Mixed Glass Former Effect in Ag2O–SeO2–TeO2 Glasses - American

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Mixed Glass Former Effect in Ag2O−SeO2−TeO2 Glasses: Dependence on Characteristic Displacement of Mobile Ions and Relative Population of Bond Vibrations A. Palui and A. Ghosh* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India ABSTRACT: We have studied and observed a mixed glass former effect in yAg2O−(1−y)[xSeO2−(1−x)TeO2] glasses for different modifier contents. We have carried out electrical measurements on these glasses in the frequency range from 10 Hz to 2 MHz and in the temperature range of 193−393 K. The crossover frequency obtained from the conductivity spectra also exhibits a mixed glass former effect similar to the conductivity. The scaling of the conductivity spectra reveals the compositionand temperature-independent nature of the ion conduction mechanism. The characteristic displacement of the silver ions has been obtained from the conductivity spectra in the framework of linear response theory, and it shows a strong dependence on the mixed former ratio. We have also studied the modification of the network structure of these glasses due to the mixing of glass formers using infrared spectra and their influence on ion dynamics. We have shown that the mixed glass former effect is associated with the relative population of bonds of structural units.

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known mixed alkali effect,13 as the ionic conductivity sometimes exhibits more than one maximum with the change in the glass former ratio.11 A mixed barrier model has been developed to explain the mixed glass-network former effect for these glasses.8 It has been proposed that different glass network formers consist of different free energy landscapes, and the change of site energy is observed due to different local geometries being involved with the mixing of different network formers. According to this model, the barrier energy for an ion jump is reduced rapidly for heterogeneous materials compared to that of the homogeneous materials.8 On the other hand, the conductivity for several mixed former glasses, such as Bi2O3− B2O3 and TeO2−B2O3 glasses, passes through a minimum value, instead of a maximum value, with the change in the mixed former ratio.14,15 It is clear that a complex scenario is associated with the mixed glass-network former effect. It has been observed in SeO2−B2O3 and SeO2−MoO3 mixed former glasses that Se−O clusters depolymerize the borate and molybdate network and create nonbridging oxygen,16,17 and the availability of more polar clusters facilitates ion migration in these glasses.17 In TeO2−P2O5 glasses some heteroatomic P− O−Te linkages are observed at a lower concentration of tellurium.12 These linkages serve as a mechanism for more effective anionic charge dispersal in the network, resulting in shallower Coulomb traps and thus enhanced ionic conductiv-

INTRODUCTION The study of ion dynamics in superionic glasses is highly important from both technological and academic points of view.1−3 It is important to determine the mechanism for ion dynamics in these glasses in order to optimize the conductivity of these glasses for technological applications. The increase in the ionic conductivity of these glasses can be achieved in various ways. The increase of alkali modifier oxide content in the glass composition is one of the ways to get enhanced ionic conductivity, as the alkali oxides depolymerize the glass network and leads to the increase in the nonbridging oxygen.4 The doping of alkali halide salts in the alkali-modified glasses increases the ionic conductivity further, as it expands the glass network, helping in faster migration of ions and reducing the activation energy needed for the conduction process.5 Addition of more than one glass former is another way to control the ionic conductivity.6 It has been observed in borophosphate glasses that the mixing of two glass formers (B2O3 and P2O5) reduces the activation energy and increases the conductivity compared to that of borate or phosphate glasses.6,7 This phenomenon, known as the mixed glass network former effect, has drawn more attention due to the simultaneous presence of different ionic sites with different potential landscapes and different microscopic and macroscopic glass properties.8 The mixed glass network former effect has been investigated for several glass systems, such as B2O3−P2O5, SeO2−B2O3, and TeO2−P2O5.9−12 In these glasses, the conductivity passes through a maximum value with the change in the mixed former ratio. The effect is observed to be less universal than the well© 2017 American Chemical Society

