Raman spectroscopic study of ternary silver tellurite glasses

Raman spectroscopic study of ternary silver tellurite glasses

Materials Research Bulletin, Vol. 34, No. 2, pp. 327–342, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408...

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Materials Research Bulletin, Vol. 34, No. 2, pp. 327–342, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00012-4

RAMAN SPECTROSCOPIC STUDY OF TERNARY SILVER TELLURITE GLASSES

B.V.R. Chowdari* and P. Pramoda Kumari Department of Physics, National University of Singapore, Singapore 119260 (Communicated by A.R. West) (Received May 3, 1998; Accepted May 8, 1998)

ABSTRACT The structure of Ag2O–MxOy–TeO2 (MxOy 5 WO3, MoO3, P2O5, and B2O3) glasses was investigated by Raman spectroscopy. Raman spectra shows that in addition to the expected TeO4 trigonal bipyramid (tbp), the TeO3 trigonal pyramid (tp), and the WO4/WO6, MoO4/MoO6, PO4, and BO3/BO4 polyhedra are present in tungstotellurite, molybdotellurite, phosphotellurite, and borotellurite glasses, respectively. The relation between the MxOy content and the intensity ratios of the deconvoluted Raman peaks I720/I660 and I780/I660 were studied. From the temperature variation study of the Raman spectra, it was observed that there is an increase in the TeO3 units with the increasing temperature. Although crystallization occurred easily during heating of the glasses in any system, the melts hardly ever crystallized during cooling. © 1999 Elsevier Science Ltd

KEYWORDS: A. glasses, A. inorganic compounds, C. Raman spectroscopy INTRODUCTION Although TeO2 is hardly ever vitrified with a conventional melt quenching technique, glasses with a wide range of composition are formed when it is combined with other metal oxides [1–3]. Recently, we synthesized the Ag2O–MxOy–TeO2 (MxOy 5 WO3, MoO3, P2O5, and B2O3) systems and characterized them from the viewpoint of thermal and electrical properties [4 –7]. They may find potential applications as solid electrolytes in electrochemical

*To whom correspondence should be addressed. Fax: 191-65-777-6126. E-mail: [email protected] edu.sg. 327

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devices such as solid-state batteries and sensors. For better understanding of their structure, we employed Raman spectroscopy, which has emerged in recent years as a powerful tool for the investigation of glass structure [8,9]. The present studies have led us to determine the proportions of the various structural groupings in these glasses and also to investigate the structural modifications with variation in composition and temperature. EXPERIMENTAL PROCEDURE The glass preparation methodology, starting from Ag2NO3, TeO2, NH4H2PO4, WO3, MoO3, and B2O3 and the glass-forming region were described in detail earlier [4 –7]. In our present study, the Raman spectra were obtained from polished samples on a Renishaw micro-Raman spectrophotometer with a new stable 780 nm laser diode system, fiber-optic probe, and GRAMS-based software system. This equipment provides high throughput (.25%) sensitivity as well as 1 mm spatial resolution and with wide wavenumber spectral range. The deconvolution of the spectra was carried out for the digitized scattering data with a microcomputer. For the temperature variation study, the thermostat of the TS 1500 was used with a water cooling system. The samples were maintained at a required temperature, between room temperature and about 500°C, for about 30 min before the measurements were carried out. The molten sample held in a stainless steel ring was illuminated by the laser light and then the Raman scattering light was passed to the spectrophotometer through a quartz glass window at the furnace wall. RESULTS AND DISCUSSION Studies at Room Temperature. The Raman spectrum of the crystalline TeO2 and the 0.30Ag2Oz0.70TeO2 glass are shown in Figures 1a and b, respectively. Figure 1a shows two strong peaks at about 400 and 650 cm21 and two weak peaks at about 240 and 600 cm21. These peaks are attributed to a TeO4 tbp structural unit in crystalline state. Figure 1b consists of only two strong peaks at about 460 and 700 cm21, with a shoulder peak at about 650 cm21. Comparison of the peaks in Figures 1a and b shows a considerable shift in the peak position and change in the width when the modifier was added to TeO2. This shift is attributed to the difference in the nature of the samples (glass or crystalline) and also to the effect of Ag2O. For the purpose of qualitative analysis, the peak at about 700 cm21 is further deconvoluted into several Gaussian peaks as shown in Figure 2 and the peak assignments are given in Table 1. The other glasses with different modifier content (,30 mol%) gave similar Raman profiles. Therefore, the peaks at 455, 610, and 660 cm21 were assigned to TeO4 tbp, while 720 and 780 cm21 were assigned to TeO3 tp structural units such as O2/2Te¢O and O1/2Te(¢O)–O2. Thus, the present binary silver tellurite glasses were composed of TeO4 tbp and TeO3 tp structural units. Similar Raman profiles are seen in other tellurite systems such as Li2O–TeO2 and Na2O–TeO2 [1,2,10]. However, the addition of MoO3 to TeO2 results in a sharp peak at about 920 cm21 and broad peaks at 820 and 350 cm21, along with peaks due to TeO4 tbp and TeO3 tp units. As the MoO3 content increases, the sharp peak becomes intense and shifts to the higher wavenumber region up to 940 cm21. Similarly, in the binary tungsten tellurite glasses (WO3–TeO2) new peaks are observed at about 930, 850, and 350 cm21, in addition to the peaks at about 770, 670, and 460 cm21 due to TeO3 tp and TeO4 tbp units. The relative intensity of the 770 cm21 peak versus the 670 cm21 peak apparently increased and the intensity of 460 cm21 peak decreased with the increase in WO3 or MoO3

