Infrared study of the effect of P 2 O 5 in the structure of lead silicate glasses

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1 Indian Journal of Pure & Applied Physics Vol. 43, February 2005, pp Infrared study of the effect of P 2 O 5 in the structure of lead silicate glasses M Rafiqul Ahsan, M Alfaz Uddin & M Golam Mortuza Department of Physics, University of Rajshahi, Rajshahi 6205, Bangladesh Received 7 June 2004; 23 September 2004; accepted 5 November 2004 The infrared spectra of xpbo (1 x z)sio 2 zp 2 O 5 glasses with a PbO:SiO 2 molar ratio of 3:2 and z=0,1,2,5,30,40 and 50 mol% have been examined in the region cm 1, together with XRD. The structure of PbO-SiO 2 binary glass is significantly different from that of P 2 O 5 substituted glasses. The low amount of P 2 O 5 containing PbO-SiO 2 glasses results in infrared spectra of the same pattern as 3:2 binary lead silica glass but the maximum peak intensities and positions are different. The spectra of high P 2 O 5 (30-50 mol%) containing glasses show an increased number of distinct peaks in the low frequency region with a convolution of several broad Gaussians in the high frequency region. All the spectra are base line corrected and deconvoluted to appropriate number of Gaussians. The peaks and peak positions of the Gaussians are assigned to Si-O-Si, Si-O-Pb, Si-O-P, P-O-P, O-P-O, P=O, P-O etc. bonds. A variation in the band positions and the relative amounts of bonds with P 2 O 5 is also shown. The result indicates that ortho- and pyro-phosphate are formed around 1-5 mol% P 2 O 5 containing glasses and meta-phosphate species from the high P 2 O 5 containing glasses. The ionic character of the phosphate groups P-O, PO 4 is well revealed by the formation of new band within cm 1. The incorporation of high contents of P 2 O 5 in the materials suggests that P 5+ occupies the network position and forms the linkage Si-O-P in the glasses. This Si-O-P bond may lead to the formation of five and six coordinated silicon in the glass matrix. X-ray diffraction pattern of the base samples, within the detection limit, shows that all are homogeneous glasses and these have not shown any evidence of phase separation on optical inspection. [Keywords: Infrared spectra, Lead silicate glasses, PbO-SiO 2, Binary glass, P 2 O 5 ] IPC Code: G01J 3/00 1 Introduction Lead silicate binary glasses are useful as solder glasses for glass to metal seal and as optical glasses. The main features of this glass are low softening point, high refractive index and large thermal expansion coefficient, which lead to a commercial importance. Owing to scientific and technological interest many researchers are attracted to study its structural details 1-4. Mainly XRD, vibrational spectroscopic and nuclear magnetic resonance (NMR) techniques are applied to the systems to investigate the bonding mechanism, nearest neighbour (nn) and next nearest neighbour (nnn) of the local order of network former silicon in PbO-SiO 2 glasses. Mydlar et al. 1 observed the range of the nearest neighbour Pb- Pb mean separation of about of 3.8Å from the XRD study of lead silicate glasses. 29 Si environment in PbO.SiO 2 glasses containing high PbO is studied by Dupree et al. 5 using NMR techniques. The structural arrangement is different from that of the alkali silicate system. The result suggested that below 30 mol% of PbO the number and deposition of [nbo] is consistent with a binary ranzuphy@yahoo.com Q 4 /Q 3 distribution. But above 30 mol% PbO statistical distribution of Q m species dominates. Also, formation of Si-O-Pb bridging units is indistinguishable from Si- O-Si and isolated SiO 4 4 groups present in glass matrix. Reviews encompassing most of the alkali binary, ternary and other glasses and glass ceramics have been studied using NMR and MAS NMR by Dupree and Holland 6. Although the structures of binary silicate and ternary phosphosilicate glasses are investigated by several researchers 7-9, but the work on divalent metal oxide silicophosphate glasses is rare. Also, numerous studies, using various techniques, on the binary divalent metal oxide phosphate glasses are available. The role of PbO in xpbo(1 x)p 2 O 5 glasses is well analyzed by Dayanand et al 10. They observed that PbO in phosphate glasses undergoes structural changes from meta-phosphate (x=0.5) to pyrophosphate (x=0.66) and to ortho-phosphate (x=0.75) depending upon the concentration of lead oxide. As this study is carried out on a binary system the lower amount of lead oxide means the higher amount of P 2 O 5 leading to the formation of meta-phosphate species. This observation is in agreement with ternary alkaliphosphosilicate system 8, where the phosphorus

