these glasses, LiNbO₃ crystallizes at higher temperature by surface nuclei. Ultimately, it is possible to produce nanostructured glasses based on KLN nanocrystals, by partial replacement of K by Li.     

EPR study of molybdenum-lead-phosphate glasses

The local symmetry and interaction between paramagnetic ions in xMoO₃(1 ? x)[2 P₂O₅PbO] glasses with 0.5 ? x ? 50 mol% are investigated by EPR spectroscopy. For x ? 10 mol% the isolated Mo⁵⁺ ions surrounded by five oxygen ligands in a square-pyramidal form (C_4v symmetry) prevail. The short range disorder in the environment of Mo⁵⁺ ions is not significantly (ΔR/R ≈ 2%). At high molybdenum content (x > 20 mol%) the dipole–dipole and superexchange coupled Mo⁵⁺ ions appear and their number increases with the MoO₃ content. These two aspects are also correlated with the network modifier and former role of molybdenum oxide in function of its concentration. Thus a strong depolymerization of the phosphate structure and the formation of P–O–Mo or Mo–O–Mo bonds in studied glasses appear.     

Thermal and optical properties of CuO–BaO–B2O3–P2O5 glasses

CuO-doped barium borophosphate glasses in a series of xCuO–(45 − x)BaO–10B₂O₃–45P₂O₅ in molar ratio with x = 0–15 mol% were prepared by a melt-quenching technique. All the glasses had excellent thermal stability against crystallization. Glass transition temperature, thermal expansion coefficient and molar volume decrease with increasing CuO concentration. The linear relationship between the absorption coefficient and CuO concentration exists for a peak wavelength in the transitions of ²A_1g → ²B_1g, ²B_2g → ²B_1g, ²E_g → ²B_1g. The relationship between the properties and glass structure evaluated by Raman spectroscopy is discussed.     

Crystallization of barium magnesium phosphate glasses determined by differential thermal analysis and X-rays diffraction

The scope of this work is to determine the crystalline phases of devitrified barium magnesium phosphate glasses and the glass composition which presents the best resistance to crystallization. Barium magnesium phosphate glasses with composition xMgO · (1 ? x)(60P₂O₅ · 40BaO) mol% (x = 0, 0.15, 0.3, 0.4, 0.5, and 0.6) were analyzed by differential thermal analysis (DTA) to evaluate the thermal stability against crystallization, and X-ray diffraction (XRD) to identify the crystalline phases formed after devitrification. The glass transition temperature (T_g) increases as the MgO content increases. The maximum temperature attributed to the crystallization peak in the DTA curve (T_c) increases when x increases in the range 0 ? x ? 0.3, and it decreases for x > 0.3. The most thermally stable glass composition against crystallization is for x = 0.3. After the devitrification, the number of coexisting crystalline phases increases as the MgO content increases. For x = 0.3 there is the coexistence of γBa(PO₃)₂ and Ba₂MgP₄O₁₃ phases for devitrified glasses. The trend of the T_c is explained based on the assumptions of changes in the Mg²⁺ coordination number and the amphoterical features of MgO.     

Preparation and characterization of telluride glasses

Chalcogenide bulk glasses Ge₂₀Se_80?xTe_x for x ∈ (0, 10) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Er and Pr, and all systems have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficient has been derived from measured transmittance and estimated reflectance. Both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te → Se substitution. Arrhenius plots of dc electrical conductivity, in the temperature range 300–450 K, are characterized by activation energies roughly equal to the half of the optical gap. Arrhenius plots for temperatures below 300 K yield much lower activation energies. The dominant low-temperature luminescence band centered at about half the band gap energy starts to quench above 200 K and a new band appears at 900 nm. The band at 900 nm, due to band to band transitions, overwhelms the spectra at room temperature. Systems doped with Er exhibit a strong luminescence due to ⁴I_13/2 → I_15/2 transition of Er³⁺ ion at 1539 nm, and Pr doped samples exhibit a relatively weak luminescence peak at 1590 nm, which we tentatively assign to ³F₃ → ³H₄ transition of Pr³⁺ ion.