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FIG. 1 The Raman spectrum of (a) the crystalline TeO2 and (b) the 0.30Ag2Oz0.70TeO2 glass.

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FIG. 2 The experimental and deconvoluted Raman spectrum for the 0.30Ag2Oz0.70TeO2 glass. content (similar to the alkali tellurite glasses). This suggests that the conversion of TeO4 tbp into TeO3 tp occurs with the gradual replacement of TeO2 by WO3 or MoO3 in the WO3–TeO2 and MoO3–TeO2 systems, respectively. The observed sharp peak at 930 cm21 and broad peaks at 850 and 350 cm21 are attributed to W–O2 and W¢O in the WO4 tetrahedra, W–O–W in WO4 or WO6 units, and corner-shared WO6 octahedra, respectively. These additional peaks observed in WO3–TeO2 systems are referred to as peaks Y, X, and Z, respectively. Figure 3 shows the Raman spectrum of a ternary silver molybdotellurite glass as an example. TABLE 1 Raman Band Assignments for the Ag2OzTeO2 Glasses Peak

Wavenumber (cm21)

Vibrational mode

A B C D E

770–780 720–760 650–665 600–615 400–500

Stretching vibrations of TeO3 tp Stretching vibrations of TeO3 tp Stretching vibrations of TeO4 tbp Stretching vibrations of TeO4 tbp Bending vibrations of Te–O–Te or O–Te–O linkages

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FIG. 3 The experimental and deconvoluted Raman spectrum of the 0.30Ag2Oz0.14MoO3z0.56TeO2 glass.

Raman spectra of 0.40Ag2Oz0.60[xP2O5z(12x)TeO2] glasses are shown in Figure 4. It is observed from Figure 4 that the initial addition of P2O5 resulted in a new broad peak at about 900 –1200 cm21 and weak peaks at about 178 and 280 cm21 due to P–O linkages in addition to the Te–O linkages. The intensities of the peaks observed in the 900 –1300 cm21 region gradually increased with increasing P2O5 content, but were weaker than those observed in the lower wavenumber region. The peak due to P¢O stretching vibration, which is usually observed in the 1330 –1450 cm21 region, was present only in the glasses with high P2O5 content. The Raman spectra of phosphotellurite glasses having low P2O5 content consists of peaks at about 780, 660, and 450 cm21 due to the continuous network composed of TeO4 tbp’s and a weak peak at about 900 –1300 cm21 due to PO4. Upon an increase of P2O5 content, new peaks are observed at about 178 and 285 cm21 and at very high P2O5 content, a weak peak due to P¢O is also observed at around 1350 –1450 cm21. Since the present glasses are formed by two network formers with modifier oxide (Ag2O), it is expected that there exist Te–O2 bonds and the peak at 285 cm21 can be assigned to the vibrations of TeO3 tp having NBO atoms. Moreover, in the present case, if most of Te atoms remain as TeO4 tbp’s or change into TeO311 polyhedra or TeO3 tp’s, the Raman peak due to P¢O stretching vibration should be observed in the 1330 –1450 cm21 range. But in practice, P¢O stretching vibration is