2 90 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 content is varied keeping alkali to silicon ratio constant. However, the role of either PbO in phosphosilicate system or P 2 O 5 in PbO-SiO 2 system is also yet to be assessed. As phosphorus plays an important role in nucleation as well as structure formation, our aim is to study the effect of P 2 O 5 on lead silica glass. The present authors have studied the bonding mechanisms of CdO-SiO 2 -P 2 O 5 glasses with the help of IR and NMR techniques 16,17. The work describes a significant structural change of silicon coordination. This has prompted the authors to investigate PbO-SiO 2 -P 2 O 5 glasses as this has not been interpreted extensively by any spectroscopic techniques. In this work, we have collected infrared spectra of xpbo(1 x)sio 2 glass with varying amounts of P 2 O 5 and qualitative and quantitative analyses of these spectra are performed to understand the bonding mechanisms, and characteristic frequencies of the vibrational chemical bonds, which are liable to the structural and spectral changes. The XRD pattern of base and heat-treated samples is also used to study the amorphocity and phases of the glass. 2 Experimental Details The base glasses are made from commercially available reagent grade PbCO 3, SiO 2 and P 2 O 5. For the binary 3PbO:2SiO 2 glasses, the powder raw material of appropriate amounts, usually 20 gm batches, were homogenized on a ball milled machine for 6-8 hr and then melted in a platinum crucible in an electric furnace. The temperature of the furnace was raised to the required temperature within 4 hr keeping the sample in the furnace and required temperature was maintained for half an hour. The glass melt was then splat cooled between two iron blocks coated with graphite. Similar procedure is followed to prepare ternary xpbo(1 x z)sio 2 zp 2 O 5 glasses. The composition, melting temperature, and optical quality of the glasses along with their nomenclature are shown in Table 1. The melting temperature of the samples in the range 5 mol%<p 2 O 5 <30 mol% is very high (>1100 C) and they become crystallized during quenching. As the samples did not form glass, they are not characterized. To prepare glass ceramics, some quantity of xpbo(1 x z)sio 2 zp 2 O 5 base glasses was first pulverized in a mortar and then each of the finely powdered base glass is heat-treated at 500 C for 12 hrs in an electric furnace. However, for comparison PbSP50 glass is also heat-treated at 550 C/24hr. A Carbolite digital furnace, model CTF12/65, controller eurotherm 91 and porcelain crucible are used for the heat treatment. To obtain IR absorption spectra of a glass sample, standard KBr pellet technique is employed. All infrared spectra of the glasses were collected on a double beam Shimadzu IR470 spectrophotometer of resolution ± 4 cm 1 for cm 1 and ±1.5 cm 1 for cm 1. The X-ray diffraction measurements of the stored sample were carried out using X-ray diffractometer system; JDX-8P, Jeol, Japan that can detect crystallinity less than 5%. The phases in the devitrified samples were identified by comparing the experimental interplanar spacing and intensity of the diffraction pattern with standard JCPDS files. 3 Results The infrared spectra of xpbo(1 x)sio 2 (PbSP0) glass and that with low and high P 2 O 5 containing glasses are shown in Fig. 1a and Fig. 1b respectively. The spectrum of PbSP0 glass consists of two main bands, one in the lower wavenumber and other in the higher wavenumber. The absorption intensity of the former region is higher than the latter. Also, both bands contain number of small peaks and shoulders with main peak and high frequency band is relatively broader than low frequency band. The introduction of small amount such as 1, 2 and 5 mol% P 2 O 5 produces a significant change in the spectra of the glass (Fig. 1a). The main difference Table 1 Melting temperature and optical quality of glasses of various compositions Samples Nominal compositions in mol% PbO SiO 2 P 2 O 5 Melting temp. in C Optical quality PbSP clear PbSP red, clear PbSP green, clear PbSP green, clear PbSP white clear PbSP white, clear PbSP white, clear XRD Amorphous