Received: February 4, 2017 Revised: April 5, 2017 Published: April 6, 2017 8738

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

Article

The Journal of Physical Chemistry C

Table 1. Glass Transition Temperature (Tg), Density (ρ), Direct Current Conductivity (σ), Activation Energy (Eσ) for Ionic Conduction, Crossover Frequency (ωc), Activation Energy (Ec) for Alternating Current Conduction, Power-Law Exponent (n), and ⟨r2(tp)⟩ for the Different Compositions of yAg2O−(1−y)[xSeO2−(1−x)TeO2] Glasses y

x

Tg (°C) (±1)

ρ (g/cm3) (±0.01)

log10 σ (Ω−1 cm−1) (±0.01) 303 K

Eσ (eV) (±0.01)

log10 ωc (rad s−1) (±0.1)

Ec (eV) (±0.01)

n (±0.01)

⟨r2(tp)⟩ (Ǻ ) (±0.05 Ǻ )

0.0 0.1 0.3 0.4 0.5 0.6

212 203 191 176 171 165

6.49 6.38 6.27 6.11 5.89 5.66

−6.99 −6.82 −6.21 −5.75 −6.15 −6.33

0.51 0.52 0.49 0.48 0.50 0.51

5.20 5.34 5.84 6.42 6.03 5.84

0.54 0.53 0.51 0.49 0.50 0.53

0.63 0.65 0.64 0.64 0.66 0.63

2.66 2.49 2.23 2.09 2.31 2.44

0.0 0.1 0.3 0.4 0.5

202 189 173 165 158

6.71 6.69 6.34 6.31 6.16

−5.64 −5.55 −5.21 −5.09 −5.31

0.47 0.47 0.45 0.44 0.47

6.74 6.75 7.09 7.25 7.03

0.47 0.47 0.45 0.44 0.45

0.61 0.65 0.63 0.64 0.64

1.80 1.75 1.56 1.37 1.60

0.3

0.4

were annealed in an oven for 2 h at 140 °C, which is approximately 50 °C below the glass transition temperature (Table 1) determined from the DSC experiment. The formation of glass for all compositions was confirmed from X-ray diffraction patterns obtained with an X-ray diffractometer (Bruker, model D8 Advanced AXS) using Cu Kα radiation. The density of these glass samples was measured with the help of the Archimedes principle using acetone as the immersion liquid (Table 1). Fourier transform infrared (FTIR) spectra of these glasses were recorded in a FTIR spectrometer (PerkinElmer, model Spectrum 100) in the wavenumber range of 400−4000 cm−1. The ac electrical measurements, such as conductance and capacitance, of the samples of diameter ∼10 mm were carried out in the frequency range of 10 Hz−2 MHz using an impedance analyzer (QuadTech, model-7600) in the temperature range of 193−393 K. Silver paste was used as the electrode material. The sample cell with parallel electrode configuration was placed inside a cryostat for measurements below room temperature. The temperature of the cryostat was controlled with a stability of ±0.1 K. The real and imaginary parts of the complex impedance have been obtained from the measured conductance and capacitance data.

ities.12 Thus, we apparently visualize that the variation of the concentration of network-forming units in mixed network former glasses profoundly influences the macroscopic properties such as the conductivity. The macroscopic conductivity can be related to the microscopic parameters, such as mean square displacements of the ions, through linear response theory.18 In a few glasses, such as sodium borophosphate and lithium borophosphate glasses, a direct relation between mean square displacement and the number of BO4 tetrahedra has been observed.6,19 It shows that the high concentration of BO4 tetrahedra, which increases the network connectivity, leads to an increase in the characteristic distance. A similar effect has been observed for lithium borobismuth glasses,14 in which the dc conductivity and the mean square displacement of mobile ions are directly correlated to the concentration of BO4 units, as the Coulomb energy is reduced with the increase of BO4 tetrahedra. These observations clearly point toward the influence of the mixed glass network former effect on the ion dynamics. Thus, the study of the mean square displacement of mobile ions gives an idea about the potential scenario the ions are moving in. The objective of the present paper is to understand the mixed glass network former effect in SeO2−TeO2 mixed former glasses for different modifier (Ag2O) contents. We have obtained the characteristic distance of ion dynamics in these glasses and correlated the mixed glass network former effect with these distances. The origin of the mixed glass network former effect is associated with the relative population of the bond vibration of SeO32− ions and TeO3 units in the molecular level.

