Structural, optical, physical and electrical properties of V2O5·SrO·B2O3 glasses

Structural, optical, physical and electrical properties of V2O5·SrO·B2O3 glasses

Spectrochimica Acta Part A 64 (2006) 196–204 Structural, optical, physical and electrical properties of V2O5·SrO·B2O3 glasses S. Sindhu, S. Sanghi ∗ ...

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Spectrochimica Acta Part A 64 (2006) 196–204

Structural, optical, physical and electrical properties of V2O5·SrO·B2O3 glasses S. Sindhu, S. Sanghi ∗ , A. Agarwal, V.P. Seth, N. Kishore Department of Applied Physics, Guru Jambheshwar University, Hisar 125001, Haryana, India Received 20 January 2005; accepted 14 June 2005

Abstract The present work aims to study the structure and variation of optical band gap, density and dc electrical conductivity in vanadium strontium borate glasses. The glass systems xV2 O5 ·(40 − x)SrO·60B2 O3 and xV2 O5 ·(60 − x)B2 O3 ·40SrO with x varying from 0 to 20 mol% were prepared by normal melt quench technique. Structural studies were made by recording IR transmission spectra. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of Urbach energy is associated with the phonon-assisted indirect transitions. The change in both density and molar volume was discussed in terms of the structural modifications that take place in the glass matrix on addition of V2 O5 . dc conductivity of the glass systems is also reported. The change of conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one. © 2005 Elsevier B.V. All rights reserved. Keywords: Infrared transmission; Optical band gap; Oxide glasses; Density; Electrical conductivity

1. Introduction Amorphous V2 O5 has attracted attention in recent years because of its potential use as cathode in solid-state devices. It shows type n semiconducting properties and the electronic conduction is caused by a phonon assisted electron hopping between V4+ and V5+ ions [1]. Boron trioxide B2 O3 is an archetypal glass forming oxide while vanadium pentaoxide V2 O5 is not easily vitrified. However, many multicomponent glasses containing V2 O5 is reported [2–5]. Most of the studies made are mainly on V2 O5 –P2 O5 glasses with V2 O5 greater than 50 mol%. Studies on glasses containing relatively low concentration of V2 O5 in presence B2 O3 are very few [5,6]. These glasses have their potential applications as optical and electrical memory switching, cathode materials for making solid-state devices and optical fiber [7,8]. In the glasses with high percentage of V2 O5 , it is considered as a glass forming oxide. In the present work effect ∗

Corresponding author. Tel.: +91 1662 263176; fax: +91 1662 276240. E-mail address: [email protected] (S. Sanghi).

1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.06.039

of addition of V2 O5 in strontium borate glasses has been studied in order to characterize the geometry of structural units of the glass network and the changes caused by the V2 O5 content on optical, physical and electrical properties. These types of glasses exhibit mixed electronic (electrons hopping along V4+ –O–V5+ paths) and ionic (Sr2+ ions) conductivity. Glasses with such mixed electrical conductivity attract scientific interest because of potential applications as solid electrolytes in electrochemical devices such as batteries, chemical sensors and smart windows [9].

2. Experimental The glass systems xV2 O5 ·(40 − x)SrO·60B2 O3 (Series I) and xV2 O5 ·(60 − x)B2 O3 ·40SrO (Series II) with six values of x in the range 0 ≤ x ≤ 20, in the steps of four were prepared by melting dry mixtures of analytical grade chemicals of H3 BO3 , SrCO3 and V2 O5 . Approximately 15 g of chemicals were thoroughly mixed to obtain homogenized batches in a porcelain crucible and then melted at 1473 K by using an