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FIG. 4 The Raman spectra of 0.40Ag2Oz0.60[xP2O5z(12x)TeO2] glasses.

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present only in glasses that contain a large amount of P2O5. Since the peaks observed in the wavenumber range of 900 –1300 cm21 have broad widths and lower intensities than the peaks observed in the lower wavenumber region, the peaks at 900 –1300 cm21 are assigned to vibrations of P–O–Te linkages, which lead to an initial increase in the glass transition temperature in these glasses. Because of the small ionic radius of P, the P51 ions have a strong electrostatic field, the PO4 group is characterized by a considerably more compact and stable structure. Hence, a small amount of P2O5 seems to successfully destroy the stability of Te in Te–O bonds. According to Sekiya et al. [11], the structure of the B2O3–TeO2 glasses involves the conversion of BO3 triangle to a BO4 tetrahedron which is concurrent with the formation of a TeO3 tp, and this cooperative structural change plays an important role in glass formation. If all the boron atoms were in triangular coordination, homogeneous glasses would not be formed in the binary B2O3–TeO2 system. The Raman spectroscopic investigation gives evidence for the formation of TeO3 tp’s in this glass system. The Raman spectra of Ag2O–B2O3–TeO2 glass samples have two strong peaks at 455 and 665 cm21 and a shoulder at about 745 cm21 as well as a very weak shoulder peak at 850 cm21. The 665 cm21 Raman band corresponds to the 635 cm21 infrared peak. The spectral profile is quite similar to that of other TeO2 glass reported earlier [11,12]. Hence, we attribute the bands observed in the ternary borotellurite glasses near 446, 602, and 653 cm21 to Te–O linkages of tbp units 350, 716, and 772 cm21 to the Te–O linkages of tp units with O1/2Te(¢O)–O2 or O2/2Te–O2 configurations, because their relative intensities increase with x value. A weak band observed at 840 cm21 may be related to a Te–O–B mode. Intensity Variation Study. The intensity ratios of the Raman peaks I720/I660 and I780/I660 may represent the ratio of the fractions of TeO3 tp and TeO4 tbp: TeO3/TeO4. Therefore, the structure of tellurite glasses may be examined based on the relation between the intensity ratio of the Raman peaks and of composition. Thus, the compositional variation of the intensity ratio of the Raman peaks I720/I660 and I780/I660 are shown in Figures 5a and b. It has been observed that in the binary systems WO3–TeO2, MoO3–TeO2, and Ag2O–TeO2, both I720/I660 and I780/I660 were found to increase with WO3, MoO3, and Ag2O content. This variation suggests that there is a continuous transformation of the TeO4 tbp to TeO3 tp units. In addition to this, in the WO3–TeO2 system the increase may also result from the formation of Te–O–W linkage. However, in the ternary Ag2O–WO3–TeO2 system, it has been noted that I780/I660 increases, whereas the I720/I660 decreases with the WO3 content for both y 5 0.30 and y 5 0.40. These variations could be visualized as follows: In a system such as Ag2O–WO3–TeO2, the WO3–TeO2 network is modified by Ag2O, which results in the transformation of TeO4 to TeO3, along with the creation of NBOs. On the other hand, the modification of the Te–O–W linkage by Ag2O may cause an observed decrease in I720/I660. The compositional variation of the intensity ratio of the Raman peaks in the phosphotellurite and borotellurite glasses are shown in Figures 6a and b. It is seen from Figure 6a that with an increase in x value for y 5 0.30, the I780/I660 increases, while the I720/I660 decreases. The intensity ratios, however, are found to increase with the initial addition of P2O5 content for y 5 0.40. This variation in Raman peak intensities suggests the structural modification of P2O5–TeO2 network by Ag2O resulting in the transformation of TeO4 to TeO3 with the formation of non-bridging oxygen atoms. On the other hand, the PO4 and TeO4 units crosslink together, leading to the formation of Te–O–P linkages. As we know, in pure TeO2 glass and also in other binary and ternary tellurite glasses a

