Optical and thermal investigations on vanadyl doped zinc lithium borate glasses

Journal of Asian Ceramic Societies 3 (2015) 234–239

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Optical and thermal investigations on vanadyl doped zinc lithium borate glasses Seema Dalal a,c , S. Khasa a,∗ , M.S. Dahiya a , Arti Yadav a , A. Agarwal b , S. Dahiya c a b c

Department of Physics, Deenbandhu Chhotu Ram University of Science and Technology, Murthal 131039, India Department of Applied Physics, Guru Jambheshwara University of Science and Technology, Hisar 125001, India Department of Physics, Baba Mast Nath University, Asthal Bohr, Rohtak 124001, India

a r t i c l e

i n f o

Article history: Received 27 February 2015 Received in revised form 19 March 2015 Accepted 31 March 2015 Available online 21 April 2015 Keywords: Optical basicity Molar refraction Optical band gap Differential thermal analysis

a b s t r a c t Using standard melt-quench technique, transition metal oxide (2 mol% of V2 O5 ) doped glasses having composition xZnO·(30 − x)Li2 O·70B2 O3 (x = 0, 2, 5, 7 and 10) are prepared. The density (D) is measured using buoyancy and found to be lying between 2.21 and 2.45 g/cm3 with an increasing trend on substituting ZnO contents in place of Li2 O. The theoretical optical basicity (th ) is calculated and found to increase with increasing inclusion of ZnO indicating an increase in the ionic character. The molar refraction (Rm ), refractive index (nr ) and molar polarizability (˛m ) are calculated and explained on the basis of structural changes. The optical absorption spectra have been used to evaluate the values of optical band gap (Eopt ) and band tailing parameter (B). It is observed that Eopt decreases with the increasing contents of ZnO in base glass matrix. The decrease in Eopt is an evidence of enhancement in the number of non-bridging oxygen atoms (NBOs) thereby increasing the four-coordinated boron atoms. The as-quenched samples in bulk form are subjected to differential thermal analysis (DTA) to assess the glass transition temperature (Tg ), which is 476 ◦ C for pure lithium borate glass. The variations suggest that the structure is being modified by the substitution of ZnO. © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction Initially, various applications of glasses were within the domain of silicate glasses (a class of oxide glasses). But with the passage of time the other oxide glasses including phosphate, borate, bismuthate, germanate, etc. were also studied with a great deal of academic, scientific, and technological interest. The non-silicate glasses proved their justifiable existence to nurture a large portion of scientific community. Out of such glasses, the borate glasses have the advantage of having highest glass forming ability among all types of glasses and they do not reach crystalline state even at the lowest cooling rates [1]. B2 O3 can be found in many commercially important glasses. Besides owing to the property of boron anomaly, it is mostly used as a dielectric material [2]. The higher

∗ Corresponding author at: Department of Physics, DCR University of Science and Technology, Murthal 131039, Sonepat, Haryana, India. Tel.: +91 98128 18900/130 2484114; fax: +91 130 2484003. E-mail addresses: [email protected], [email protected] (S. Khasa). Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

bond strength, lower cationic size and smaller heat of fusion also make B2 O3 as a strong glass former [3]. The borate glasses are presumed to be an arrangement of irregular network of BO3 triangles with each oxygen atom being shared by two boron atoms [4]. The borate glasses containing alkali metal ions (particularly Li+ ) have been studied with great interest for applications in solid state batteries, fast ion conductors, etc. [5–9]. The addition of alkali metals at low concentrations to pure borate glasses is presumed to convert the triangular borate units into the tetrahedral coordinated borate units without the creation of NBOs unlike their silicate counterparts. But this is not the case for borate glasses containing high alkali concentrations where the trends in physical properties (such as Tg ) just reverse, suggesting the creation of non-bridging oxygen atoms (NBOs) which is termed as the “boron anomaly” [10]. The thermal stability of the oxide glasses is also enhanced by the addition of Li2 O due to the increase in NBO bonding. Substituting ZnO makes these glasses promising materials for photonic and optoelectronic applications [11]. Depending upon the nature of bonding between metal and oxygen atom, ZnO can play the role of a modifier or a glass former [12]. The study of adding ZnO to oxide glasses is also important due to their non-toxicity, non-hygroscopic nature, lower cost of production, higher polarizability and lower

http://dx.doi.org/10.1016/j.jascer.2015.03.004 2187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