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electrical muffle furnace for half an hour. The mixture was shaken frequently to ensure homogeneity. The glasses were obtained by pouring the melts onto a stainless steel plate and immediately quenched by pressing with another plate. The glasses were finely polished to a thickness of 0.5–1.5 mm for the measurements of various properties. Infrared transmission spectra were recorded at room temperature on a Shimadzu FTIR-8001 PC spectrophotometer over the range 400–4000 cm−1 . The powdered samples were thoroughly mixed with dry KBr in a ratio of 1:20 and then pellets were formed under a pressure of 7–8 ton. The optical absorption spectra were recorded in the wavelength range 350–1000 nm at room temperature using PerkinElmer UV/VIS spectrometer (Lambda 20). The density (D) of the glasses was determined at room temperature using Archimede’s method with xylene as the buoyant liquid. The molar volume (VM ) of each glass sample was calculated using the formula [10].  xi Mi (1) VM = D where xi is the molar fraction and Mi is the molecular weight of the ith component. For measuring dc conductivity (σ), disks of thickness about 1 mm were coated with silver paint to serve as electrodes. A constant voltage was applied and the current was measured using Keithley 6485 picoammeter in the temperature range 373–573 K.

3. Results 3.1. Infrared transmission spectra Figs. 1(a) and 2(a) show the IR transmission spectra for all the glass samples over the range 400–4000 cm−1 respectively for Series I and II. For clarity, the spectra over the range 400–1100 cm−1 is separately shown in Figs. 1(b) and 2(b). The infrared spectra of these glasses arise largely from the modified borate networks. According to the Krogh Moe’s [11], the structure of the boron oxide glass consists of a random network of planer BO3 triangles with a certain fraction of six-membered (boroxol) rings. X-ray and neutron diffraction data suggests that glass structure consist of a random network of BO3 triangles without boroxol rings. Similar findings have also been reported from the molecular dynamic studies [12]. The vibrational modes of the borate network are seen to be mainly active in three infrared spectral regions, which are similar to those reported by several workers [13–15]. (i) The first group of bands, which occur at 1200– 1600 cm−1 , is due to the asymmetric stretching relaxation of the B O band of trigonal BO3 units. (ii) The second group lies between 800 and 1200 cm−1 and is due to the B O bond stretching of the tetrahedral BO4 units.

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(iii) The third group is observed around 700 cm−1 and is due to the bending of B O B linkages in the borate networks. 3.2. Optical band gap The absorption as a function of incident photon energy for different compositions of V2 O5 is shown in Fig. 3(a) and (b) for both the series respectively. The absorption coefficient α(ν), below and near the edge of each curve was determined at different wavelengths using the relation [16]     1 I0 α(ν) = (2) ln d It where ‘I0 ’ and ‘It ’ are the intensities of the incident and transmitted beams, respectively and ‘d’ corresponds to thickness of each sample. The factor ln(I0 /It ) is the absorbance, ‘A’. The data is related to the optical band gap energy, Eg , through the following general relation proposed by Davis and Mott [17] αhν = [B(hν − Eg )]r

(3)

where the index ‘r’ takes different values depending on the mechanism of interband transitions and ‘B’ is a constant called band tailing parameter, ‘hν’ is the incident photon energy. In various glass systems Eq. (3) depicts a straight-line for r = 2 and is associated with indirect allowed transitions. The variation of (αhν)1/2 with hν (Tauc’s plot) is shown in Fig. 4(a) and (b) for some of the glass samples of the two series respectively [18]. To estimate the values of Eg , the linear region of the curves is extrapolated to meet the hν axis at (αhν)1/2 = 0 and are listed in Tables 1 and 2 for all the compositions of V2 O5 . The same data was fitted in Eq. (3) for r = 3 which corresponds to indirect forbidden transitions and calculated values of Eg , in this case are also included in Tables 1 and 2. The values of ‘B’ calculated from the linear portion of the Tauc’s plot for r = 2 and 3 are also presented in the tables for both the series. The absorption at lower photon energy usually follows Urbach rule [19] given by   νh α(ν) = αo exp (4) E where ‘αo ’ is a constant and E is the Urbach energy, corresponds to the optical transitions between localized tail states adjacent to the valance band and the extended states in the conduction band above the mobility edge. The logarithm of the absorption coefficient was plotted as a function of photon energy for various compositions of V2 O5 for both the series. Urbach plots for some of the samples of Series I are shown in Fig. 5 and the values of E have been calculated from the reciprocal of the slopes of the linear portion of these curves (Tables 1 and 2).