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RESULTS AND DISCUSSION Direct Current Conductivity. The dc ionic conductivity (σ) was obtained from the complex impedance plots for all glass compositions. The variation of the dc conductivity with the reciprocal temperature is shown in Figure 1 for different glass samples. It is noted that the ionic conductivity for all compositions follows the Arrhenius relation of the form

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σ = (σ0/T ) exp[−Eσ /kBT ]

EXPERIMENTAL SECTION Glasses of compositions yAg2O−(1−y)[xSeO2−(1−x)TeO2] were prepared within the glass-forming limits (x = 0.0−0.6 for y = 0.3, and x = 0.0−0.5 for y = 0.4) by quenching of the melts. The mixtures of appropriate amounts of the reagent-grade chemicals AgNO3, SeO2, and TeO2 (Sigma-Aldrich Chem. Co.) were first preheated in alumina crucibles at 400 °C for 2 h for denitrogenation of AgNO3 and then melted in the temperature range of 580−650 °C, depending on compositions. The melts were kept at that temperature for 1 h to equilibrate and then rapidly quenched between two aluminum plates. Glass samples of thickness of about 0.5 mm were obtained. The glass samples

(1)

where σ0 is the pre-exponential factor, Eσ is the activation energy, T is the absolute temperature, and kB is the Boltzmann constant. The activation energy (Eσ) was obtained from the least-squares straight line fits of the data presented in Figure 1 and is shown in Table 1 for different glass samples. Parts a and b of Figure 2 respectively show the dependence of the ionic conductivity at T = 303 K and the activation energy on the mixed former ratio for different modifier contents. It is noted that the ionic conductivity shows a maximum and the activation energy a minimum for x = 0.4. It is also noted that the ionic conductivity is higher for glasses containing higher modifier 8739

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

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The Journal of Physical Chemistry C

TeO2 single network former glasses.21 Thus, the mixing of two different glass network formers enhances the conductivity, and the mixed glass network former effect comes into play. Alternating Current Conductivity Spectra. The frequency dependence of the real part σ′(ω) of the complex conductivity at several temperatures is shown in Figure 3a for a

Figure 1. Ionic conductivity shown as a function of reciprocal temperature for different glass compositions of yAg2O−(1−y)[xSeO2−(1−x)TeO2]. The solid lines are the least-squares linear fits to the data.

Figure 3. (a) Alternating current conductivity spectra at several temperatures for the glass composition 0.3Ag2O−0.7(0.3SeO2− 0.7TeO2). Solid lines are fit to eq 2. (b) Alternating current conductivity spectra at T = 273 K for different compositions of yAg2O−(1−y)[xSeO2−(1−x)TeO2] glasses.

composition, while Figure 3b shows the same at T = 273 K for different glass compositions. It is observed in all cases that at low frequencies and low temperatures the ac conductivity is almost independent of frequency and corresponds to the dc conductivity, while the ac conductivity shows a dispersive behavior with the increase in frequency. It may be noted that the ac conductivity at low frequencies and higher temperatures is influenced by the electrode polarization effect. The frequency dependence of the real part of complex conductivity can be well-described by the following power-law model22,23

Figure 2. (a) Dependence of the ionic conductivity on glass former ratio for different Ag2O contents at T = 303 K and (b) dependence of the activation energy Eσ on the glass former ratio for different Ag2O contents for yAg2O−(1−y)[xSeO2−(1−x)TeO2] glasses. Solid lines are guides to the eye.