S. Dalal et al. / Journal of Asian Ceramic Societies 3 (2015) 234–239

melting point [13–15]. Due to these aspects, the lithium borate glasses containing zinc ions have been studied for their structure and properties by many authors [16–20]. Incorporation of vanadium oxide doping assists to analyze different structural and physical properties of the base glass matrix. In recent past, vanadyl doped glasses have remained a subject of interest for many researchers [5–7,21]. Mixing vanadium ions in small quantities in the glass matrices makes them suitable for use in memory and switching devices [6]. As V4+ , vanadium is usually coordinated to six ligands which forms an octahedral complex. With oxygen as ligand, one V O bond of this complex becomes very distinct and termed as vanadyl ion (VO2+ ) [22]. In a previous work, one of the authors studied electron paramagnetic resonance (EPR) of the vanadyl ion in CoO·ZnO·B2 O3 glasses [21] and an improvement in the octahedral symmetry of the V4+ site is observed. Gahlot et al. [20] studied ZnO doped alkali bismuthate glasses and found that optical band gap and dc conductivity has a decreasing trend. After considering above mentioned properties and literature survey, authors have prepared a series of lithium borate glasses doped with V2 O5 containing different amounts of ZnO and studied the physical, thermal and optical properties of glasses. 2. Experimental The starting materials used were analar grade chemicals Li2 CO3 , ZnO, H3 BO3 and V2 O5 obtained from Loba Chemie. The detailed preparation technique is available in literature [23]. The prepared samples in the compositional range of xZnO·(30 − x)Li2 O·70B2 O3 (x = 0, 2, 5, 7 and 10) containing 2 mol% of V2 O5 were abbreviated as ZLBV1–5 respectively. The density (D) of the prepared samples in the bulk form was measured using buoyancy with xylene as immersion liquid. Data obtained for density were used to calculate the molar volume (Vm ) [24]. The theoretical optical basicity (th ) was calculated by the method explained in literature [25]. The samples obtained in the form of slices were polished to optical quality for UV–vis spectroscopic measurements. The optical absorption measurements were then carried out in the wavelength range of 200–800 nm at an ambient temperature on a UV–vis spectrophotometer (Shimadzu UV2401). The optical absorption data were used to calculate optical band gap and band tailing [26]. The differential scanning calorimetric (DSC) studies of the samples in the bulk form amounting to 30–40 mg were carried out on a simultaneous thermal analyzer (Perkin Elmer STA6000) [27,28].

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consistent. This may be because of two reasons: first due to the fact that Vm depends on both D and molecular weight and the second because of the role of ZnO which is not clearly defined for oxide glasses (as it can act as both glass former and modifier) [14]. The optical basicity has been studied by Duffy and Ingram [32,33] by relating it with the spectroscopic probe ion data or the Pauling electro-negativity [34] and the values have been predicted for the optical basicity of many oxide systems. Another way to calculate the theoretical optical basicity (th ) for multi-component glass systems was proposed by Duffy and Ingram [35,36], in which th is calculated by the algebraic sum of basicity values of each component in glass stoichiometry. However the values calculated using this method reflect only the trends in the optical basicity rather than true basicity as determined experimentally, reason being why in some cases the theoretical and experimental values of the optical basicity deviates from each-other. So the concept proposed by Duffy and Ingram [35,36] represents the bulk density and does not assist in obtaining information about the cation (B3+ here) coordination number. To understand the role of basicity in obtaining information about the changes governed in the glass matrix by the addition of ZnO, th was calculated whose values are reported in Table 1. These values are in good agreement with the values of th obtained for alkali borate glasses [37,38] and the increasing trends in optical basicity suggest an enhancement in the ionic character of prepared glasses on substituting ZnO in place of Li2 O. To understand the role of trends in basicity it is important to relate it with the oxide ion polarizability (˛2− o , also termed as oxide ion activity [39]) through an intrinsic relationship proposed by Duffy [40] as: th = 1.67(1 − (1/˛2− o )). This relationship suggests that the trends in basicity are similar to polarizability. The values of polarizability calculated by this relation are also reported in Table 1. Extensive studies have been done by researchers [41–43] to study the variation in polarizability with substance in crystalline and amorphous materials and it was concluded that the polarizing power of cation increases with increase in its positive charge. Hence the trends in optical basicity in present study are in accordance with the expectations. Moreover, th assists us in understanding the structure modification in the vanadyl ion doped in the host glass. An increase in th leads to a decrease in oxygen covalency resulting in an enhancement in sigma bonding which decreases the positive charge on V4+ . This leads to an increase in bond length of

