Optical and thermal investigations on vanadyl doped zinc lithiumborate glasses

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ptical and thermal investigations on vanadyl doped zinc lithiumorate glasses

eema Dalala,c, S. Khasaa,∗, M.S. Dahiyaa, Arti Yadava, A. Agarwalb, S. Dahiyac

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

r t i c l e i n f o

rticle history:eceived 27 February 2015eceived in revised form 19 March 2015ccepted 31 March 2015vailable online xxx

eywords:ptical basicityolar refractionptical band gap

a b s t r a c t

Using standard melt-quench technique, transition metal oxide (2 mol% of V2O5) doped glasses havingcomposition xZnO·(30 − x)Li2O·70B2O3 (x = 0, 2, 5, 7 and 10) are prepared. The density (D) is measuredusing buoyancy and found to be lying between 2.21 and 2.45 g/cm3 with an increasing trend on substitut-ing ZnO contents in place of Li2O. The theoretical optical basicity (�th) is calculated and found to increasewith 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 structuralchanges. 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 inbase glass matrix. The decrease in Eopt is an evidence of enhancement in the number of non-bridging

ifferential thermal analysis oxygen atoms (NBOs) thereby increasing the four-coordinated boron atoms. The as-quenched samplesin bulk form are subjected to differential thermal analysis (DTA) to assess the glass transition tempera-ture (Tg), which is 476 ◦C for pure lithium borate glass. The variations suggest that the structure is beingmodified by the substitution of ZnO.

© 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting byElsevier B.V. All rights reserved.

. Introduction

Initially, various applications of glasses were within the domainf silicate glasses (a class of oxide glasses). But with the passagef time the other oxide glasses including phosphate, borate, bis-uthate, germanate, etc. were also studied with a great deal of

cademic, scientific, and technological interest. The non-silicatelasses proved their justifiable existence to nurture a large portionf scientific community. Out of such glasses, the borate glassesave the advantage of having highest glass forming ability amongll types of glasses and they do not reach crystalline state even athe lowest cooling rates [1]. B2O3 can be found in many commer-ially important glasses. Besides owing to the property of boronnomaly, it is mostly used as a dielectric material [2]. The higher

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∗ Corresponding author at: Department of Physics, DCR University of Science andechnology, Murthal 131039, Sonepat, Haryana, India.el.: +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

eramic Society.

ttp://dx.doi.org/10.1016/j.jascer.2015.03.004187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Produc

bond strength, lower cationic size and smaller heat of fusion alsomake B2O3 as a strong glass former [3]. The borate glasses are pre-sumed to be an arrangement of irregular network of BO3 triangleswith 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 statebatteries, fast ion conductors, etc. [5–9]. The addition of alkalimetals at low concentrations to pure borate glasses is presumedto convert the triangular borate units into the tetrahedral coordi-nated borate units without the creation of NBOs unlike their silicatecounterparts. But this is not the case for borate glasses containinghigh alkali concentrations where the trends in physical properties(such as Tg) just reverse, suggesting the creation of non-bridgingoxygen atoms (NBOs) which is termed as the “boron anomaly”[10]. The thermal stability of the oxide glasses is also enhancedby the addition of Li2O due to the increase in NBO bonding. Substi-tuting ZnO makes these glasses promising materials for photonicand optoelectronic applications [11]. Depending upon the nature of

l investigations on vanadyl doped zinc lithium borate glasses, J.04

bonding between metal and oxygen atom, ZnO can play the role ofa modifier or a glass former [12]. The study of adding ZnO to oxideglasses is also important due to their non-toxicity, non-hygroscopicnature, lower cost of production, higher polarizability and lower

tion and hosting by Elsevier B.V. All rights reserved.

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the structure modification in the vanadyl ion doped in the hostglass. An increase in �th leads to a decrease in oxygen covalencyresulting in an enhancement in sigma bonding which decreases thepositive charge on V4+. This leads to an increase in bond length of

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elting point [13–15]. Due to these aspects, the lithium boratelasses containing zinc ions have been studied for their structurend properties by many authors [16–20].

Incorporation of vanadium oxide doping assists to analyze dif-erent structural and physical properties of the base glass matrix.n recent past, vanadyl doped glasses have remained a subject ofnterest for many researchers [5–7,21]. Mixing vanadium ions inmall quantities in the glass matrices makes them suitable for usen memory and switching devices [6]. As V4+, vanadium is usu-lly coordinated to six ligands which forms an octahedral complex.ith oxygen as ligand, one V O bond of this complex becomes very

istinct and termed as vanadyl ion (VO2+) [22]. In a previous work,ne of the authors studied electron paramagnetic resonance (EPR)f the vanadyl ion in CoO·ZnO·B2O3 glasses [21] and an improve-ent in the octahedral symmetry of the V4+ site is observed. Gahlot

t al. [20] studied ZnO doped alkali bismuthate glasses and foundhat optical band gap and dc conductivity has a decreasing trend.fter considering above mentioned properties and literature sur-ey, authors have prepared a series of lithium borate glasses dopedith V2O5 containing different amounts of ZnO and studied thehysical, thermal and optical properties of glasses.

