Electrical Mobility of Silver Ion in Ag2OâB2O

Article pubs.acs.org/JPCB

Electrical Mobility of Silver Ion in Ag2O−B2O3−P2O5−TeO2 Glasses Kristina Sklepić,† Maryna Vorokhta,‡ Petr Mošner,‡ Ladislav Koudelka,‡ and Andrea Moguš-Milanković*,† †

Ruđer Bošković Institute, 10000 Zagreb, Croatia Department of General and Inorganic Chemistry, University of Pardubice, Faculty of Chemical Technology, 53210 Pardubice, Czech Republic

‡

ABSTRACT: The effect of adding TeO2 into (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2, with 0−80 mol % TeO2 glass, on the structural changes and electrical properties has been investigated. DSC and thermodilatomery were used to study their thermal behavior, structure was studied by Raman spectroscopy, and electrical properties have been studied by impedance spectroscopy over a wide temperature and frequency range. The introduction of TeO2 as a third glass former to the glass network causes the structural transformation from TeO3 (tp) to TeO4 (tbp) which contributes to the changes in conductivity. The glasses with low TeO2 content show only a slow decrease in dc conductivity with addition of TeO2 due to the increase of the number of nonbridging oxygens, which increases the mobility of Ag+ ions. The steep decrease in conductivity for glasses containing more than 40 mol % TeO2 is a result of decrease of the Ag2O content and stronger cross-linkage in glass network through the formation of more Te−eqOax−Te bonds in TeO4 tbp units. The glasses obey ac conductivity scaling with respect to temperature, implying that the dynamic process is not temperature dependent. On the other hand, the scaling of the spectra for different glass compositions showed the deviations from the Summerfield scaling because of the local structural disorder which occurs as a result of the structural modifications in the tellurite glass network.

1. INTRODUCTION Tellurite glasses have attracted considerable interest because of their outstanding electrical and optical properties such as high dielectric constant, high refractive index, and wide infrared transmittance. They also exhibit a low glass transition and melting temperature and high thermal expansion coefficient and are less hygroscopic compared to the phosphate and other oxide glasses. Due to these advantageous properties, the tellurium dioxide-based glasses are considered as promising materials for nonlinear optical applications, optical switching devices, erasable recording materials, laser hosts, sensors, and spectroscopic devices.1−5 Tellurium dioxide is a conditional glass former, which means that, without the addition of other components, it does not transform to the glassy state under conventional quenching conditions. Therefore, modifiers or glass forming agents such as alkali oxides or halides and heavy metal or different nonmetal oxides are used to improve the tellurite glass forming ability and its stability.1 Addition of boron oxide to tellurite or phosphate glasses enhances their thermal and chemical stability.1,6,7 It is worth noting that the role and type of modifier oxides in vitreous transition of tellurite melts is extremely important for changing of the tellurite forming units. The basic structural unit in crystalline TeO2 is a TeO4 trigonal bipyramid (tbp). One equatorial site of the Te sp3d hybrid orbitals in this bipyramid is occupied by a lone pair of electrons, and the other two equatorial and both axial sites are occupied by oxygen atoms. When modifiers, like alkali or silver © 2014 American Chemical Society

oxides, are introduced into the tellurite network, a structural transformation from TeO4 trigonal bipyramids (tbp) to TeO3 trigonal pyramids (tp) takes place.8,9 The transition goes through an intermediarte, TeO3+1 polyhedra, and results in the creation of nonbridging oxygen (NBO) atoms. Many binary and ternary glass systems with different content of borate, phosphate, and tellurite oxides as glass formers, containing alkali or silver oxides as modifiers, have been investigated during the last few decades due to their numerous potential applications as optical materials.10−15 Previous reports on the binary silver tellurite glasses have suggested that the structural transition along with the increase in the number of NBOs is responsible for the increase of the conductivity with increasing silver oxide content.10 In many previously studied systems, it has been observed that ionic conductivity increases when the network former is progressively substituted by another while the network modifier content is held constant. This effect is known as the mixed network former effect, and it was also observed in several ternary alkali or silver boro- and phosphotellurite glass systems.11−13 According to Chowdari and Pramoda Kumari,14 the increase in the conductivity of silver containing phosphotellurite glasses is almost 2 orders of magnitude when the TeO2 is progressively substituted for P2O5 at constant Received: July 23, 2014 Revised: September 16, 2014 Published: September 22, 2014 12050