3. Results and discussion 3.1. Density (D), molar volume (Vm ) and theoretical optical basicity (th ) Density is an important intrinsic property for providing information on short range structure of oxide glasses. Although it is not possible to conclude exactly about the atomic arrangements from experimental data of densities, yet density remains to be a fundamental test for a short range order model [29]. The measured values of D and calculated values of Vm for all prepared compositions are reported in Table 1. It can be noticed from Table 1 that D is increasing with increasing content of ZnO which is an expected result as the relative molecular weight of ZnO is higher than that of Li2 O. Obtained values of density have same order as reported by El-Alaily et al. for lithium borate glasses (containing ∼10 mol% of Li2 O) [30]. The obtained values of Vm (as observed from Table 1) are in between 26.57 and 27.36 cm3 /mol which are in good agreement with those reported for lithium borate glasses [10]. The alkali/alkaline oxides are known to be acting as glass modifiers in oxide glasses [31]. It is observed from Table 1 that the variations in molar volume are not

Fig. 1. Optical absorption curves for samples ZLBV1–5 (inset: cut-off intercept for sample ZLBV1).

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Table 1 Mole fraction of ZnO (x), density (D), molar volume (Vm ), theoretical optical basicity (th ), oxide ion polarizability (˛2− o ), refractive index (nr ), mole fraction (Rm ), and molar polarizability (˛m ) for glasses with composition xZnO·(30 − x)Li2 O·70B2 O3 . Sample code

x

D (g/cm3 )

Vm (cm3 /mol)

th

˛2− o

nr

Rm (cm3 /mol)

˛m (×10−24 cm3 )

ZLBV1 ZLBV2 ZLBV3 ZLBV4 ZLBV5

0 2 5 7 10

2.21 2.29 2.33 2.34 2.45

27.26 26.69 26.94 27.18 26.57

0.503 0.505 0.507 0.509 0.511

1.431 1.433 1.436 1.438 1.441

1.466 1.476 1.480 1.482 1.495

7.55 7.52 7.65 7.75 7.74

2.99 2.98 3.03 3.07 3.07

Fig. 2. Tauc plots corresponding to indirect allowed Mott transitions (solid lines are x-intercepts yielding band-gap).

vanadyl oxygen and improve the octahedral nature of the V4+ O6 complex [38]. For glasses having density between 1 and 3 g/cm3 , Told [44] established a relation to calculate the refractive index (nr ) as: nr =

D + 10.4 8.6

(1)

The molar refraction (Rm ) is exclusively related to nr for isotropic substances as [45]: Rm =

Vm (n2r − 1) n2r + 2

(2)

The molar polarizability (˛m ) was also related with Rm by Lorentz–Lorenz [46–48] equation as: Rm =