content. It is to be noted that the variation of the conductivity for the present TeO2−SeO2 mixed former glasses with mixed former ratio is quite different from that of the TeO2−B2O3 glasses.20 It is reported for these borotellurite glasses that initially the conductivity of these glasses decreases slightly with the increase of B2O3 content but shows an increase for higher content of B2O3.20 It is also observed that the present mixed former glasses exhibit higher ionic conductivity than the Ag2O−

σ ′(ω) = σ[1 + (ω/ωc)n ]

(2)

where ωc is the crossover frequency, which denotes a transition from the dc ionic conductivity to the dispersive region, and n is the power-law exponent having values in between 0 < n ≤ 1. The conductivity spectra σ′(ω) of all the glass samples for different temperatures were fitted to eq 2 to obtain the parameters σ, ωc, and n from the best fits. It was noted that the 8740

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

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The Journal of Physical Chemistry C value of σ obtained from the fits is very close to that of the dc conductivity obtained from complex impedance plot (Figure 2a). It was observed that the reciprocal temperature dependence of ωc for all the present glasses also followed an Arrhenius relation with activation energy Ec. It is noted in Table 1 that the activation energy Ec for ωc is very close to that (Eσ) of the dc conductivity, signifying a common conduction mechanism for both dc and ac conduction. It is also noted in Table 1 that the ωc increases with an increase of SeO2 content, and a maximum value is obtained for x = 0.4 (y = 0.3, 0.4), similar to the dc conductivity. It has been reported earlier that the power-law exponent n is related to the dimensionality of the conduction pathway.24 The value of n ∼ 0.65 corresponds to a threedimensional conduction, whereas n ∼ 0.50 corresponds to a two-dimensional conduction.24 The conduction pathways available for ion transport decrease with the decrease of the dimensionality, which decreases the mobility and the conductivity. Thus, for the present glass series n is almost independent of composition and possesses an average value of n ∼ 0.63 (Table 1), which corresponds to a three-dimensional conduction.24 To get further insight into ion dynamics, we have checked whether the ac conductivity follows the time−temperature superposition principle. In that case, the conductivity spectra of a particular composition should overlap perfectly onto the master curve with proper scaling. Various workers have proposed different scaling models for the conductivity spectra. 25−27 For the present glass compositions, the modification of network structure is due to the change of the glass former ratio, resulting in a change in potential landscape in which the mobile ions are moving. As the glass network structure changes, the hopping distance for the mobile ions also changes. This change in hopping length can be accounted for by the scaling formalism proposed by Sidebottom,27 which takes into account the hopping contribution to the permittivity, since the hop of the ion between anionic sites is analogous to the rotation of a permanent dipole.27 The scaling formalism proposed by Sidebottom is given by27 ⎛ ωε Δε ⎞ σ ′(ω) = F⎜ 0 ⎟ ⎝ σ ⎠ σ

Figure 4. (a) Scaled master curves of the ac conductivity spectra according to the Sidebottom scaling law of the glass composition 0.3Ag2O−0.7(0.1SeO2−0.9TeO2) for several temperatures. (b) Scaling of the conductivity spectra for different glass former ratios for the composition 0.3Ag2O−0.7[xSeO2−(1−x)TeO2].

mean square displacement ⟨r2(t)⟩ of mobile ions by the following equation18,28

(3)

σ ′(ω) =

where ε0 is the permittivity of free space and Δε is the dielectric strength of the material, which was obtained from the dielectric spectra. Following eq 3, the scaling of the conductivity spectra for a particular composition at different temperatures is shown in Figure 4a. The same scaling is also shown in Figure 4b for different glass former ratios. It is noted that scaling of the conductivity spectra for different glasses containing different mixed former ratios is almost perfect. The near-perfect superposition of different conductivity spectra onto a single master curve signifies that the ion conduction process follows a common mechanism and it is independent of temperature as well as composition for the present glass system. The dependence of the conduction mechanism on the variation of hopping length in scaling is well-incorporated by considering the dielectric strength factor. Characteristic Displacement of Mobile Ions and Its Correlation with Ion Dynamics. We have analyzed the conductivity spectra of these glasses in the framework of the linear response theory.18 In the framework of this theory, the frequency-dependent conductivity is expressed in terms of the