dx.doi.org/10.1021/jp5073796 | J. Phys. Chem. B 2014, 118, 12050−12058

The Journal of Physical Chemistry B

Article

Table 1. Composition and Selected Properties for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2, x = 0−80 mol %, Glasses glass composition (mol %)

a

sample

Ag2O

B2O3

P2O5

TeO2

ρ (g cm−3) (±0.2)

VM (cm3 mol−1) (±0.2)

Tga (K) (±3)

Te-0 Te-20 Te-40 Te-60 Te-80

50 40 30 20 10

10 8 6 4 2

40 32 24 16 8

0 20 40 60 80

4.58 4.89 5.06 5.27 5.42

39.2 35.9 33.9 31.8 30.2

513 544 565 572 582

Tg from DSC curves.

temperature, Tg, was determined as the midpoint of the change in heat capacity, cp, in the glass transition region, and the crystallization temperature, Tc, values were obtained from the onset of the first crystallization peak in the DSC curves. Dilatometric measurements were carried out on the bulk samples with dimensions of ∼5 × 5 × 20 mm3 in static air atmosphere at a heating rate of 5 K min−1 using an Al2O3 sample holder and subjected to a compressive force of 0.20 N. Dilatometric curves were evaluated by Proteus software. The glass transition temperature, Tg, was obtained from the change in the slope of the elongation vs temperature plot, whereas the dilatometric softening temperature, Td, was determined from the maximum of the expansion trace corresponding to the onset of viscous deformation under applied load. The coefficient of thermal expansion, α, was determined as a mean value in the temperature range from 423 to 523 K. 2.3. Raman Spectroscopy. The Raman spectra were recorded on bulk samples at room temperature using a HoribaJobin Yvon LaBRam HR spectrometer. The spectra were collected in backscattering geometry under excitation with Nd:YAG laser radiation (532 nm) at a power of 10 mW taking 10 scans with an exposition time of 2 s. The laser was focused through a 10× objective onto the samples. The obtained Raman spectra were deconvoluted using a Gaussian function. The position and intensity of each component band were determined from the deconvoluted Raman spectra. 2.4. Electrical Measurements. Samples for the electrical measurements were cut into ∼1 mm thick discs and polished. Gold electrodes, 7 mm in diameter, were deposited onto both sides of the samples using a Sputter Coater SC7620. Electrical conductivity and dielectric property measurements were performed using an impedance analyzer (Novocontrol Alpha-AN Dielectric Spectrometer, Novocontrol Technologies GmbH & Co. KG, Germany) in the frequency range from 0.01 Hz to 1 MHz at temperatures between 183 and 513 K. The temperature was controlled to an accuracy of ±0.20 K. The obtained complex impedance data, Z*(ω), were presented as Nyquist plots where each point of this curve represents values of the real, Z′(ω), and imaginary, Z″(ω), part of the complex impedance at an exact angular frequency ω (ω = 2πf). Equivalent circuits modeling was used to analyze the impedance spectra, and the corresponding parameters were determined by the complex nonlinear least-squares (CNLLSQ) fitting procedure.

modifier content. The observed enhancement in the conductivity can be attributed to the expansion of the glass network due to the replacement of smaller phosphorus atoms by tellurium atoms which have a larger ionic radius. In this case, the formation of nonbridging oxygens causes an increase in the size of the interstitial windows and facilitates the diffusion of Ag+ ions. Moreover, several studies have been carried out to compare the conductivity of alkali and silver ion conducting glasses. Kumar et al.15 suggested that silver containing glasses have higher conductivities and lower energy barriers than lithium glasses because of the lower ionic potential of Ag+ ions. Silver glasses also have high molar volumes, and the activation barrier is decreased because the activated state during BO−NBO switching is more easily accommodated. In the present work, the electrical properties of the quaternary Ag2O−B2O3−P2O5−TeO2 glass system were investigated. The influence of structural modifications induced by addition of tellurium dioxide into the borophosphate network on ionic conductivity and silver ion transport properties has been analyzed in detail.