4NA ˛m 3

(3)

with NA as the Avogadro’s number. The values of nr , Rm , and ˛m calculated using the above mentioned theoretical approaches are reported in Table 1. These values

are in agreement with those reported for borate glasses [26]. It can be observed that the trends in ˛m are somewhat similar to that of ˛2− o which indicates that the ionic character of lithium borate glasses is enhanced by the addition of ZnO in place of Li2 O. 3.2. Optical absorption measurements Fig. 1 describes the optical absorption spectra of samples ZLBV1 to ZLBV5 recorded at ambient temperature. It can be observed from Fig. 1 that the absorption edges are non-sharp which depicts that the prepared compositions are glassy in nature. The absorption edge is signified by the cut-off wavelength (cut-off ). The values of cut-off are determined from the optical absorption data which are reported in Table 2. It can be seen that cut-off shows a red-shift (towards the higher wavelength) with increasing concentration of ZnO. The region near the absorption edge can be used to calculate the optical absorption co-efficient (˛()) by using the value of sample thickness (t) and the absorbance (A) [49] as: ˛() = A/t. The absorbance A in this relation depends upon intensity of incident

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Fig. 3. Tauc plots corresponding to indirect allowed Mott transitions (solid lines are x-intercepts yielding band-gap).

Table 2 Cut-off wavelength (cut-off ), optical band gap (Eopt ), band tailing (B), Urbach energy (E) and glass transition temperature (Tg ) for samples ZLBV1–5. Sample code

cut-off (nm)

ZLBV1 ZLBV2 ZLBV3 ZLBV4 ZLBV5

414 413 418 419 432

B ((cm eV)−1/r )

Eopt (eV) r=2

r=3

r=2

r=3

2.82 2.80 2.79 2.74 2.76

2.66 2.63 2.62 2.55 2.58

24.74 22.56 19.45 21.45 21.83

8.06 7.49 7.53 7.02 7.06

(I0 ) and transmitted (It ) radiations and is given as the natural logarithm of the ratio I0 /It . Another form of absorption co-efficient as a function of photon energy (h), relating ˛() with the optical band gap (Eopt ) and band tailing (B) was given by Davis and Mott [50] as: ˛() =

B(h − Eopt ) h

r

(4)

The above equation holds good for all direct and indirect transitions and r is the index, value of which signifies the type of transition involved in the matter. r can have values 2, 3, 1/2 and 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions respectively. It was suggested by Tauc [51] that only indirect transitions are valid for amorphous materials. Eq. (4) can be used to generate Tauc plots (h vs. (˛h)r ) corresponding to r = 2 and 3. These curves are represented by Figs. 2 and 3 and are similar to those obtained for lithium borate and zinc borate glasses [52,53]. From these plots ‘Eopt ’ is calculated by the intercept of the plot on x-axis where (˛h)r = 0. The band tailing

E (eV)

Tg (◦ C)

0.243 0.235 0.272 0.249 0.262

476 480 487 494 480

parameter (B) is calculated by the slope of linear portion of the plots. The values of Eopt and B calculated for both indirect allowed and forbidden transitions are reported in Table 2. The variations in Eopt can be attributed to the structural changes governed in the glass matrix through the substitution of ZnO in place of Li2 O. These variations synchronize with the variations in Eg as observed from Table 1. The NBOs that are formed by breaking of metal-oxygen bond are presumed to contribute in the valence band maximum (VBM) and shifts it a bit upper as the non-bridging orbitals are more energetic than bonding orbitals [54]. This results in the reduction of the band gap. These variations are also supported by the corresponding red shift in the absorption edge. Similar trends in optical properties were also observed by Sanghi et al. while studying borate glasses [26]. One more point to notice is that for sample ZLBV5 the value of Eopt is more as compared to samples having lower ZnO concentration. This may be due to the participation of ZnO in the network formation alongside network modification as ZnO can act as both network former and modifier [14].

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Fig. 4. Urbach plots for samples ZLBV1–5 (inset: tangent to calculate Urbach’s energy for sample ZLBV1).

Fig. 5. DTA thermographs for samples ZLBV1–5 (arrows mark the glass transition temperature).