. Experimental

The starting materials used were analar grade chemicals Li2CO3,nO, H3BO3 and V2O5 obtained from Loba Chemie. The detailedreparation technique is available in literature [23]. The preparedamples in the compositional range of xZnO·(30 − x)Li2O·70B2O3x = 0, 2, 5, 7 and 10) containing 2 mol% of V2O5 were abbreviateds ZLBV1–5 respectively. The density (D) of the prepared samplesn the bulk form was measured using buoyancy with xylene asmmersion liquid. Data obtained for density were used to calcu-ate the molar volume (Vm) [24]. The theoretical optical basicity�th) was calculated by the method explained in literature [25]. Theamples obtained in the form of slices were polished to optical qual-ty for UV–vis spectroscopic measurements. The optical absorption

easurements were then carried out in the wavelength range of00–800 nm at an ambient temperature on a UV–vis spectropho-ometer (Shimadzu UV2401). The optical absorption data weresed to calculate optical band gap and band tailing [26]. The differ-ntial scanning calorimetric (DSC) studies of the samples in the bulkorm amounting to 30–40 mg were carried out on a simultaneoushermal analyzer (Perkin Elmer STA6000) [27,28].

. Results and discussion

.1. Density (D), molar volume (Vm) and theoretical opticalasicity (�th)

Density is an important intrinsic property for providing infor-ation on short range structure of oxide glasses. Although it is not

ossible to conclude exactly about the atomic arrangements fromxperimental data of densities, yet density remains to be a funda-ental test for a short range order model [29]. The measured values

f D and calculated values of Vm for all prepared compositions areeported in Table 1. It can be noticed from Table 1 that D is increas-ng with increasing content of ZnO which is an expected result ashe relative molecular weight of ZnO is higher than that of Li2O.btained values of density have same order as reported by El-Alailyt al. for lithium borate glasses (containing ∼10 mol% of Li2O) [30].he obtained values of Vm (as observed from Table 1) are in between

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6.57 and 27.36 cm3/mol which are in good agreement with thoseeported for lithium borate glasses [10]. The alkali/alkaline oxidesre known to be acting as glass modifiers in oxide glasses [31]. It isbserved from Table 1 that the variations in molar volume are not

PRESSic Societies xxx (2015) xxx–xxx

consistent. This may be because of two reasons: first due to the factthat Vm depends on both D and molecular weight and the secondbecause of the role of ZnO which is not clearly defined for oxideglasses (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 thePauling electro-negativity [34] and the values have been predictedfor the optical basicity of many oxide systems. Another way to cal-culate the theoretical optical basicity (�th) for multi-componentglass systems was proposed by Duffy and Ingram [35,36], in which�th is calculated by the algebraic sum of basicity values of eachcomponent in glass stoichiometry. However the values calculatedusing this method reflect only the trends in the optical basicityrather than true basicity as determined experimentally, reasonbeing why in some cases the theoretical and experimental valuesof the optical basicity deviates from each-other. So the conceptproposed by Duffy and Ingram [35,36] represents the bulk den-sity and does not assist in obtaining information about the cation(B3+ here) coordination number. To understand the role of basicityin obtaining information about the changes governed in the glassmatrix by the addition of ZnO, �th was calculated whose valuesare reported in Table 1. These values are in good agreement withthe values of �th obtained for alkali borate glasses [37,38] and theincreasing trends in optical basicity suggest an enhancement in theionic character of prepared glasses on substituting ZnO in place ofLi2O. To understand the role of trends in basicity it is importantto relate it with the oxide ion polarizability (˛2−

o , also termed asoxide ion activity [39]) through an intrinsic relationship proposedby Duffy [40] as: �th = 1.67(1 − (1/˛2−

o )). This relationship sug-gests that the trends in basicity are similar to polarizability. Thevalues of polarizability calculated by this relation are also reportedin Table 1. Extensive studies have been done by researchers [41–43]to study the variation in polarizability with substance in crystallineand amorphous materials and it was concluded that the polarizingpower of cation increases with increase in its positive charge. Hencethe trends in optical basicity in present study are in accordancewith the expectations. Moreover, �th assists us in understanding

l investigations on vanadyl doped zinc lithium borate glasses, J.04

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

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Table 1Mole 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 molarpolarizability (˛m) for glasses with composition xZnO·(30 − x)Li2O·70B2O3.