2. EXPERIMENTAL SECTION 2.1. Glass Preparation and Density Measurements. Homogeneous glasses with the composition (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2, x = 0−80 mol %, were prepared from the appropriate mixture of analytical grade AgNO3, H3BO3, H3PO4, and TeO2 using a total batch weight of 10 g. The starting mixtures were slowly heated in covered platinum crucibles up to 873 K for 2 h to remove water and subsequently melted for 30 min at temperatures ranging from 1173 to 1523 K relying on the sample composition. Samples with higher TeO2 content were melted at lower temperatures. The melts were quenched in air by pouring into graphite molds. Obtained glasses were then transferred to the relaxation furnace and annealed for 30 min at a temperature of 5 K below their glass transition temperature, Tg. The glass density, ρ, was determined on bulk samples by the Archimedes’ method using toluene as the immersion liquid. The molar volume, VM, was calculated as VM = M/ρ, where M is the average molar weight of the glass composition aAg2O − bB2O3 − cP2O5 − dTeO2 calculated for a + b + c + d = 1. 2.2. Differential Scanning Calorimetry and Dilatometry. The thermal behavior of glasses was investigated using DTA 404 PC (NETZSCH) operating in the DSC mode and with the horizontal pushrod dilatometer DIL 402PC (NETZSCH). DSC measurements were performed on powder samples (100 mg) with a mean diameter of ∼10 μm placed in an open platinum crucible under a flowing nitrogen atmosphere at a heating rate of 10 K min−1. The value of the glass transition

3. RESULTS 3.1. Density and Molar Volume. Glasses in the compositional series (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2, with 0−80 mol % TeO2, were prepared 12051



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crystallize in several steps. The crystallization temperature, Tc (onset of the first crystallization peak), reveals a maximum in the glass with x = 30 mol % TeO2. This glass also has the highest thermal stability and the lowest tendency toward crystallization, as revealed from the ΔT = Tc − Tg criterion. The values of glass transition temperature, Tg, obtained from DSC curves, increase with increasing TeO2 content from 513 K for Te-0 to 582 K for Te-80. Horizontal thermodilatometry was used to investigate the thermal expansion of the glasses. The compositional dependence of the glass transition temperature, Tg, the dilatometric softening temperature, T d, and the thermal expansion coefficient, α, on TeO2 content is shown in Figure 3. The

and studied. The chemical composition of the prepared glasses and the values of glass density, ρ, molar volume, VM, and glass transition temperature, Tg, are listed in Table 1. Glass density increases with increasing TeO2 content, whereas molar volume decreases from 39.2 to 30.8 cm3 mol−1. The compositional dependence of the glass density, ρ, and molar volume, VM, is shown in Figure 1.

Figure 1. Compositional dependence of the density, ρ, and molar volume, VM, of the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses.

3.2. Thermal Properties. The DSC thermograms of all glasses are shown in Figure 2. From the DSC curves, it is evident that all glasses (their solidified melts) crystallize on heating within the range of Tc = 673−743 K and some of them

Figure 3. Compositional dependence of the glass transition temperature, Tg, dilatation softening temperature, Td, and thermal expansion coefficient, α, of the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses determined by horizontal thermodilatometry.