3 AHSAN et al.: INFRARED STUDY OF LEAD SILICATE GLASSES 91 Fig. 1(a) Infrared spectra of xpbo(1 x z)sio 2 zp 2 O 5 glasses (z= 0, 1, 2 and 5 mol%); (b Infrared spectra of xpbo(1 x z) SiO 2 zp 2 O 5 glasses (z= 30, 40 and 50 mol %) among the spectra of binary and ternary glasses lies in the absorption intensity of the two main bands. The P 2 O 5, even 1 mol% (PbSP1), results in decrease in intensities of the bands relative to that of PbSP0. however, further addition of P 2 O 5 increases the intensities in PbSP2 and PbSP5 material. In 1 mol% P 2 O 5 (PbSP1) glass three distinct shoulders appear in the 750 cm 1 to 580 cm 1 region and they become weaker in 2 mol% P 2 O 5 (PbSP2) glass and almost disappear in the 5 mol% P 2 O 5 (PbSP5) glass. The infrared spectra obtained from glasses with 30, 40 and 50 mol% P 2 O 5 (PbSP30, PbSP40 and PbSP50) show two broad overlapped Gaussian absorption bands in the low and high frequency regions. In this case variation of absorption is remarkably different from glasses with low content of P 2 O 5. The spectrum of PbSP30 glass consists of one sharp peak with some shoulders in the low frequency region and in the high frequency region there is a broad Gaussian band with high absorption associated with small peak around 1120 cm 1, which becomes distinct in the remaining glasses. In the spectrum of PbSP40 and PbSP50 glasses, the low frequency band is resolved into three peaks of almost equal absorption and a new band is developed in the frequency range cm 1. The high frequency band is similar to the spectrum of PbSP30 glass, but the absorption intensities are different. There is a general tendency of shifting of position towards high frequency with the addition of P 2 O 5 in the PbSP0 glass. When a resultant spectrum consists of more than one band, either the peak or the shoulder positions do not appear at the center of gravity of the individual band, rather they often move towards the center of gravity of all the bands present in the spectrum. Therefore, the deconvolution of the experimental spectrum provides with the exact position of the absorption band. The band position and widths obtained from the deconvolution are thus considered to be true representation of the spectra. On the basis of close inspection of the spectral shape, a number of Gaussians are fitted to it. To deconvolute at first the peak position, full width at half maximum and peak height are first chosen by eye estimation to use them as input parameters and the program itself varied them to fit the experimental spectra with least chi-square value. So all the spectra are deconvoluted to several Gaussians according to their spectral shape. The deconvoluted parameters describing each band position, and amplitude are presented in Table 2. 4 Discussion 4.1 Spectra of xpbo(1 x) SiO 2 glass Apparently it seems that there are two distinct bands in the low and high frequency regions of the spectrum but the close inspection reveals several shoulders in the pattern. The two Gaussian bands of each of the spectra are deconvoluted to 6 and 4 Gaussians depending on the basis of spectral shape to estimate the band position and relative amounts of various bonding mechanisms. A typical diagram of the deconvoluted spectrum of the glass is shown in Fig. 2. The deconvoluted band positions and bandwidths are considered to be true representation of the spectra. The assignments of these bands are carried out by comparing its position with those of related glass and crystalline phases and are summarized in Table 3. The weak band observed at 520 cm 1 is ascribed to Si-O vibration 18. The strong bands at 565, 586 and 597 cm 1 in the spectrum can be resulted from the Si- O-Pb asymmetric bending mode. In vitreous silica and titanium alkalisilicate glasses, Bell et al. and Kyoshi Kusabiraki 19,20 obtained the frequency of Si- O-Si bending motion at 730 cm 1 and cm 1

4 92 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 Table 2(a) Deconvolution parameters in the low frequency region Name of the glasses A1 ±0.05 A2 ±0.10 A3 ±0.50 A4 ±0.20 A5 ±0.02 Parameters* A6 ±0.05 XB1 XB2 XB3 XB4 XB5 XB6 PbSP PbSP PbSP PbSP PbSP PbSP PbSP Name of the glasses A1 ±0.05 A2 ±0.05 Table 2(b) Deconvolution parameters in the high frequency region A3 ±0.04 A4 ±0.02 A5 ±0.01 Parameters PbSP PbSP PbSP PbSP PbSP PbSP PbSP *A= Peak height of the final Gaussians in arbitrary unit XB= Peak position of the final Gaussians in cm 1 XB1 XB2 XB3 XB4 XB5 Fig. 2 A typical deconvoluted spectra with residual respectively, where oxygen atom moves approximately at right angles to the Si-O-Si planes. Again the oxygen rocking Si-O-Si bending vibration is observed in the frequency 18,19 region cm 1. The frequency of Si-O-Pb asymmetric bending mode lies between these ranges. The formation of the bands can be explained as Pb atom is heavier than Si atom and each Pb ion reacts with silica to produce two nonbridging oxygen and the network becomes depolymerised. The three bands within the closed range are due to the asymmetry in the Si-O-Pb bond distribution because a range of environments can exist in the glassy materials. The medium absorption band at 652 and 740 cm 1 can be assigned to Si-O-Si bending motion 19,20. The strong band at 1150 cm 1 can be assigned to Si-O-Si stretching vibration. This frequency is higher than that of Si-O-Si stretching bond as observed by several researchers 18,20 within cm 1. The frequency increase can be interpreted as the decrease in the average Si-O-Si angle 21. The high frequency band can be observed by a role of cation Pb ++ in glass structure. Bobkova et al. 22 observed intense band in the range cm 1 with a maxima at 1180 cm 1 for the different role of Cd 2+ ion. A similar result was obtained by Naboru et al. 23 in borate glasses for Pb 2+ ion. On the basis of these results, the strong band with a maximum at 1260cm 1 can be ascribed to O-Pb-O bonds. For the O-H stretching and bending vibration of water of crystallization all the hydrate