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Fig. 1. (a) Infrared transmission spectra of the glass system xV2 O5 ·(40 − x)SrO·60B2 O3 (400–4000 cm−1 ) and (b) infrared transmission spectra of the glass system xV2 O5 ·(40 − x)SrO·60B2 O3 (400–1100 cm−1 ).

3.3. Density and molar volume Density (D) and molar volumes (VM ) of the glass samples were measured as explained earlier and the calcu-

lated values are presented in Tables 1 and 2. The variation of D and VM with increasing V2 O5 content is shown respectively in Fig. 6(a) and (b) for both the series.

Table 1 Optical band gap (Eg ), band tailing parameter (B), Urbach energy (E), density (D) and molar volume (VM ) for the glass system xV2 O5 ·(40 − x)SrO·60B2 O3 Glass no.

x (mol%)

Eg (eV)

Sr1 Sr2 Sr3 Sr4 Sr5 Sr6

0 4 8 12 16 20

2.56 1.97 1.76 1.33 – –

B (cm eV)−1/2

Eg (eV)

9.04 9.03 10.17 9.69 – –

2.48 1.53 1.04 0.61 – –

r=2

B (cm−1/3 eV−2/3 )

E (eV)

D (g/cm3 )

VM (cm3 /mol)

4.44 2.97 2.49 2.48 – –

0.44 0.58 0.40 0.55 – –

3.94 3.06 3.05 2.93 2.83 2.77

20.89 27.88 28.99 31.33 33.46 35.32

r=3

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Fig. 2. (a) Infrared transmission spectra of the glass system xV2 O5 ·(60 − x)B2 O3 ·40SrO (400–4000 cm−1 ) and (b) infrared transmission spectra of the glass system xV2 O5 ·(60 − x)B2 O3 ·40SrO (400–1100 cm−1 ). Table 2 Optical band gap (Eg ), band tailing parameter (B), Urbach energy (E), density (D) and molar volume (VM ) for the glass system xV2 O5 ·(60 − x)B2 O3 ·40SrO Glass no.

x (mol%)

Eg (eV)

B (cm eV)−1/2

r=2 Sr7a Sr8 Sr9 Sr10 Sr11 Sr12 a

0 4 8 12 16 20

2.56 1.44 1.27 1.01 – –

Sr1 and Sr7 have the same composition.

Eg (eV)

B (cm−1/3 eV−2/3 )

E (eV)

D (g/cm3 )

VM (cm3 /mol)

4.44 2.51 2.81 2.52 – –

0.44 0.87 0.83 0.77 – –

3.94 3.21 3.17 3.15 3.12 3.07

20.89 27.08 28.82 30.46 32.19 34.24

r=3 9.04 5.55 5.39 6.12 – –

2.48 1.30 1.26 0.77 – –

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Fig. 3. (a) Optical absorption as a function of wavelength for the glass system xV2 O5 ·(40 − x)SrO·60B2 O3 and (b) optical absorption as a function of wavelength for the glass system xV2 O5 ·(60 − x)B2 O3 ·40SrO.

Fig. 4. (a) Tauc’s plots for xV2 O5 ·(40 − x)SrO·60B2 O3 glasses (r = 2) and (b) Tauc’s plots for xV2 O5 ·(60 − x)B2 O3 ·40SrO glasses (r = 2).

3.4. dc conductivity The temperature dependence of dc electrical conductivity (σ) for both the series are presented respectively in Fig. 7(a) and (b). This dependence of the dc conductivity obeys the well-known Arrhenius formula   W σ = σo exp − (5) kT

The values of ‘W’ and ‘σ o ’ were evaluated by using the least square fitting of the experimental data with the relation

where ‘W’ is the activation energy for conduction and is the average value of the heights of the potential energy barriers that the mobile alkaline ions must overcome in its jumps. The pre-exponential factor, ‘σ o ’ contains several constants including the vibrational frequency of the mobile ion. ‘k’ is the Boltzmann constant.