glass transition temperature, Tg, and the dilatometric softening temperature, Td, increase with increasing TeO2 content. The thermal expansion coefficient, α, continuously decreases with increasing TeO2 content from 22.3 to 19.0 ppm K−1 for Te-80. 3.3. Structural Properties. The structural modifications of the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glass, 0−80 mol % TeO2, were determined by Raman spectroscopy. Raman spectra of the glasses are shown in Figure 4, and the assignments of the bands are given in Table 2. The investigated quaternary glass system contains three different glass network formers: boron oxide (B2O3), phosphorus pentoxide (P2O5), and tellurium dioxide (TeO2). Thus, the analysis of the Raman spectra is complex due to the overlapping of their bands. The efficiency of Raman scattering of borate structural units is considerably weaker in comparison with phosphate and tellurite units. Therefore, at low TeO2 content, the dominant bands are those corresponding to the phosphate structural units, whereas, at high TeO2 content, vibrational bands of phosphate units are suppressed as the vibrations of tellurite structural units prevail. In the spectrum of the TeO2 free glass, the phosphate content is the highest. The bands in the high frequency region between 950 and 1300 cm−1 correspond to the symmetric stretching vibration of nonbridging oxygen atoms in metaphosphate (Q2) and diphosphate units (Q1).16,17 The strong band at 1086 cm−1 is attributed to the symmetric stretching mode of the O−P−O nonbridging bond in the Q1 structure. The bands at 1121 and 1195 cm−1 are assigned to the

Figure 2. DSC curves of the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses. 12052



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intensive vibrational bands of tellurite units as the TeO2 content increases. In the spectral range between 400 and 850 cm−1, Figure 4, the bands of different TeO vibrations characteristic for the two structural units formed in tellurium oxide glasses, TeO3 (tp) and TeO4 (tbp), are observed. In order to obtain more information on the participation and connectivity of these individual tellurite units in the glass structure, the Raman spectra were deconvoluted. Figure 5 shows a deconvolution of the Raman spectra for Te-40, Te-60, and Te-80 glasses in the vibrational range from 400 to 850 cm−1. Five bands are deconvoluted according to the literature data.20−24 The band at 775 cm−1 (band A) corresponds to the symmetric stretching mode of TeO bonds between tellurium and bridging oxygens in the TeO3 (tp) or in the TeO3+1 units. Band B at 735 cm−1 is associated with the stretching mode of the TeO bond (Te O and TeO−) between tellurium and nonbridging oxygens in the TeO3 (tp) structural units.20,21 Band C at 670 cm−1 is related to the continuous network composed of TeO4 (tbp), whereas the band at 605 cm−1 (band D) is assigned to the asymmetric stretching mode of TeeqOaxTe bridges with different TeO bond lengths in TeO4 (tbp) units.20,21 As the TeO2 content increases, the new band at 460 cm−1 (band E) associated with the bending mode of TeeqOaxTe linkages appeared. Considering the role of Ag2O in these glasses, it should be pointed out that the intensity of the bands at 775 and 735 cm−1 increases with increasing Ag2O content. On the other hand, the intensity of the bands related to the stretching vibrations of TeO4 (tbp) at 670 cm−1 and bending vibrations in tellurite units at 470 cm−1 decreases as the Ag2O content increases. This variation in Raman band intensities suggests that the conversion of the TeO4 (tbp) units to the TeO3 tp units is caused by the addition of the Ag2O. 3.4. Electrical Properties. The complex impedance spectra for different glass compositions measured at 333 K are shown in Figure 6. The impedance plots consist of a high-frequency semicircle and a low-frequency spur for glasses with high Ag2O content, Figure 6a. The impedance plot for the Te-60 consists of a single semicircle, whereas the spectrum for the Te-80 shows only the beginning of the semicircle as a result of the limit of the instrument and the very low conductivity, Figure 6b and c. The radius of the semicircle which is related to the bulk behavior decreases with increasing temperature, indicating that the ion conduction is thermally activated, Figure 7. Since the experimental data exhibit a depressed semicircle with the center below the real axis, the constant-phase element (CPE) and resistance (R) are used in the equivalent circuit. The CPE

Figure 4. Raman spectra of the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses.