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FIG. 5 The compositional variation of the intensity ratio of the Raman peaks I720/I660 and I780/I660 for the binary Ag2O–TeO2, WO3–TeO2, and MoO3–TeO2 glasses and the ternary 0.30Ag2Oz0.70[xMO3z(12x)TeO2] glass, where M 5 W and Mo.

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FIG. 6 The compositional variation of the intensity ratio of the Raman peaks I720/I660 and I780/I660 in the 0.30Ag2Oz0.70[xMxOyz(12x)TeO2] glass, where MxOy 5 P2O5 and B2O3. 335

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Raman band observed at about 770 –780 cm21 has been attributed to the Te–O mode of tp units; whereas in borate glasses a Raman band observed near 770 cm21 has been assigned to borate groups involving one or two BO4 units [12]. Therefore, one may assign the 772 cm21 band to such groups, since the difference of 2 cm21 in band position is within experimental uncertainty. It is seen from Figure 6b that I780/I660 increases with x values, indicating that there is a small contribution of BO4 vibrations to the 772 cm21 peak in addition to the normal Te–O vibrations in tellurite glasses. Thus, the change in structural modifications of the glass network is different with the incorporation of different network formers. The relative scattering efficiencies of various structural groupings that are present in the tungstotellurite and molybdotellurite glasses were determined and are plotted in Figure 7. Although there are equal amounts of the two structural groupings in the mixture, for example, 0.30Ag2Oz0.35WO3z0.35TeO2 glass, the peak area associated with WO4 tetrahedra is about three times greater than that associated with TeO4 tbp. Thus, WO4 has a scattering efficiency about three times greater than that of TeO4 tbp. The proportion of a structural grouping is obtained by dividing the associated Gaussian peak area by the corresponding relative scattering efficiency. Figure 7 shows that the proportion of TeO4 tbp decreases while that of WO4 or MoO3 increases as x is increased from 0.0 to 0.6. As we know, the addition of a modifier oxide or a network former to TeO4 results in the transformation of TeO4 to TeO3 with NBO. However, the relative scattering efficiency of TeO3 was found to decrease. A plausible explanation for this is that in a pure TeO2 glass the Raman spectrum consists of peaks at 720 and 780 cm21, although it is composed of only TeO4 tbp structural units. This indicates that there is a contribution of TeO4 tbp to the peak that was assigned to TeO3 tp. The scattering efficiency of TeO4 may be predominant when compared with that of TeO3, thus causing a decrease in the relative scattering efficiency of TeO3 tp with the incorporation of MoO3 or WO3. Temperature Variation Study of Raman Spectra. The local structure of tellurite glasses, which is important in relation to the tellurite glass structure, has been studied recently by using high-temperature Raman spectroscopy [2]. It was found that the Raman spectra did not show significant changes with change in temperature, and the major structural units present in the melt and the corresponding glasses were not very different. It was also found that in some tellurite systems crystallization occurs easily by the heating of glasses, but it hardly ever occurs when the corresponding melts are cooled [12]. This behavior may be related to the structural change of the glasses with crystallization and melting. Hence, the comparison of the structural units that are present in glasses, precipitated crystals, and melts should lead to a clear understanding of the crystallization behavior observed in tellurite system. Figures 8a and b show the temperature dependence of Raman spectra in tungstotellurite glasses when heated from glass to molten state and cooled from molten state to glass, respectively. At ambient temperature (i.e., at 25°C), six main peaks are observed at around 930, 850, 770, 670, 460, and 350 cm21 in the glass spectrum. As we have already discussed in earlier sections, the peaks at about 930, 850, and 350 cm21 are attributed to the W–O linkages, and the peaks at about 770, 670, and 460 cm21 are attributed to the Te–O linkages. Though the spectrum at 232°C is very similar to that at 25°C, the relative intensity of the 770 and 670 cm21 peaks is larger and the intensity of the 460 cm21 peak is smaller in the spectrum at 232°C, compared to that in the spectrum at 25°C. Sharp peaks at about 800, 700, 650, 400, 330, and 240 cm21 are observed at 400°C, which is in between crystallization