NV q 2 ω 2 6kBTHR

∫0

∞

r 2(t ) sin(ω t) dt

(4)

where NV is the number density, which is calculated from the density (Table 1) and molar mass;29 q is the charge of the mobile ion; kB is the Boltzmann constant; T is the absolute temperature; and HR is the Haven ratio, which is defined as the ratio of the tracer diffusion coefficient to the conductivity diffusion coefficient. HR indicates the degree of correlation between successive hops.28 In our case we have assumed that all ions are mobile, as over long time scale all ions are mobile.12 Therefore, HR can be considered as unity for the present glasses.30 The linear response theory allows us to extract ⟨r2(t)⟩ from the conductivity spectra by taking the Fourier transform of eq 4 and hence ⟨r 2(t )⟩ =

12kBT 2

NV q π 0

∫0

t

dt ′

∫0

∞

σ ′(ω) sin(ωt ′) dω ω

(5)

Since the charge carrier density is found to be almost independent of temperature, we have chosen a fixed value of NV for a particular composition, followed by other authors,19 and 8741

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

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The Journal of Physical Chemistry C obtained ⟨r2(t)⟩ from eq 5 using the experimental conductivity spectra (Figure 3). Figure 5 shows the time dependence of

Figure 6. Variation of the characteristic length ⟨r2(tp)⟩ with mixed former ratio for different values of modifier contents for yAg2O−(1− y)[xSeO2−(1−x)TeO2] glasses. Solid lines are guides to the eye.

mention here that, for modestly large alkali ion concentrations of other glasses,19,29 a similar low value of ⟨r2(tp)⟩ (∼1 Å) has been reported, while a higher value of ⟨r2(tp)⟩ (∼3.9 Å) is observed for low-alkali-containing glasses.31 The results for the present glasses are similar to those results: the values of ⟨r2(tp)⟩ are lower for higher Ag2O content (y = 0.4). Correlation of Mixed Former Effect with Relative Population of Bond Vibrations. To correlate further the displacement of mobile ions with the modification of network structure due to mixing of glass formers, we have analyzed the FTIR spectra for the glass compositions, presented in Figure 7a, in the wavenumber range 400−4000 cm−1. It is observed that major vibration bands are located in the wavenumber range 500−980 cm−1, which results from the superposition of several vibration bands. We have deconvoluted the spectra in the wavenumber range 500−980 cm−1 to extract the individual contributions of the different vibration bands. A Gaussian function has been used to find the foremost contributing bands. Figure 7b shows the deconvolution of the band at 500−980 cm−1 for a typical composition. It is observed that a significant contribution to the structural modification is found for the bands centered at around ∼610−620, ∼750−770, and ∼870− 875 cm−1. The absorption bands located around ∼610−620 and ∼750−770 cm−1 are assigned to the stretching vibration of the Te−O−Te bridges between TeO4 tetragonal bipyramidal units and Te−O bending vibration in TeO3 trigonal pyramidal units,32−34 respectively. The band located in the range ∼870− 880 cm−1 is attributed to the vibration mode of the Se−O−Se bond of SeO32− ions.35 To obtain the individual contribution of vibration bands to the resulting spectra, we have taken the ratio of the area under the corresponding band to the total area of all the bands, obtained from Gaussian fits. In fact, this relative area signifies the strength of the corresponding bond. It was observed that the relative population of Te−O−Te bridges in TeO4 tetragonal bipyramidal units decreased with an increase of glass network former ratio (x ≤ 0.4) but increased at higher SeO2 content and showed a similar trend to ⟨r2(tp)⟩. These results can be explained as follows: the number of TeO4 groups decreases because some tetragonal TeO4 bipyramidal units transform into trigonal TeO3 pyramidal units.32 It has been observed in various single former glasses that the dc conductivity depends on the structural transformation.36 For

Figure 5. Mean square displacement curve at different temperatures for the composition 0.4Ag2O−0.6(0.3SeO2−0.7TeO2). Solid lines are the best fits at subdiffusive and diffusive regions. The characteristic time tp is represented by •. The inset shows the mean square displacement curve at T = 273 K for several glass compositions of yAg2O−(1−y)[xSeO2−(1−x)TeO2].