symmetric stretching of the P−O and (PO2) in Q2 units, respectively. The appearance of a weak band at 999 cm−1 indicates the presence of some Q0 units as a result of the depolymerization of the phosphate network. The bands at 646 and 717 cm−1 in the middle range of the spectrum are attributed to the symmetric stretching of the P−O−P bridging bonds in Q2 and Q1 structures,16 respectively. The barely detectable bands in the range between 350 and 530 cm−1 can be assigned to the bending of phosphate units characteristic for a network dominated by pyrophosphate units.18,19 With increasing TeO2 content, the bands related to the phosphate network become weaker and broader and shift to the lower wavenumbers. The most significant changes in the Raman spectra are observed in the middle frequency range between 400 and 850 cm−1, Table 2. The bands assigned to the symmetric stretching vibrations of oxygen atoms in P−O−P bridging bonds between metaphosphate (Q2) and diphosphate (Q1) units are gradually overlapped and replaced by more Table 2. Raman Band Assignments wavenumber (cm−1) 1195 1121 1086 999 775 (A) 735 (B) 670 (C) 605 (D) 470 (E)

vibrational mode 2

reference

(PO2)symm stretch (NBO) in Q (P−O)symm stretch in Q2 (PO3)symm stretch (NBO) in Q1 (PO4)symm stretch (NBO) in Q0 stretching vibrations of TeO3 (tp)a stretching vibrations of TeO3 (tp)a overlapping with (P−O−P)symm stretch (BO) in Q1 stretching vibrations of TeO4 (tbp)b overlapping with (P−O−P)symm stretch (BO) in Q2 stretching vibrations of TeO4 (tbp)b bending vibrations of Te−O−Te or O−Te−O linkages

16, 16, 16, 16, 20, 16, 16, 20, 20,

17 17 17 17 22 20, 22 20, 22 22 22

a,b

Different vibrational modes, explained in detail in the text. 12053



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Figure 7. Complex impedance plot measured for the Te-40 glass at different temperatures.

fitting of the measured impedance data. The theoretical curves (line) obtained by the fitting procedure using ZView software are in excellent agreement with experimental (scatter) data, as can be seen in Figures 6 and 7. The dc conductivity values, listed in Table 3, were calculated using the following equation, σdc = d/(R × A), where R is the resistance of the sample, d is the sample thickness, and A is the electrode area. Table 3. Selected Electrical and Dielectric Properties for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2, x = 0− 80 mol %, Glasses sample Te-0 Te-20 Te-40 Te-60 Te-80

Figure 5. Deconvolution of the Raman spectra for Te-40, Te-60, and Te-80 glasses in the 400−850 cm−1 region.

σdc [(Ω cm)−1] at 333 K 2.82 6.22 9.59 2.58 1.92

× × × × ×

10−5 10−6 10−7 10−9 10−14

Edc (kJ mol−1) 44.41 45.66 49.98 66.47 105.80

τM″ (s) at 303 K 2.70 1.21 9.89 7.06

× × × ×

10−7 10−6 10−6 10−3

EM″ (kJ mol−1) 43.69 44.49 48.84 64.88 106.14

The frequency dependence of the ac conductivity for the Te40 glass shows a typical behavior of the ionically conducting glass, as can be seen in Figure 8. The frequency independent plateau observed at low frequency region is assigned to the long-range translational motion of ions and corresponds to the dc conductivity, whereas at higher frequencies there is a

represents an empirical impedance function, ZCPE* = A(jω)−α, where both A and α are the constants. The parameters of the equivalent circuit, CPE and R, were acquired using CNLLSQ

Figure 6. Complex impedance plots for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses measured at 333 K. 12054



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revealed deviations in the crossover regime from dc conductivity to dispersive conductivity. The calculated dc conductivity variation with 1/T for all glasses investigated is shown in Figure 10. The activation

Figure 8. Frequency dependence of the ac conductivity at different temperatures for the Te-40 glass.