The band structure and energy band gap in crystalline and amorphous materials can also be estimated by the study of optical absorption data. In optical absorption, the photon having energy greater than the band gap is absorbed and this leaves behind an absorption edge with an exponential increase in absorption coefficient, ˛(). The increase in ˛() is related to the energy h through the relation [55]:

variation of optical basicity and polarizability the ionic character of the glass is enhanced with increasing content of ZnO which may be a possible reason for increase in the value of Tg . For sample ZLBV5 a decrease in glass transition temperature may be due to the participation of ZnO in network formation which might be converting the coordination number of boron from four to three and thereby decreasing the connectivity and hence the network strength of the glass matrix.

˛() = ˛0 eh/E

(5)

where ˛0 is a constant and E is the Urbach energy. E has been calculated from the reciprocal of the slope of the Urbach’s plot in the region of lower photon energy. The Urbach’s plots (i.e. h vs. ln(˛)) are shown in Fig. 4. The Urbach energy can yield the information about the disorder effects in amorphous or crystalline systems and the lack of long-range order in glassy/amorphous materials is associated with the tailing of density of states [56,57]. The materials having larger values of E are believed to have greater tendency to convert weak bond into defects. The obtained values of E for the present glass system lie between 0.235 and 0.272 eV and are reported in Table 2. These values have same order as reported for oxide glasses in literature [58,59]. 3.3. Differential thermal analysis (DTA) The typical DTA thermographs of all prepared compositions in bulk form recorded at a constant heating rate of 20 ◦ C/min are shown in Fig. 5. The reason for recording all thermographs at same heating rate is the heating rate dependence of characteristic temperatures [60,28]. An endothermic shift (or upward shift) around 480 ◦ C is evidence from all the DTA thermographs. This shift represents the glass transition temperature (Tg ) or the softening temperature [61] as the change in viscosity is maximum at this temperature. To find Tg , a point is taken at exactly half of the baseline shift and the corresponding value of temperature from xaxis is taken as Tg (the uncertainty in Tg is ±2 ◦ C). Obtained values of Tg are reported in Table 2. Availability of single peak due to Tg for all compositions depicts that prepared samples are highly homogeneous in nature [62]. It can be seen from Table 2 that Tg increases up to 7 mol% doping on the ZnO in place of Li2 O in base glass and decreases thereafter. Studies reveal that Tg depends on oxygen density of the network [63]. The increase in Tg is also characterized by the ionic strength of the constituents [64]. As observed from the

4. Conclusions Vanadyl doped ZnO·Li2 O·B2 O3 glasses were prepared successfully by melt-quench technique. The density was increasing with increase in ZnO concentration. Theoretical optical basicity, oxide ion polarizability, refractive index, mole fraction and molar polarizability were increasing with increasing inclusion of ZnO indicating an increase in the ionic character of prepared glass compositions. A non-sharp absorption edge signifying glassy nature was observed in optical absorption spectra. A red shift in cut-off wavelength indicating increase in NBOs was observed from optical absorption analysis. Optical band gap was decreasing up to 7 mol% of ZnO and increase beyond 7 mol% and similar kind of behaviour was observed for glass transition temperature i.e. Tg . The variations in optical band gap and glass transition temperature suggest that ZnO is acting as glass modifier up to 7 mol% and network former thereafter. Acknowledgements The authors would like to acknowledge University Grant Commission (UGC), New Delhi for providing financial and experimental support under Major Research Project (File No. 40-461/2011 (SR)). Coordinator Central Instrumentation Laboratory (CIL) DCRUST Murthal is also acknowledged for providing thermal and optical absorption measurement facilities. Author M.S. Dahiya is thankful to Department of Science and Technology (DST), New Delhi for providing financial support under INSPIRE Fellowship. Author Arti Yadav is thankful to CSIR, New Delhi for providing financial support under Junior Research Fellowship. References [1] K.J. Rao, Structural Chemistry of Glasses, Elsevier Publishing (2002), pp. 478.

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