5 AHSAN et al.: INFRARED STUDY OF LEAD SILICATE GLASSES 93 Table 3 Deconvoluted band position and assigned chemical bonds of the glass system xpbo(1 x z)sio 2 zp 2 O 5 Samples Low frequency region Band positions Chemical in cm 1 bonds High frequency region Band positions Chemical bonds in cm 1 PbSP0 PbSP1 PbSP2 PbSP5 PbSP30 PbSP40 PbSP Si-O 1150 Si-O-Si 565 Si-O-Pb 1260 O-Pb-O 586 Si-O-Pb 1453 OH 597 Si-O-Pb 1910 OH 652 Si-O-Si 740 Si-O-Si 500 Si-O 1160 PO O-P-O, O=P-O 1220 O-Pb-O 580 Si-O-Pb 1450 OH 590 P-O-Pb 2090 OH 620 O-P-O 700 P-O-P 500 Si-O 1170 PO O-P-O, O=P-O 1230 O-Pb-O 590 Si-O-Pb 1340 P=O 620 P-O-Pb 1870 OH 630 O-P-O 705 P-O-P 500 Si-O 1178 P-O 570 O-P-O, O=P-O 1230 O-Pb-O 595 Si-O-Pb 1320 P=O 630 P-O-Pb 1880 OH 670 O-P-O 720 P-O-P 515 O-P-O,O=P-O 1140 P-O 570 O-P-O,O=P-O 1357 P=O 600 P-O-Pb 1768 OH 640 Si-O-P 2334 P-O-H 690 O-P-O 2890 P-O-H 770 P-O-P 520 O-P-O,O=P-O 1150 P-O 600 P-O-Pb 1470 P=O 672 Si-O-P 1880 OH 735 O-P-O 2437 P-O-H 810 P-O-P 2900 P-O-H 950 PO O-P-O,O=P-O 1154 P-O 610 P-O-Pb 1480 P=O 675 Si-O-P 1900 OH 745 O-P-O 2460 P-O-H 820 P-O-P 2930 P-O-H 960 PO 4 salts absorb 24 near 1640 cm 1 and therefore the broad Gaussian having maxima at 1453 cm 1 and 1910 cm 1 may be due to absorption of moisture. This atmospheric moisture can be easily absorbed by the pellet in recording IR spectra causing the appearance of IR bands belonging to O-H bending mode. 4.2 Spectra of xpbo(1 x)sio 2 glass with 1-5 mol% P 2 O 5 For the glasses in the ternary system, the molar ratio PbO:SiO 2 were kept 1.5, while P 2 O 5 substituted systematically into the glass system. Effect of P 2 O 5 is obvious from the change in spectral shape due to the addition of small amount of P 2 O 5. Though the deconvoluted band positions are almost similar to those in lead silicate binary glass, the slight shift in band position toward high frequency and abrupt change in the relative height of the peak (Fig. 1a) indicate the structural change. As in binary glass, the band assignment is based on the comparison of the reported data of absorption frequencies of glassy and

6 94 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 crystalline compounds related to the PbSP1-PbSP5 glasses (Table 3). The weak band observed at 500 cm 1 in PbSP1- PbSP5 glasses can be assigned to Si-O bending vibration 25,26. Other weak bands at 560, 565 and 570 cm 1 in the above glasses are due to the harmonics of bending O-P-O and O=P-O vibration as reported by Dayanand et al 10. The strong absorption bands at 580, 590 and 595 cm 1 in the PbSP1-PbSP5 glasses respectively can be assigned to Si-O-Pb vibrations and relatively weaker bands in the above glasses at 590, 620 and 630 cm 1 may be due to P-O-Pb vibrations. Since the electronegativity of Pb and Si is identical and each lead ion produces two non bridging oxygen, so one can assume Si-O-Pb bond in cm 1 region in lieu of Si-O-Si bending vibration. Again the ratio of P 2 O 5 /PbO+SiO 2 is increased systematically keeping the ratio of PbO/SiO 2 constant and the Pb and P atoms are heavier than silicon atom, so we can expect P-O-Pb vibrations in the lower cm 1 than Si-O-Si vibrations. The frequency of P- O-Pb vibration is also lower than the stretching frequency of P-O-P vibration. The possible explanation of the formation of this band is that a covalent bond between the non-bridging oxygen ions and the lead ion gave rise to P-O-Pb unit in the system and the assigned frequency of the P-O-Pb unit is lower than the P-O-P stretching frequency 27. The plateau like absorption at 620, 630 and 670 cm 1 are attributed to O-P-O bending vibration as observed by Chakraborty 28 in both devitrified and untreated silicophosphate glasses. Formation of this band indicates that phosphorus may act as a network former. If so, the charge of the network forming ion is imbalanced, and then O-P-O arrangement will be unstable and PO 4 tetrahedra will bear an excess unit positive charge. This discrepancy can be eliminated if one of the oxygen ions around P 5+ were doubly bonded to the central cation. Thus, it is highly likely that an O=P-O