Sample code x D (g/cm3) Vm (cm3/mol) �th ˛2−o nr Rm (cm3/mol) ˛m (×10−24 cm3)

ZLBV1 0 2.21 27.26 0.503 1.431 1.466 7.55 2.99ZLBV2 2 2.29 26.69 0.505 1.433 1.476 7.52 2.98ZLBV3 5 2.33 26.94 0.507 1.436 1.480 7.65 3.03ZLBV4 7 2.34 27.18 0.509 1.438 1.482 7.75 3.07ZLBV5 10 2.45 26.57 0.511 1.441 1.495 7.74 3.07

tt tran

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t

Fig. 2. Tauc plots corresponding to indirect allowed Mo

anadyl oxygen and improve the octahedral nature of the V4+O6omplex [38].

For glasses having density between 1 and 3 g/cm3, Told [44]stablished a relation to calculate the refractive index (nr) as:

r = D + 10.48.6

(1)

he molar refraction (Rm) is exclusively related to nr for isotropicubstances as [45]:

m = Vm(n2r − 1)

n2r + 2

(2)

he molar polarizability (˛m) was also related with Rm byorentz–Lorenz [46–48] equation as:

= 4�NA˛m (3)

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m 3

ith NA as the Avogadro’s number.The values of nr, Rm, and ˛m calculated using the above men-

ioned theoretical approaches are reported in Table 1. These values

sitions (solid lines are x-intercepts yielding band-gap).

are in agreement with those reported for borate glasses [26]. It canbe observed that the trends in ˛m are somewhat similar to thatof ˛2−

o which indicates that the ionic character of lithium borateglasses is enhanced by the addition of ZnO in place of Li2O.

3.2. Optical absorption measurements

Fig. 1 describes the optical absorption spectra of samples ZLBV1to ZLBV5 recorded at ambient temperature. It can be observed fromFig. 1 that the absorption edges are non-sharp which depicts thatthe prepared compositions are glassy in nature. The absorptionedge is signified by the cut-off wavelength (�cut-off). The values of�cut-off are determined from the optical absorption data which arereported in Table 2. It can be seen that �cut-off shows a red-shift(towards the higher wavelength) with increasing concentration of

l investigations on vanadyl doped zinc lithium borate glasses, J.04

ZnO. The region near the absorption edge can be used to calcu-late the optical absorption co-efficient (˛(�)) by using the value ofsample thickness (t) and the absorbance (A) [49] as: ˛(�) = A/t. Theabsorbance 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 2Cut-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) Eopt (eV) B ((cm eV)−1/r) �E (eV) Tg (◦C)

r = 2 r = 3 r = 2 r = 3

ZLBV1 414 2.82 2.66 24.74 8.06 0.243 476ZLBV2 413 2.80 2.63 22.56 7.49 0.235 480ZLBV3 418 2.79 2.62 19.45 7.53 0.272 487

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ZLBV4 419 2.74 2.55

ZLBV5 432 2.76 2.58

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

(�) = B(h� − Eopt)r

h�(4)

he above equation holds good for all direct and indirect transitionsnd r is the index, value of which signifies the type of transitionnvolved in the matter. r can have values 2, 3, 1/2 and 1/3 cor-esponding to indirect allowed, indirect forbidden, direct allowednd direct forbidden transitions respectively. It was suggested byauc [51] that only indirect transitions are valid for amorphousaterials. Eq. (4) can be used to generate Tauc plots (h� vs. (˛h�)r)

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orresponding to r = 2 and 3. These curves are represented byigs. 2 and 3 and are similar to those obtained for lithium borate andinc borate glasses [52,53]. From these plots ‘Eopt’ is calculated byhe intercept of the plot on x-axis where (˛h�)r = 0. The band tailing

21.45 7.02 0.249 49421.83 7.06 0.262 480

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 for-bidden transitions are reported in Table 2. The variations in Eopt canbe attributed to the structural changes governed in the glass matrixthrough the substitution of ZnO in place of Li2O. These variationssynchronize with the variations in Eg as observed from Table 1.The NBOs that are formed by breaking of metal-oxygen bond arepresumed to contribute in the valence band maximum (VBM) andshifts it a bit upper as the non-bridging orbitals are more energeticthan bonding orbitals [54]. This results in the reduction of the bandgap. These variations are also supported by the corresponding redshift in the absorption edge. Similar trends in optical propertieswere also observed by Sanghi et al. while studying borate glasses[26]. One more point to notice is that for sample ZLBV5 the value

l investigations on vanadyl doped zinc lithium borate glasses, J.04

of Eopt is more as compared to samples having lower ZnO concen-tration. This may be due to the participation of ZnO in the networkformation alongside network modification as ZnO can act as bothnetwork former and modifier [14].