The calculated values of ‘σ’ (at two different temperatures 473 and 573 K), log σ o and W are displayed in Tables 3 and 4 for the two series, respectively. It is observed that dc electrical conductivity continuously increases and the activation energy decreases with the addition of V2 O5 content in both the series.

 log σ = log σo −

W 1000k



1000 T



1 2.303

 (6)

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Table 3 dc conductivity (σ), log σ o , and activation energy (W) for the glass system xV2 O5 ·(40 − x)SrO·60B2 O3 Glass no. Sr1 Sr2 Sr3 Sr4 Sr5 Sr6

(S m−1 )

x (mol%)

σ

0 4 8 12 16 20

3.97 × 10−10

at 473 K

1.08 × 10−9 1.17 × 10−9 2.26 × 10−9 5.10 × 10−9 5.98 × 10−8

σ

at 573 K

(S m−1 )

1.42 × 10−7 1.46 × 10−7 1.82 × 10−7 3.82 × 10−7 6.90 × 10−7 1.32 × 10−5

log σ o (S m−1 )

W (eV)

2.32 2.02 0.71 0.95 1.86 3.68

1.11 1.04 0.91 0.72 0.66 0.63

log σ o (S m−1 )

W (eV)

2.32 1.27 1.45 0.73 0.72 0.95

1.11 0.86 0.84 0.77 0.64 0.56

Table 4 dc conductivity (σ), log σ o , and activation energy (W) for the glass system xV2 O5 ·(60 − x)B2 O3 ·40SrO Glass no. Sr7a Sr8 Sr9 Sr10 Sr11 Sr12 a

(S m−1 )

x (mol%)

σ

0 4 8 12 16 20

3.97 × 10−10

at 473 K

9.95 × 10−10 1.87 × 10−9 2.89 × 10−9 3.58 × 10−9 4.05 × 10−9

σ

at 573 K

(S m−1 )

1.42 × 10−7 1.52 × 10−7 2.31 × 10−7 3.37 × 10−7 5.57 × 10−7 7.01 × 10−7

Sr1 and Sr7 have the same composition.

4. Discussion 4.1. Infrared transmission spectra The IR transmission spectra depict the broad composite bands extending from 3200 to 3600 cm−1 and are obtained in all the glasses (Figs. 1 and 2). These are attributed to hydroxyl or water groups [20,21]. The band at 2700–3000 cm−1 originates from hydrogen bonding. According to Borrellie et al. [22,23] these bands are due to hygroscopic nature of powdered glass samples. Hence it can be safely concluded that the samples are hygroscope in nature. The OH− groups form nonbridging oxygen sites and should contribute to those formed upon introducing into the glasses. These bands are obtained in all the glasses almost at the same position.

Fig. 5. Urbach plot for xV2 O5 ·(40 − x)SrO·60B2 O3 glasses.

Fig. 6. (a) Variation of density with mol% of V2 O5 for both the series and (b) variation of molar volume with mol% of V2 O5 for both the series.

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glasses of higher SrO contents, it is known that asymmetric stretching vibrations of pyro-(B2 OO4 4− ) and orthoborate (BO3 3− ) units also contribute to infrared transmission above 1200 cm−1 [13,24]. Therefore the evolution of the 800–1200 cm−1 envelope with 0 < SrO ≤ 0.40 signals the progressive change of boron coordination number from 3 to 4. A transmission band around 1000–1050 cm−1 (clearly seen in Figs. 1(b) and 2(b)) was observed in all the samples. This is assigned to V O stretching mode. The position of band remained unchanged by changing the composition of V2 O5 . With the addition of V2 O5 a band appears at around 900–950 cm−1 which becomes more broad and deep with increase in V2 O5 content. It is known that the characteristic vibrations of the isolated vanadium–oxygen bonds in the IR spectrum are in the 900–1020 cm−1 [28]. The high frequency band in 950–980 cm−1 range is assigned to the vibrations of VO2 groups of the VO4 polyhedra, while the intense band at 1020 cm−1 is related to the vibrations of the VO2 groups of the VO5 group [29]. This indicates the formation of VO5 groups in addition to VO4 group as the content of V2 O5 is increased. A sharp dip at around 650–700 cm−1 is also observed in all the glasses, which is assigned to the bending of B O B linkages in the borate networks. The band around 450–600 cm−1 of V2 O5 is retained in almost all the samples. The results obtained in our glass systems are in very good accordance with the already reported results for the V2 O5 –B2 O3 glasses [30–32]. 4.2. Optical band gap