bond is formed. Although there is no remarkable variation in the spectral shape on addition of small amount of P 2 O 5 (1, 2 and 5 mol%), the variation of intensity indicates that P 5+ may occupy the lattice site rather than interstitial position. A general trend of shifting of the band position towards high frequency is observed with the addition of phosphorus in the materials. This shifting is also caused by the change of local field of the Si-O-Pb bonding due to P 5+ ion. The bands at 700, 705 and 720cm 1 are for P-O-P stretching vibrations 10,29. This shift in band position is due to change in bond angle and concentration 30 of P 2 O 5. In the high frequency region, the very strong bands at 1160 cm 1 and 1170 cm 1 arise from the stretching vibration of PO 4 mode as observed by Dayanand and Corbridge 10,24 at 910, 990 and 981, 1056 cm 1 respectively. The shift in frequency is due to the asymmetry of PO 4. The strong band at 1178 cm 1 in PbSP5 can be attributed to P-O stretching band 10,31. The absence of P-O bond in PbSP1 and PbSP2 glasses indicates that PO 4 dominates the system or exists in monomer form. So, the presence of PO 4 group in PbSP1 and PbSP2 glasses suggests the formation of lead orthophosphate in this region and the presence of P-O bond may be identified as the characteristic of lead pyrophosphate species. These two phosphate species scavenge PbO from silicate network and repolymerise Si-O-Si bonding. The strong bands at 1220 and 1230 cm 1 can be attributed to O-Pb-O stretching vibration as observed by Bovkova 22. The bands at 1340 and 1320 cm 1 can be assigned to P=O stretching mode. This band is not observed in PbSP1 glass. The presence of P=O bonds in the PbSP2 and PbSP5 glasses can be accounted for the increase of P 2 O 5 concentration. The broad Gaussian bands at 1450, 2090, 1870 and 1880 cm 1 may be due to O-H bending vibration. Since P 2 O 5 can absorb atmospheric moisture easily for this reason these bands may form. The gradual increase in intensity of the high frequency bands arises from the scavenging of Pb by phosphorus from the silica network. The shifting of band frequency towards high frequency may indicate the change of phosphate species, namely ortho-phosphate to pyro-phosphate. Since the ortho-phosphate and pyro-phosphate species are monomer and dimer respectively, the increase in amount of them cannot shift the band positions and this is what has been observed in the samples. The meta-phosphate species does not form at this level of phosphorus. As the samples containing P 2 O 5 are hygroscopic, the broad absorption pattern in the range of 1600 to 3200 cm 1 indicates the presence of water of crystallization which is probably loosely held in the structure Spectra of xpbo(1 x)sio 2 glass with mol% P 2 O 5 The observed variation in the spectra with high amount of P 2 O 5 (PbSP30, PbSP40 and PbSP50) indicates a considerable structural change in the samples. Although the absorption bands obtained with 30 mol% P 2 O 5 are broader in comparison to low amounts of P 2 O 5 containing glasses (Fig. 1b), the

7 AHSAN et al.: INFRARED STUDY OF LEAD SILICATE GLASSES 95 spectra are informative. The PbSP40 and PbSP50 glass spectra are very much similar. The development of three bands in PbSP40 and PbSP50 glasses is only possible when the tetrahedral network along with associated species becomes relatively more amorphous. Similar effect is also observed for the high frequency band in all the glasses. The shifting of band positions towards high frequency region caused by the incorporation of P 5+ in the structure indicates a structural change. The new bands developed in the region cm 1 in the PbSP40 and PbSP50 glasses justify this hypothesis. Considering the spectral shape, the spectra are deconvoluted to 6 and 5 Gaussians in the low and high frequency regions respectively and the entire deconvoluted band positions and assigned chemical bonds are shown in Table 3. In PbSP30-PbSP50 glasses, the bands obtained at 515, 570, 545 and 560 cm 1 may be due to O-P-O, O=P-O bending vibration 10. The strong bands at 600 and 610 cm 1 can be assigned to P-O-Pb asymmetric vibration. Another set of strong bands at 640, 672 and 675 cm 1 can be attributed to Si-O-P bending motion of SiO 6 octahedra rather than Si-O-Si tetrahedra as observed by Chakraborty et al 28. The band positions for these three samples are shifted towards the more positive side indicating Si-O-P bonding instead of Si- O-Si bond. Thus, P 5+ occupies Si 4+ lattice site by breaking the SiO 4 tetrahedral network. Similar effect is also observed in alkali-phosphosilicate system by Dupree et al 32. The absorptions at 690, 735 and 745 cm 1 in the above glasses represent O-P-O bending vibration 28. The