⟨r2(t)⟩ at several temperatures for a composition, while the inset of Figure 5 shows the time dependence of ⟨r2(t)⟩ for different compositions at a fixed temperature. It is noted that at a short time scale ⟨r2(t)⟩ shows subdiffusive behavior due to the localized motion of the ions and follows the relation ⟨r2(t)⟩ ∼ t1‑n, where n (n ∼ 0.63) is the power-law exponent, which is almost independent of temperature. On the other hand, at a longer time scale ⟨r2(t)⟩ reflects a diffusive random motion of the ion and is proportional to t (i.e., ⟨r2(t)⟩ ∼ t). The dynamics of ions in the glass network can be considered as if the mobile ions are moving in a three-dimensional potential scenario and the heights of these potential barriers vary from site to site.28 At a shorter time scale, when a mobile ion tries to hop to the neighboring high-energy state, it finds a large barrier for the forward hop. As the barrier height for the backward hop is low, the mobile ion has a high probability of backward hop. But at a longer time scale the ion is able to perform a successful hop to the neighboring high-energy site. Thus, the correlated forward−backward motion is connected with the shorter time scale, whereas a successful random motion of mobile ions can only occur at longer time. Hence, a crossover happens from subdiffusive to diffusive ion motion at a certain characteristic time t = tp, where the mean square displacement of the charge carrier is ⟨r2(tp)⟩, which is the minimum distance a mobile ion has to cover for a successful hop to the neighboring high-energy site in the conduction pathway. We have calculated the value of ⟨r2(tp)⟩ following the procedure reported elsewhere.28 Figure 6 shows the dependence of ⟨r2(tp)⟩ on the mixed former ratio for both modifier contents. It is noted that ⟨r2(tp)⟩ shows a minimum for x = 0.4 for both modifier contents (y = 0.3 and 0.4) unlike the conductivity. As ⟨r2(tp)⟩ decreases, Ag+ ions have to travel only a small distance before their motion becomes diffusive, and therefore, the conductivity increases. It is worthwhile to 8742

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

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The Journal of Physical Chemistry C

Figure 7. (a) FTIR spectra for several compositions of 0.3Ag2O− 0.7[xSeO2−(1−x)TeO2] glasses in the wavenumber range 400−4000 cm−1. (b) Deconvolution of the FTIR spectrum in the wavenumber range 500−980 cm−1 is shown for the glass composition 0.4Ag2O− 0.6(0.1SeO2−0.9TeO2). The dotted lines show the deconvoluted bands.

Figure 8. (a) Variation of the relative population for the vibration band (TeO3) centered at 760 cm−1 with mixed former ratio for different Ag2O contents. (b) Variation of relative population for the vibration band (SeO32−) centered at 875 cm−1 with mixed former ratio for different Ag2O contents for yAg2O−(1−y)[xSeO2−(1−x)TeO2] glasses. Solids lines are guides to the eye.