Fig. 7. (a) Variation of log σ as a function of reciprocal of temperature for xV2 O5 ·(40 − x)SrO·60B2 O3 glasses and (b) variation of log σ as a function of reciprocal of temperature for xV2 O5 ·(60 − x)B2 O3 ·40SrO glasses.

In order to understand the effect of addition of SrO on the structure of borate network, first we consider the mid infrared region (500–1600 cm−1 ) where the vibrations of boron oxygen arrangement are active. In particular, transmission band in the 800–1200 cm−1 range can be attributed to the B O (O represents bridging oxygen) stretching vibration of B O4 tetrahedron [24,25–27]. Glassy B2 O3 is known to consist of boroxol rings and independent BO3 triangles and show no transmission in the mid IR region. In the present glass systems a broad band at 800–1200 cm−1 is found in each glass. The high frequency profile (1200–1600 cm−1 ) also originates for every glass, which is attributed to the stretching vibration of B O and B O− bonds in borate triangular units, which are of the BO3 and BO2 O− type for glasses below the metaborate stoichiometry [24,25–27]. For

From the analysis of optical absorption spectra it is found that optical absorption edge is not sharply defined in the glass systems under reference, which clearly indicates their amorphous nature. It is also observed that the fundamental absorption edge and cutoff wavelength shift towards red with the increase in the content of V2 O5 for both the series. The calculated values of Eg (Tables 1 and 2) lie in the same range as reported for semiconductors. It is clear from Fig. 4(a) and (b) that there is a clear tendency that the Eg becomes smaller with increasing V2 O5 for all the glass samples. This behavior may be associated with the structural changes that are taking place caused by the addition of V2 O5 content into the strontium borate glasses. Boron oxide is well known conventional network former. It consists of a random three-dimensional network of sixmembered boroxol rings [33]. When one molecule of SrO is introduced into the borate matrix, coordination number of two boron atoms changes from 3 to 4. As the concentration of SrO reaches greater than 33 mol%, nonbridging oxygens (NBOs) would start to form [34]. It is well reported that at low concentration V2 O5 acts as network modifier in place of network former [35]. At x = 0 (40SrO·60B2 O3 ), NBOs are already present and with the addition of V2 O5 (as modifier), concentration of NBOs increases. With the further addition of V2 O5 we get the six-membered rings with only one BO4 tetrahedron. The six membered rings with one BO4 units can

