Thermal and FTIR Properties of Semiconducting SnO 2-PbO-V 2 O 5 Glass System

Melt quenched “SnO2(50−x)PbO:50V2O5” glass system containing x = 5, 10, 15 in molar ratio has been investigated. Effects of heating rate, glass transition, crystallization, melting temperature and infrared spectra of SnO2 substituted PbO-V2O5 glass system are reported. XRD results show that perfect vitrification has been achieved for all the glass samples after annealing at 150 ̊C. DSC results have indicated that eutectic composition of the lead metavanadate has been maintained for all the glass systems up to 15 mole% of substitution. IR spectra for a SnO2 substitution of 5 mole% V=O stretching frequency occur at 966 cm−1 without appearance of any additional peak. But for 10 mole% and 15 mole% SnO2 substituted samples, additional peaks appear at 1023 and 1005 cm−1 indicating the effect of SnO2 in the vanadate crystalline matrix such that there is an elongation of V=O bond. Since the crystalline matrix is affected, we can expect similar effect in the glass matrix also.


Introduction
Transport properties of semiconducting glasses are very interesting and provide useful information on the conduction mechanisms.Modern glasses are of crucial importance for electronics and have been widely used in industry, space explorations, computer memory units, etc. IR spectroscopy is a particularly suitable method for the structural studies of vanadate system in glass and crystalline forms because of the characteristic vanadate group's vibrational frequencies which can be easily identified.Structural models for PbO-V 2 O 5 glass systems have been discussed on the basis of IR spectroscopy [1] by comparing with those of known crystal structures.From IR and NMR spectroscopy, Hayakwa et al. [2] have proposed the existence of VO 4 tetrahedral and VO 5 trigonal bipyramids.The local structure around Pb in lead vanadate glasses mainly consists of PbO 3 trigonal and PbO 4 square pyramids.This study does not contradict the results obtained in earlier IR studies [3] that 50PbO:50V 2 O 5 glass system contains affected VO 5 groups but gives additional information regarding the local environment of vanadium as well as lead ions.The lead ions are located between the vanadate chains and layers.These ions affect the isolated V=O bonds by elongating them, thus reducing their frequency.In the present studies IR spectra have been recorded in order to verify if the substituent metal oxide is replacing PbO in the glass network of the equimolar lead vanadate i.e., 50PbO:50V 2 O 5 glass system.

Experimental
The glasses were obtained by melting a chemically pure PbO, V 2 O 5 and SnO 2 in amounts varying from 5 to 15 mole% PbO and SnO 2 in glazed silica crucibles in the 800˚C -1200˚C temperature range.Vitrification was achieved by rapid cooling of the melt using a roller technique.The glasses were subjected to crystallization at 380˚C.The prepared samples were grounded into fine powder for X-ray, DSC and FT-IR studies.X-ray studies were carried out on a PAN Alytic X'Pert-PRO diffractrometer using CuK α radiation at 1.5418Å and diffractrometer settings in the 2θ range from 10˚C -70˚C by changing the 2θ with a step size of 0.020.Differential Scanning calorimetry investigation of glass specimens was performed using DuPont, USA make model 2000 DSC instrument.DSC scans were conducted using 5 -10 mg ground as-cast glass specimens which heats up at the rate of 10˚C/min between 0˚C and 600˚C in a platinum crucible and alumina powder was used as the reference material.The density of the glass samples was determined by the Archimedes principle, using toluene as immersion liquid.In the present studies IR spectra were recorded for both the vitreous and non-vitreous samples in order to study the effect of a different metal oxide substitution in the place of PbO in the lead metavanadate glass systems.