plateau like absorptions at 770, 810 and 820 cm 1 are due to the P-O-P stretching vibration 29,31,33. In this case, the phosphorus reflects similar shifting as Si-O-P bond. Other bands at 950 and 960 cm 1 in PbSP40-PbSP50 glasses can be Compositions P-O-P stretching vibration cm 1 Table 4 Important absorption bands for meta-phosphate glasses P-O-P bending vibration cm 1 ν 3 PO 4 cm 1 ascribed to stretching vibration of PO 4 mode 10,11,31. The new bands appearing at 1140, 1150 and 1154 cm 1 in PbSP30-PbSP50 glasses can be assigned to P-O stretching vibration 34. The strong bands at 1357, 1470 and 1480 cm 1 can be considered as the characteristic of P=O stretching bond. This assignment of P=O bond in the phosphoryl group is consistent with Hudgen and Martin 35. Also, Higazy et al. 29 observed this band frequency within cm 1 range. Actually in these glasses this band frequency is high. This shift may be attributed to the increase in the bond order of P=O bond due to the localization of P=O bond in the meta-phosphate unit. Also, these positions are significantly different from P=O bond of low phosphate containing glasses (PbSP2 and PbSP5). The polymerisation of chain structure could shift the P=O vibrational position and this is only feasible for the formation of meta-phosphate structure. Therefore, the polymer, instead of monomer and dimer, forms at this level of P 2 O 5. For comparison, the main absorption bands obtained for the composition up to the metaphosphate glasses along with the observed bands in the present work are shown in Table 4. The strong overlapped Gaussians having maximum at 1768,1880 and 1900 cm 1 may be due to O-H bending vibration 36,37. The other strong bands at 2334, 2890, 2437, 2900, 2460 and 2930 cm 1 can also be assigned to P-O-H stretching mode Variation in band position and intensity with 1-50 mol% P 2 O 5 In general, molecular groups P=O, P-O, PO 4, P-O-P bending, P-O-P stretching and P-O are identified as a characteristic of phosphate glasses. The frequency of absorption of each of these bands depends on composition and combination of bonding to other groups. The vibrational modes of these bands P-O cm 1 P=O H 2 O Ref. cm 1 V 2 O 5 -P 2 O CoO 3 -P 2 O CdO-P 2 O xpbo(1 x) P 2 O 5 721, xcdo(1 x z)sio 2 zp 2 O 5 z=30-50 mol% xpbo(1 x z)sio 2 zp 2 O 5 z=30-50 mol% This work

8 96 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 are well defined. The possible infrared vibrational modes in P 2 O 5 correspond to P=O bonds, bridge vibration of P-O-P chains of the glass lattice and P-O- P, O-P-O deformations and exhibit infrared bands below 1300 cm 1. Also, Wong 37 revealed three vibrational groups P-OH, P=O and P-O-P for vitreous P 2 O 5 infrared absorption. Corbridge 38 suggested P=O band between cm 1 and Hudgen et al. 35 observed P=O as a large intense band at 1390 cm 1. The variation in band position with P 2 O 5 concentration is shown in Fig. 3. Also, the variation of the intensity, i.e. relative area of the assigned bands, with error bars, is shown in Fig. 4 and its data are presented in Table 5. The band positions for P-O-P, O-P-O, and P-O-Pb shift to higher wavenumber abruptly up to PbSP5 and is almost unchanged at PbSP40 and PbSP50 glasses. This change in position could be due to the change in Gibb s free energy affected by the formation of one species to another. The two species may also form at this level of P 2 O 5 but it is too early to be dogmatic. Nevertheless, if this is so, it can be said that one species forms at the expense of the other. For higher concentration of P 2 O 5, the metaphosphate species may occur in the material as observed by Dupree et al. 32 and Dayanand et al 10. The variation in intensity of P-O-P, O-P-O and P-O-Pb vibrational mode is shown in Fig. 4. In all the cases, the intensity gradually decreases to a minimum at around 5 mol% and then increases with further increase of P 2 O 5 concentration. This is due to the depolymerisation of silica network by lead and active role of phosphorus as a network former. This behaviour is a clear indication of the formation of phosphate species. The variation of band position (Fig. 3) and intensity (Fig. 4) of Si-O-Pb and O-Pb-O are due to the effect of Pb ++ in the silica network. Obviously, as the number of Si-O-Pb group decreases with the increase of P 2 O 5, the frequency and intensity of Si-O-P mode should increase with P 2 O 5. This is observed in the composition for Si-O-P bonds (Figs 3 and 4). The formation and variation of these bands may be due to P 5+ occupying the SiO 4+ lattice site by breaking tetrahedral network. Also, the frequency of Si-O-P is higher than Si-O-Si band. This can be interpreted in terms of different force constants arising from octahedral