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be in triborate or pentaborate forms thereby causing a further increase in NBOs and the glass structure becomes more randomized. So the addition of V2 O5 causes the breaking of the regular structure of the rings (borate and boroxol) and the appearance of orthoborate groups takes place. As a result of this, band gap decreases. The analysis of optical spectra of all the samples show that the fitting curves of Fig. 4(a) and (b) give very good fit for r = 2, indicating the occurrence of indirect allowed transitions. The same data was applied for r = 3, which correspond to indirect forbidden transition. It has been observed that both provide a good fit for the optical data. Therefore it is concluded that whatever is the mechanism of transition, the glass systems under study behave as an indirect gap semiconductor. The values of ‘B’ lies between 10.17 and 9.03 (cm eV)−1/2 and between 4.44 and 2.48 cm−1/3 eV−2/3 for Series I and between 9.04 and 5.39 (cm eV)−1/2 and between 4.44 and 2.51 cm−1/3 eV−2/3 for Series II corresponding to r = 2 and 3, respectively. These values are in good agreement with the reported values [36–38]. The value of E for the present glass systems lie between 0.44–0.55 and 0.44–0.77 eV, respectively for the two series. However, this is comparatively a small range and some conclusions can be extracted from it [39]. It is generally assumed that the exponential tail observed in various materials must have the same physical origin and this origin can be attributed to the phononassisted indirect electronic transitions [39,40]. These results obtained are in accordance with those reported for inorganic glasses. 4.3. Density and molar volume Fig. 6(a) shows that density (D) decreases with increasing V2 O5 content in both the series. Although the relative molecular mass of V2 O5 is higher than for SrO and B2 O3 , but V2 O5 contributes in the density to decrease. It is known that introducing one molecule of SrO into the B2 O3 matrix converts two BO3 units into two BO4 units. This process continues up to about 33 mol% of SrO where the number of BO4 units reaches its maximum value. The structure of base glass (40SrO·60B2 O3 ) is characterized by large number of NBOs and therefore relatively more open. As explained earlier addition of V2 O5 randomized the structure, which causes the density to decrease for both the series. The linear increase in VM with V2 O5 (Fig. 6(b)) can be explained on the same terms. As the relative molecular mass of V2 O5 is higher than that for SrO and B2 O3 , so increase in VM is an expected result. The increase in VM may indicate that the volume of NBO sites produced by the modifier V2 O5 is greater than that produced by an equivalent quantity of SrO [10].

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polaron hopping conduction [41–45]. It is expected that the conductivity of these types of glasses will consist of a mixture of ionic and electronic conduction. Generally ionic conduction depends on the alkaline concentrations and alkaline ion mobility while electronic conduction is described by small polaron theory. The conduction in the base glass is considered due to migration of Sr ions within the matrix [46]. The activation energy of base glass and the glass containing 4 mol% of V2 O5 are higher than that of other glasses. There is a fast decrease in W between 4 and 8 mol% of V2 O5 . The decrease in activation energy for both the series may reflect the change from ionic to polaronic conduction upon introducing V2 O5 into the glassy matrix. V2 O5 should be included in the glass matrix either as glass former or as glass modifier. If V2 O5 enters the structure as a glass former then the conductivity should decrease due to increasing concentration of bridging oxygen ions [35]. But increase in conductivity reveals that in the present glass system V2 O5 plays the role of network modifier in place of network former.

5. Conclusions The structural, optical, physical and electrical properties of the V2 O5 ·SrO·B2 O3 glasses have been studied. The infrared spectra of both the series were recorded over a continuous spectral range (400–4000 cm−1 ) to study their structure systematically. No boroxol ring formation was observed in the structure of these glasses. The conversion of three-fold to four-fold coordinated boron took place with the addition of V2 O5 . The values of different optical parameters, viz. optical band gap, Urbach energy, band tailing parameter have been reported. The optical band gap decreases with increase in V2 O5 content due to increase in concentration of nonbridging oxygens. The change in both density and molar volume was discussed in terms of the structural modifications that take place in glass matrix upon replacing SrO (or B2 O3 ) by V2 O5 . The density of all the glasses decreases linearly with x and is in accordance with the network-modifying behavior of V2 O5 . Molar volume is found to increase with the addition of V2 O5 . From the theoretical fitting of the experimental absorption coefficients, for all the glass samples it is concluded that both indirect allowed and indirect forbidden transitions are involved. Hence the present glass system behaves as an indirect gap semiconductor. dc conductivity increases with the addition of V2 O5 due to the structural changes caused by V2 O5 . Activation energy shows the reverse trend. The change of conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one.

4.4. dc conductivity Acknowledgements The deviation from linearity in the log σ versus (1/T) curves (Fig. 7) indicates that activation energy is temperature dependent, which is a characteristic feature for the small

Authors are thankful to UGC, CSIR and DST (FIST Scheme), New Delhi for providing financial support.

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