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ig. 4. Urbach plots for samples ZLBV1–5 (inset: tangent to calculate Urbach’snergy for sample ZLBV1).

The band structure and energy band gap in crystalline and amor-hous materials can also be estimated by the study of opticalbsorption data. In optical absorption, the photon having energyreater than the band gap is absorbed and this leaves behind anbsorption edge with an exponential increase in absorption co-fficient, ˛(�). The increase in ˛(�) is related to the energy h�hrough the relation [55]:

(�) = ˛0eh�/�E (5)

here ˛0 is a constant and �E is the Urbach energy. �E has beenalculated from the reciprocal of the slope of the Urbach’s plot in theegion of lower photon energy. The Urbach’s plots (i.e. h� vs. ln(˛))re shown in Fig. 4. The Urbach energy can yield the informationbout the disorder effects in amorphous or crystalline systems andhe lack of long-range order in glassy/amorphous materials is asso-iated with the tailing of density of states [56,57]. The materialsaving larger values of �E are believed to have greater tendencyo convert weak bond into defects. The obtained values of �E forhe present glass system lie between 0.235 and 0.272 eV and areeported in Table 2. These values have same order as reported forxide glasses in literature [58,59].

.3. Differential thermal analysis (DTA)

The typical DTA thermographs of all prepared compositions inulk form recorded at a constant heating rate of 20 ◦C/min arehown in Fig. 5. The reason for recording all thermographs atame heating rate is the heating rate dependence of characteris-ic temperatures [60,28]. An endothermic shift (or upward shift)round 480 ◦C is evidence from all the DTA thermographs. Thishift represents the glass transition temperature (Tg) or the soft-ning temperature [61] as the change in viscosity is maximum athis temperature. To find Tg, a point is taken at exactly half of theaseline shift and the corresponding value of temperature from x-xis is taken as Tg (the uncertainty in Tg is ±2 ◦C). Obtained valuesf Tg are reported in Table 2. Availability of single peak due to Tg forll compositions depicts that prepared samples are highly homo-eneous in nature [62]. It can be seen from Table 2 that Tg increases

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p to 7 mol% doping on the ZnO in place of Li2O in base glass andecreases thereafter. Studies reveal that Tg depends on oxygen den-ity of the network [63]. The increase in Tg is also characterized byhe ionic strength of the constituents [64]. As observed from the

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

variation of optical basicity and polarizability the ionic character ofthe glass is enhanced with increasing content of ZnO which may bea possible reason for increase in the value of Tg. For sample ZLBV5a decrease in glass transition temperature may be due to the par-ticipation of ZnO in network formation which might be convertingthe coordination number of boron from four to three and therebydecreasing the connectivity and hence the network strength of theglass matrix.

4. Conclusions

Vanadyl doped ZnO·Li2O·B2O3 glasses were prepared success-fully by melt-quench technique. The density was increasing withincrease in ZnO concentration. Theoretical optical basicity, oxideion polarizability, refractive index, mole fraction and molar polariz-ability were increasing with increasing inclusion of ZnO indicatingan increase in the ionic character of prepared glass compositions. Anon-sharp absorption edge signifying glassy nature was observedin optical absorption spectra. A red shift in cut-off wavelengthindicating increase in NBOs was observed from optical absorptionanalysis. Optical band gap was decreasing up to 7 mol% of ZnO andincrease beyond 7 mol% and similar kind of behaviour was observedfor glass transition temperature i.e. Tg. The variations in opticalband gap and glass transition temperature suggest that ZnO is act-ing as glass modifier up to 7 mol% and network former thereafter.

Acknowledgements

The authors would like to acknowledge University Grant Com-mission (UGC), New Delhi for providing financial and experimentalsupport under Major Research Project (File No. 40-461/2011 (SR)).Coordinator Central Instrumentation Laboratory (CIL) DCRUSTMurthal is also acknowledged for providing thermal and opticalabsorption measurement facilities. Author M.S. Dahiya is thank-ful to Department of Science and Technology (DST), New Delhi forproviding financial support under INSPIRE Fellowship. Author ArtiYadav is thankful to CSIR, New Delhi for providing financial supportunder Junior Research Fellowship.

l investigations on vanadyl doped zinc lithium borate glasses, J.04

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