Results and Discussion
The X-ray diffractograms annealed at 150˚C containing (x = 5, 10, 15 mole%) of SnO 2 showed no trace of crystallinity and are shown in Figure 1.
DSC investigations were conducted on the SnO 2 -PbO-V 2 O 5 glasses.The DSC patterns for these glass systems shown in Figure 2 are slightly different when compared to the un-substituted system [4].Values of glass transition temperature T g , crystallization temperature T c , melting temperature T m , glass forming tendency K g and densities of the xSnO 2 (50−x)PbO:50V 2 O 5 are given in Table 1.Glass Composition (mole%) There is a slight change in T g along with an increase in the number of crystallization peaks, the T g values decrease with increasing SnO 2 contents, these results suggest that SnO 2 acts as a network modifier where as PbO acts as a network former [5].Crystallization temperature T c is the maximum temperature of the exotherm and onset of crystallization temperature, T x is the beginning of the first exothermic reaction where the crystallization starts.As seen in Figure 2 up to x = 15 mole% there is only one endothermic peak corresponding to melting point.This indicates that the substituted samples behave like the eutectic composition upto x = 15 mole%.In order to understand the devitrification tendency and thermal stability of the glass samples, glass forming tendency values K g are calculated using the below equation.
Lower K g suggests higher tendency of crystallization and lower thermal stability.K g represents the temperature interval during nucleation [6].These values indicate that the 5SnO 2 45PbO:50V 2 O 5 glass samples have lowest thermal stability among four compositions, with a K g value of 0.099.The densities seem to be increasing with an increase in SnO 2 substitution.The IR spectra for the glass as well as crystalline forms are for xSnO 2 (50−x)PbO:50V 2 O 5 glass systems given in Figure 3.The observed IR band positions are given in Table 2 and Table 3.
A consequence of disorder in the amorphous or glass systems when compared to crystalline forms is the   breakdown of the wave vector selection (k-selection) rules, which allows electromagnetic radiation to couple with vibrations other than k = 0.As a result, unlike the crystalline case in which narrow well defined lines are observed, broad and diffuse bands representing a continuum of IR absorption result.Even though bands due to individual, localized structural units are observable, the identification of IR spectra of glasses alone is rather difficult unless crystalline spectra are also present.Hence the present discussion mainly corresponds to the IR bands observed in the devitrified samples.SnO 2 substituted samples for x = 10 and 15 mole% , there is an indication of additional bands at 1023 cm −1 beside 966 cm −1 and 1005 cm −1 beside 962 cm −1 the appearance of additional bands can be understood as the splitting of the original V=O stretching band into affected and unaffected components.The affected components arises due to the effect of dopant i.e.SnO 2 in the present case on V=O bond the glass matrix.In this case from the IR spectral evidence we can summarize the following.As SnO 2 is substituted for PbO, SnO 2 is replacing PbO in the glass network in such a way that it affects V=O bond frequency, as evident by the shift in V=O band (at 1023 cm −1 ) whose intensity increases with the increase of SnO 2 concentration.The affected V=O band (at 966 cm −1 ) indicates the presence of meta vanadate phase containing PbO in the usual evidence we can summarize the following.As SnO 2 is substituted for PbO, SnO 2 is replacing PbO in the glass network in such a way that it affects V=O bond frequency, as evident by the shift in V=O band (at 1023 cm −1 ) whose intensity increases with the increase of SnO 2 concentration.The affected V=O band (at 966 cm −1 ) indicates the presence of metavanadate phase containing PbO in the usual glass network.The occurrence of the band at 1005 cm −1 is due to the indirect effect of SnO 2 on the V=O band in the new stable phase that is being formed.In x(TiO 2 ):(100−x)(V 2 O 5 ) glass containing x = 20 mole%, Dimitriev et al. [3] reported that the isolated V=O band is unaffected at 1020 cm −1 .It was observed by Dimitrov et al. [1] that in the IR spectra of xPbO:(1−x)V 2 O 5 glass systems as x is varied from 0 to 75 mole% , there appears a new band in the range 970 -955 cm −1 beside 1020 cm −1 band.It has been suggested that [3] Pb 2+ ions occupy a position between the V-O-V layers.They exercise a direct influence of the isolated V=O bonds of VO 5 groups according to the scheme Pb 2+ …O=V 5+ .This causes an elongation of the affected V=O bond and a decrease in the vibrational frequency to 970 -950 cm −1 .There may be unaffected V=O bonds whose vibrational frequency is still around 1020 cm −1 .With the increase of PbO, their number decreases and for eutectic composition 50PbO:50V 2 O 5 (i.e.PbV 2 O 6 metavanadate) only one type of VO 5 polyhedron results as is evidenced by the presence of a single high frequency band in the region 955 -970 cm −1 [7].This band is observed at 966 cm −1 in 5SnO 2 :45PbO:50V 2 O 5 system.As SnO 2 concentration is increased to 10 mol% and 15 mole%, the appearance of 1023 cm −1 band indicates that SnO 2 is replacing PbO in such a way that the V=O bond in the glass network of the eutectic composition is elongated [8].

Conclusion
Perfect vitrification has been achieved for all the glass samples as can be seen from their X-ray diffractograms of the prepared samples after annealing at 150˚C for two hours.DSC recordings show that eutectic composition of the lead metavanadate has been maintained for all the glass systems up to 15 mole% of substitution.The DSC data also indicate that all the glass systems are characterized by more than one crystallization peak.This can be thought as an evidence for the existence of more than one meta-stable phase in the glass systems.The dopant SnO 2 is not divalent oxide like PbO.Besides SnO 2 is known to be glass former unlike PbO which is replacing PbO in the glass network in such a way that the eutectic composition is maintained.In the present studies the IR spectra of SnO 2 substituted samples of 5 mole% V=O stretching frequency occur at 966 cm −1 without the appearance of any additional peak.But for 10 mole% and 15 mole% SnO 2 substituted samples, additional peaks appear at 1023 and 1005 cm −1 indicating the effect of SnO 2 in the vanadate crystalline matrix such that there is an elongation of V=O bond.Since the crystalline matrix is affected, we can expect similar effect in the glass matrix also.