Figure 10. Temperature dependence of the dc conductivity (full symbol), σdc, and the relaxation frequency (open symbol), f M″, for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses.

crossover into a dispersive regime of the conductivity. The onset of the dispersive region shifts toward higher frequencies with increasing temperature. In the conductivity plots, the slopes observed at low frequency regions and higher temperatures are attributed to the electrode polarization effect. Other glass compositions also showed a similar behavior. For better comprehension of the dynamics of mobile Ag+ ions, the Summerfield scaling25 of the ac conductivity spectra has been used. Conductivity master curves have been generated by normalizing the conductivity axis by σdc and the frequency axis by the product σdc·T.26 The scaling method can be referred to as the time−temperature superposition principle, since it gives the dimensionless ac conductivity as a function of dimensionless frequency. The conductivity master curves of all glasses investigated are exhibited in Figure 9. The conductivity isotherms for Te-40 glass are superimposed onto a single master curve, suggesting that the dynamic process is not temperature dependent, as can be seen from the inset to Figure 9. However, the application of the Summerfield scaling law on different glass compositions

energy for dc conductivity, Edc, for each glass was determined from the slope of log σdcT vs 1/T using the equation σdcT = σ0 exp(−Edc/kBT), where σdc is the dc conductivity, σ0 is the preexponent, kB is the Boltzmann constant, and T is the temperature (K). The activation energies, Edc, for all of the glasses are also listed in Table 3. Figure 11 shows the compositional dependence of dc conductivity, σdc, at 333 K and the activation energy, Edc, for

Figure 11. Compositional dependence of dc conductivity, σdc, at 333 K and activation energy, Edc, for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses.

all glasses. σdc increases from 1.92 × 10−14 to 2.82 × 10−5 (Ω cm)−1, whereas Edc decreases from 105.8 to 44.4 kJ mol−1 with increasing Ag2O and decreasing TeO2 content. 3.5. Dielectric Properties. The conductivity data of the glasses have been analyzed using the electrical modulus formalism. The electric modulus M*(ω) is defined as the reciprocal of the complex dielectric constant ε*(ω): 1 M *(ω) = = M′(ω) + iM″(ω) ε*(ω) (1)

Figure 9. Scaled conductivity spectra for the (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glasses. Inset: conductivity master curves at different temperatures for the Te-40 glass. 12055



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M′(ω) =

ε′(ω) (ε′(ω)) + (ε″(ω))2

(2)

M″(ω) =

ε″(ω) (ε′(ω))2 + (ε″(ω))2

(3)

2

Consequently, the Tg values should decrease with increasing TeO2 content, but for glasses studied in this paper, the glass transition temperature increases. The observed increase in Tg can be explained in terms of the formation of highly crosslinked structural units, resulting in more closely packed glass structure with improved thermal stability. The increase of the compactness of glass structure is also confirmed by examining the changes in molar volume values. VM is proportional to the molecular weight of the glass and inversely proportional to the glass density. The decrease in VM observed with incorporation of TeO2 into the glass network suggests the reduction of the free space within the glass network. On the other hand, the glass transition temperature, Tg, decreases, whereas the molar volume increases with addition of Ag2O into the glass network. The decrease of the Tg can be attributed to the formation of nonbridging oxygens (NBOs) that arise as a result of the gradual modification of the TeO4 (tbp) units to TeO3 (tp) units. Apart from the structural modification of the tellurite structural units, the addition of Ag2O also induces the depolymerization of phosphate chains in the glass network structure, thereby reducing the network rigidity and hence lowering the glass transition temperature. Structural alterations caused by reduction of Ag+ ion content are observed in the Raman spectra. Changes in the Raman spectra correspond to the variations in the type and number of TeO2 structural units caused by the addition of tellurium oxide. In the glasses with low TeO2 content up to 40 mol %, the intensity of bands related to the stretching vibrations of nonbridging Te−O− bonds in TeO3 (tp) increases (bands A, B).21,30 With further addition of TeO2 content up to 80 mol %, the tellurium atoms are incorporated into the borophosphate network. The most prominent band at 670 cm−1 for glasses containing high TeO2 content corresponds to the Te−eqOax− Te bonds in TeO4 tbp units, indicating an increase in the number of TeO4 (tbp) in the glass structure. Also, the appearance of the band at 460 cm−1 attributed to the bending of Te−eqOax−Te linkages at low TeO2 content and its significant increase with further addition of TeO2 suggests an increase in connectivity between TeO3 and TeO4 units.21 The Raman results indicate that the increase in TeO2 content in the investigated glasses results in a progressive transformation of the basic structural units of TeO3 (tp) to TeO4 (tbp) followed by the increased number of bridging oxygens. Such a structural change, associated with the formation of bridging oxygens that introduce stronger covalent bonds, makes the glasses progressively stronger, which is manifested in an increasing Tg, Figure 3. The electrical conductivity of ionically conducting glasses is a sensitive property that depends upon both the concentration and the mobility of the charge carriers. Furthermore, the structural modifications determine the potential barriers for the transport of mobile ions and consequently the mobile ions concentration. The observed decrease in the dc conductivity, σdc, and the corresponding increase in the activation energy, Edc, is attributed to the increase of TeO2 content and decrease of Ag2O content. With gradual introduction of TeO2 content to the silver borophosphate glass network, the structural transformation from TeO3 (tp) to TeO4 (tbp) occurs followed by the creation of more bridging oxygens in highly asymmetric Te−eqOax−Te bonds of TeO4 (tbp), as can be seen from deconvoluted Raman spectra in Figure 5.