environment of silicon and respective bond angle. This also suggests that new species either phosphate or silicate or both of them may be grouped in the materials. In alkali-silicophosphate system for 30 mol% P 2 O 5 octahedral silicate, SiP 2 O 7, forms along with alkali-metaphosphate species 8. It may also be reckoned that in lead silicophosphate system such octahedral silicon can occur. The variation of band position and intensity of P=O in the high P 2 O 5 containing glasses show a positive correlation, i.e. shifts towards more positive side (Figs 3 and 4) with Fig. 3 Variation of bond position with P 2 O 5 concentration Fig. 4 Variation of relative area of various bands with P 2 O 5 concentration

9 AHSAN et al.: INFRARED STUDY OF LEAD SILICATE GLASSES 97 Table 5 Relative amounts of various chemical bonds P 2 O 5 Si-O-Pb ±1.67 O-Pb-O ±0.07 P-O ±0.06 PO 4 ±0.08 P=O ±0.57 Si-O-P ±0.86 P-O-H ±2.52 P-O-P ±1.19 O-P-O ±1.52 P-O-Pb ±2.80 O-H ± the increase of P 2 O 5 concentration. This nature indicates that the cationic (Pb ++ ) effect weakens with the increase of P 2 O Ionic character of xpbo(1 x z)sio 2 zp 2 O 5 glasses The ionic character of phosphate group can be explained by the formation of P-O and PO 4 groups. In the present work, the band appearing at cm 1 is attributed to P-O stretching vibrations, which is basically present in P 2 O 5. The band position of P-O group is nearly unaffected with the increase of P 2 O 5. The slight increase in band position (Fig. 3) can be attributed to shortening of P-O bond due to the increase in the network packing. This also suggests that the phosphate species that formed at around 30 mol% P 2 O 5 remain unchanged at 40 and 50 mol% and breakdown of covalent vitreous network into small ionic groups PO 4, P 2 O 2 6 and P 2 O 4 7 might be expected. The absence of P-O bond in PbSP1 and PbSP2 indicates near total absence of network P 2 O 5. Thus we conclude that new phosphate species form at this level of P 2 O 5. In the low phosphate glasses, the band PO 4 is observed in the high frequency region and shifts to lower wavenumber with the increase of P 2 O 5 and this band is not observed in PbSP5 glass. The disappearance of PO 4 stretching mode in the above glasses indicates the change of structural unit, say, orthophosphate to pyrophosphate. In PbSP40 glass, this band is observed at 950 cm 1 and shifts to 960 cm 1 in PbSP50 glass. This shifting of band position toward high frequency confirms the formation of meta-phosphate species in PbSP40 and PbSP50 glasses where the homogeneous glass phase exists. These changes in the infrared band can be explained as follows. The gradual increase of PO 4 tetrahedra and hence the increase of bridging oxygen with the increase of P 2 O 5 and PO 4 tend to change to PO 3 ionic group. So the bands at 950 and 960 cm 1 in the infrared spectrum of meta-phosphate glass can be attributed to PO 3. Similar results have been observed by several researchers 36,39 within cm 1. Khafagy et al. 31 observed ν 3 of PO 4 in V 2 O 5 -P 2 O 5 glass at 930 cm 1. The variation in intensity in Fig. 4, certainly suggests that the structure of the glass changes in such a way that phosphate tetrahedra dominates the network. 4.6 Hygroscopicity of phosphate glass The collection of infrared spectra by KBr pellet technique is likely to be influenced by atmospheric moisture causing the formation of the infrared bands belonging to H 2 O molecules. Further, P 2 O 5 can absorb moisture easily from the atmosphere. So the hygroscopic nature of the xpbo(1 x z)sio 2 zp 2 O 5 glass system should permit one to refer them as xpbo(1 x z)sio 2 zp 2 O 5.H 2 O compositions. In the high phosphate glasses volatile P 4 O 10 exists and as much as 25 mol% water can retain 40. All the hygroscopic glasses absorb near 1600cm 1 and 3600 cm 1 for O-H bending and stretching vibration respectively. O-H stretching vibration occurs due to the absorbed water, which has no role in the network. In this work, the vibrational band for O-H bending mode lies between 1768 to 2090 cm 1. This variation indicates that the water seems to be nearly free or loosely held by the glass network. The intensity of the band increases with the increase of P 2 O 5 /PbO+SiO 2 ratio. This indicates the glass with large amount of P 2 O 5 contains more absorbed water than those with smaller amounts. The total amount of water can, therefore, be estimated from the flat Gaussian in the O-H region. 4.7 XRD of base and heat-treated samples X-ray diffraction pattern of the base samples, within the detection limit (<5%), shows that all are homogeneous glasses. Moreover, optical inspection did