example, in lithium−tellurite glasses, TeO4 trigonal bipyramids transform to TeO3 trigonal pyramids.36 In Li2O−B2O3 glasses, a conversion of BO4 to BO3 unit of glass network has been observed.37 Also the formation of P−O−Te linkages has been observed in TeO2−P2O5 glasses, which serve as a mechanism for more effective anionic charge dispersal in the network, resulting in shallower Coulomb traps and thus enhanced ionic conductivities.12 The increase of TeO3 and BO3 units in the glass network indicates that the concentration of nonbridging oxygen increases, which helps in faster ion migration. Thus, a destruction of the three-dimensional structure of TeO4 unit happens with the introduction of the selenium oxide, and TeO3 isolated group increases in proportion.35 Figure 8a shows the composition dependence of the relative population of Te−O bond in TeO3 pyramidal units, in which one oxygen involved in the double bond (TeO) is nonbridging and the other two oxygen atoms are involved in bridging bonds.32 The increase in the nonbridging oxygen increases the availability of hopping sites for Ag+ ions in the glass network. Therefore, there is a decrease in the distance that the ion has to travel to access the neighboring high-energy site. As a result, the mobility of the ions increases, and hence, an increase in the conductivity is observed. The dependence of the relative population of the band at ∼875 cm−1 for SeO32− ions is shown in Figure 8b. For the binary glass (x = 0), no bands are present in the higher wavenumber region (>825 cm−1). With the increase of SeO2

content, the band at ∼875 cm−1 starts dominating for both modifier contents (y = 0.3 and 0.4), but it has been observed that at higher mixed former ratio, the strength for the band at ∼875 cm−1 of SeO32− ions decreases. Initially, the vibration of SeO32− ions is predominant, as the polarizability as well as the availability of the number of nonbridging hopping sites facilitates faster ion conduction. However, for higher mixed former ratio, the SeO32− ions have a tendency to bond with Ag+ ions and form isolated Ag2SeO3 crystalline structure, which in turn reduces the availability of the effective nonbridging hopping site.16 It has been observed in various selenium− borate38 and selenium−molybdate16 glasses that, when sufficient Ag+ ions are present in the glass structure, a part of SeO2 participates in the formation of silver−selenite immobile structure. In a few selenite glasses containing Cu2+ ions, the formation of a composite copper−selenite structure has been observed.39 The feeble band observed at around 820 cm−1 corresponds to the Ag2SeO3 unit,38 which grows at a higher value of x. Figure 9 shows the growth of the silver-selenite structure with composition in the glass matrix. A sharp increase is observed for glasses containing higher SeO2 amounts (x = 0.5, 0.6, y = 0.3), so the decrease in the relative population of SeO32− ions and the decrease in free volume due to the increase in isolated Ag2SeO3 crystalline structures collectively decrease 8743

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(EMR/2015/000149) of SERB-DST is also thankfully acknowledged.

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Figure 9. Variation of relative area for the vibration band (Ag2SeO3) centered at 820 cm−1 with mixed network former ratio for different modifier ratios of the glass series yAg2O−(1−y)[xSeO2−(1−x)TeO2]. Solids lines are guides to the eye.

the conductivity. Thus, the modification of glass structure at the molecular level due to the variation of the glass former ratio is well-connected with the ion dynamics in the present glass system.

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CONCLUSIONS Ion dynamics in Ag+ ion conducting selenium−tellurite mixed former glasses has been studied by varying the modifier content as well as the network former ratio. The ionic conductivity gradually increases with the increase of SeO2 content but decreases at higher mixed former ratio. The scaling of the conductivity spectra shows that the ion conduction mechanism is independent of temperature as well as composition. The characteristic distance ⟨r2(tp)⟩, which signifies a crossover from subdiffusive nonrandom to diffusive random motion, shows a strong dependence on the glass former ratio. The dependence of ⟨r2(tp)⟩ on the mixed former ratio is associated with the relative population of different structural units. The relative population of SeO32− units is directly correlated to the ionic conductivity. At higher SeO2 content, the relative population of SeO32− ions decreases due to the formation of isolated Ag2SeO3 crystalline structures, which reduces the available free volume for ion migration as well as the ion concentration, and consequently, a decrease in the conductivity is observed at higher selenium content for these glasses.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Ghosh: 0000-0003-4713-9854 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the J.C. Bose fellowship grant (SB/ S2/JCB-33/2014) of the Department of Science and Technology (DST), Government of India is thankfully acknowledged. The financial support from the project grant 8744

DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745

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DOI: 10.1021/acs.jpcc.7b01121 J. Phys. Chem. C 2017, 121, 8738−8745