Using this electrical modulus formalism has several advantages. It provides a quantitative description of the observed electrical relaxation in which the electrode polarization effects are minimized. The frequency dependence of the complex electric modulus (real and imaginary part) for the Te-40 glass at selected temperatures is shown in Figure 12. The peak in M″(ω) spectra

Figure 12. Frequency dependence of the real (M′) and imaginary (M″) part of the electrical modulus at different temperatures (183− 333 K) for the Te-40 glass.

corresponds to the relaxation frequency, f M″ = (2πτM″)−1, where τM″ is the relaxation time, which can be determinated from the positions of the M″(ω) maximum,27 as is shown in Figure 12. The relaxation maximum shifts to higher frequencies with increasing temperature from 183 to 333 K, which is consistent with a thermally activated behavior. The relaxation time values, τM″, listed in Table 3, decrease with increasing Ag2O content. Finally, the relaxation frequency, f M″, for all glasses has been plotted as a function of inverse temperature and is represented by an Arrhenius equation, f M″T = f 0M″ exp(−EM″/kBT), where EM″ is the activation energy for the electrical relaxation, Figure 10. The activation energies, EM″, determined from the slopes of log(f M″T) vs 1/T, are given in Table 3 for all glasses. The activation energies for the electrical relaxation, EM″, and for the dc conductivity, Edc, are almost identical, suggesting the close agreement between these two processes.

4. DISCUSSION The glass transition temperature, Tg, depends mostly upon three factors, the strength of the bonds between oxygen and the network atoms, the degree of cross-linking of the network, and the density of packing of the oxygen atoms.28 In the compositional series (100 − x)[0.5Ag2O − 0.1B2O3 − 0.4P2O5] − xTeO2 glass, x = 0−80 mol %, the weaker Te− O bonds (bond dissociation energy D0298(O−Te) ≈ 376 kJ mol−1) are replacing stronger P−O bonds (D0298(O−P) ≈ 599 kJ mol−1) and B−O bonds (D0298(O−B) ≈ 809 kJ mol−1).29 12056



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small and their mobility is assisted by the increased number of the NBOs formed in TeO3 (tp). Thus, the relaxation time, τM″, decreases as Ag+ ions become mobile at higher frequencies. With increasing TeO2 and decreasing Ag2O content, due to smaller amounts of nonbridging oxygens available, the Ag+ ions are mobile over a long distance at lower frequencies, which results in large relaxation times.