10 98 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 not show any evidence of phase separation as well. The diffraction pattern of PbSP1-PbSP5 heat treated samples showed the crystallinity in the material and the observed phases are Pb 3 Si 2 O 7, Pb 3 Si 3 O 11 and Pb 2 P 2 O 7. Formation of Pb 3 (PO 4 ) 2 is not clear in these samples. But the PbSP40 and PbSP50 samples did not show any crystallinity except amorphous halo. The sample PbSP50 heat-treated at 550 C/24 hr shows the different result from that of other samples. In this case possible phases are SiP 2 O 7, Pb 2 (PO 3 ) 4 and PbP 2 O 6. 5 Conclusion The glass system xpbo(1 x z)sio 2 zp 2 O 5 was prepared by conventional method, where the ratio PbO/SiO 2 is 1.5 and the concentration of P 2 O 5 was 0,1,2,5, 30,40 and 50 mol%. The IR band positions in the glasses have a general tendency to shift towards the high frequency region with the increase of P 2 O 5 concentration. The phosphate glasses exhibit welldefined IR bands characteristic of molecular groups identified as P=O, P-O, PO 4, P-O-P and harmonics of bending O-P-O and O=P-O vibrations. The frequency of these groups is dependent on its bonding to other groups in the glass network The variation in band position and relative intensities with P 2 O 5 concentration suggests that the glass system with z = 1-2 mol% are orthophosphate compound, with z = 5 mol%, pyrophosphate compound and with z = mol%, metaphosphate compound. The fact that the IR band due to P=O bond is observed in the glasses PbSP30-PbSP50 glasses suggests that Pb ++ does not act as a glass former and no complete rupture of the glassy network by Pb ++ takes place. Rather the cation enters the network interstitially acting more as a network modifier. The effect of P 2 O 5 is obvious for certain bonding mechanisms where P 5+ plays a significant role. A major change is observed for O-Pb-O and Si-O-Pb bond positions at low P 2 O 5 concentration indicating the scavenging of Pb ++ ion. The almost constant band positions P=O, P-O, P-O-P, Si-O-P for higher amounts of P 2 O 5 (> 30 mol%) represent a stable local environment. Incorporation of small and large amounts of P 2 O 5 in the material suggests that P 5+ occupies the network former position and once the P 5+ goes to the silicate network, it remains as Si-O-P. The Si-O-P bond may lead to the octahedral silicon in the glass matrix. Ionic character is well observed in the glasses containing 30 mol% P 2 O 5 by the formation of P-O and PO 4 groups in the frequency region cm 1. The characteristic frequency related to the ionic P-O and PO 4 groups shifts to higher wavenumber with the increase of P 2 O 5. The formation of O-H band around the region cm 1 expresses the hygroscopic nature of the glass and provides a wealth of information about the structural units. The significant changes in the spectral pattern for 0 z 5 and 30 z 50 prove unambiguously that there must be structural changes which lead to the physical and chemical properties of the glasses. Further NMR experimentation on 29 Si and 31 P nuclei of the materials is on the way to present the results. X-ray diffraction pattern of the low P 2 O 5 containing heat-treated samples show the possible phases Pb 3 Si 2 O 7, Pb 3 Si 3 O 11 and Pb 2 P 2 O 7 and that in the high P 2 O 5 containing glasses are SiP 2 O 7, Pb 2 (PO 3 ) 4 and PbP 2 O 6. Acknowledgement We wish to thank the Chairman, Department of Chemistry, University of Dhaka, Dhaka, and the Chairman, Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, for their kind help to use their laboratory for IR spectrophotometry and X-ray diffractrometry. We are also thankful to the Faculty of Science, Rajshahi University, Rajshahi for a research grant. References 1 Mydlar M F, Kreidle N J, Hendren J K & Clayton G T, Phys Chem Glasses, 11(6) (1970) Rabinovitch F M, J Mat Sci, 11 (1976) Smets B M I & Lommen I P A, J Non-Cryst Solids, 48 (1982) Khalifa F A, EL- Hadi Z A, EL- Keshen A A & Moustafa F A, Indian J of Pure & Appl Phys, 34 (1996) Dupree R, Ford N & Holland D, Phys Chem Glasses, 28(2) (1987) Dupree R & Holland D, In glasses and glass ceramics, Edited by Lewis M H, Chapman and Hall, CH.1 (1989)1. 7 Weeding T L, de Jong B H W S, Veeman W S & Aitken B G, Nature, 318 (1985) Dupree R, Holland D & Mortuza M G, Nature, 328 (1987) Frurukawa T, Brawer S & White W B, J Mat Sci, 13 (1978) Dayanand C, Bhikshamaiah G, Jaya Tyagaraju V, Salagram M, Krishana Murthy A S R, J Mat Sci, 31 (1996) Ghauri M A & Hogarth C A, J Mat Sci, 19 (1984) Walter G, Hoppe U & Kranold R, Phys Chem Glasses, 35(6) (1994) Meyer K, Phys Chem Glasses, 39(2) (1998) Dupree R, Holland D & Mortuza M G, Phys Chem Glasses, 29(1) (1988) Hoppe U, Walter G, Kranold R, Stachel D & Barz A, J Noncryst Solids, 192 (1995) 28.

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