The dc conductivity decreases for 2 orders of magnitude from 2.82 × 10−5 to 9.59 × 10−7 (Ω cm)−1 as TeO2 increases up to 40 mol %. On the other hand, for glasses containing a higher TeO2 content up to 80 mol %, the dc conductivity decreases significantly for 7 orders of magnitude from 9.59 × 10−7 to 1.92 × 10−14 (Ω cm)−1, Figure 11. It should be noted that, in the glasses with low TeO2, the Ag2O content is high although decreases from 50 to 30 mol %. In this compositional range, the TeO3 tp structural units contain a large amount of nonbridging oxygens which maintains Ag+ ions mobile. Although the concentration of Ag+ ions decreases, the formation of a continuous network composed of isolated TeO3 tp units is responsible for the relatively high dc conductivity. The sharp decrease in dc conductivity for glasses containing a high content of TeO2 is related more to the structural transition of TeO3 (tp) to TeO4 (tbp) units than to the decrease of Ag2O content from 30 to 10 mol %. It seems that with increasing TeO2 content the strength of chemical bonds in the glass network increases, which consequently weakens the mobility of Ag+ ions. Universal conductivity curves were obtained by applying the Summerfield scaling law. The analysis of the scaled data, Figure 9, shows that the deviation from Summerfield scaling occurs in the crossover regime from dc to dispersive conductivity, while the scaling principle is valid at higher frequencies. This suggests that the ion transport mechanism is universal at shorter time scale, but in the crossover region, it depends on glass composition. Similar observations have been found for some binary telluride glasses, where the deviation from scaling is attributed to the specific characteristics of tellurite glasses.10,31 For present glasses, the scaling behavior depends on the adjustment of the glass network influenced by the transformation of tellurite structure. It is proposed that, by the incorporation of tellurite into a glass network, a local environment becomes constrained which leads to the dispersion in shorter time region and is in agreement with the dimensionality of the local cation conduction space proposed by Sidebottom.32 The divergence in the scaled data for these glasses is probably caused by the structural disorder and transition in the tellurite glass network. Information about the relaxation mechanisms can be provided by using the conductivity relaxation model, where a dielectric modulus is defined as M*(ω) = 1/ε*(ω).27 In the low-frequency region, the dispersion of M′(ω) is primarily a result of the conductivity relaxation. The contribution of electrode polarization to M′(ω) at lower frequencies is negligible, and therefore, M′(ω) approaches zero. The variation of M″(ω) with frequency for the glasses investigated is shown in Figure 12. A single asymmetric peak occurs in the M″(ω) spectrum with the peak maximum positioned approximately in the middle of the dispersion region of M′(ω). The relaxation times, τM″, were determined from the frequencies corresponding to the M″(ω) maxima, and respective activation energies, EM″, were calculated as shown in Figure 12 and listed in Table 3. It can be seen from Table 3 that the relaxation time values, τM″, decrease as the TeO2 content decreases due to the lower Edc and higher conductivities, σdc. For TeO2 free glass, the relaxation time, τM″, becomes the lowest, 2.70 × 10−7 s, which corresponds to the highest conductivity values. Such behavior can be explained by interpreting the influence of compositional changes on the dc conductivity. In glasses with high Ag2O content, the jump distance for the Ag+ ions is

5. CONCLUSION The influence of structural changes induced by the addition of TeO2 content to the Ag2O−B2O3−P2O5 glass on the electrical transport properties and relaxation behavior has been studied. The gradual introduction of the third glass former, TeO2, to the glass network causes the structural transformation from TeO3 (tp) to TeO4 (tbp) which is responsible for the changes in conductivity. The observed slow decrease in dc conductivity with addition of TeO2 is attributed to the increase of the number of nonbridging oxygens, which facilitates movement of Ag+ ions resulting in high values of dc conductivity. The steep decrease in conductivity for glasses containing a high TeO2 content is a result of both decrease of the Ag2O content and stronger cross-linkage in glass network through the formation of more Te−eqOax−Te bonds in TeO4 tbp units. The present glasses obey scaling of the ac conductivity with respect to temperature but show deviations from scaling for various glass compositions. The origin of this deviation is a local structural disorder caused by structural transition in the tellurite glass network.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: ++385-1-4561-149. Fax: + +385-1-4680-114. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Croatian Ministry for Science, Education and Sport (Grant No. 098-0982929-2916). The Czech authors are grateful for the financial support from the Grant Agency of the Czech Republic (Grant No. 13-00355S). The authors thank Prof. Z. Cernosek for the Raman spectra measurements.

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REFERENCES

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