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FIG. 7 Compositional dependence of the proportions of the various structural groupings in the 0.30Ag2Oz0.70[xMO3z(12x)TeO2] glass, where M 5 W and Mo. 337

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FIG. 8 Temperature variation study of Raman spectra of 0.30Ag2Oz0.14WO3z0.56TeO2 glass (a) from glass to molten state and (b) from molten state to glass.

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temperature (Tc 5 348°C) and melting temperature (Tm 5 440°C). The peaks at 800, 700, and 330 cm21 are known to be characteristic of WO3 crystal, and the 650 and 400 cm21 peaks are due to TeO2 crystal. Further heating up to 450°C, which is higher than Tm, yielded a spectrum similar to glass spectra at lower temperature, in which the 930 cm21 peak due to W–O terminal bonds is observed again. The peak due to TeO3 appeared to be more intense than that the peak due to TeO4 units. During cooling from the molten state, although the outline of the spectra did not change, the relative intensity of the TeO3 and TeO4 peaks decreased with an increase in 460 cm21 peak as the temperature gradually decreased. The final spectrum is in good agreement with that for the glass before heating. The temperature dependence of silver molybdotellurite glass samples is shown in Figure 9a and b for heating and cooling cycles, respectively. The main peaks are observed at 25°C at about 920, 770, 670, 450, and 320 cm21. While the peaks at about 920 and 320 cm21 are attributed to Mo–O bonds, the peaks at about 770, 670, and 450 cm21 are ascribed to Te–O bonds. Though both the spectra at 212 and 25°C are similar to those in the WO2 system, the relative intensity of the TeO3 to TeO4 peaks was found to be larger, with the disappearance of the peak at 450 cm21. At the temperature between Tc (5 310°C) and Tm (5 430°C), sharp peaks are observed at about 900, 874, 720, 626, and 230 cm21, which are due to MoO3 and TeO2 crystals. On heating further to 450°C, which is slightly higher than Tm, the resulting spectrum obtained is similar to the glass spectra at lower temperatures. The glass samples with different Ag2O content (40 mol%) showed similar behavior in the case of molybdotellurite; whereas in the case of tungstotellurite glasses, the sample remained crystallized on cooling. The temperature variations of Raman spectra of the silver phosphotellurite glasses are shown in Figures 10a and b, in which the heating process from glass to molten state and the cooling process from molten state to glass, respectively, are depicted. Three main peaks are observed in the spectrum of the glass before heating. With the increase of temperature to 284°C, just above the Tg, the intensity of the peak at about 770 cm21 due to the TeO3 tp units tends to be greater than that for the 670 cm21 peak due to the TeO4 tbp units. With heating to 350°C, which is between the crystallization and melt temperature, several sharp peaks at 780, 650, and 400 cm21 are observed. These peak positions agree well with the frequencies reported for the TeO2 crystal. Further heating to 450°C, which is higher than Tm, generated three broad peaks at about 770, 670, and 460 cm21 instead of sharp peaks due to the TeO2 crystal. It is to be noted that the observed relative intensity of the 770 cm21 peak vs. the 670 cm21 peak is much larger and the intensity of the 460 cm21 peaks is much smaller in the melt compared with those in the glassy state at lower temperatures. In Figure 10b, which depicts the cooling process, the spectrum change is continuous from melt to glass, suggesting that the equilibrium liquid was converted through an undercooled liquid to the glassy state without any crystallization. There are three main peaks, at about 770, 670, and 460 cm21, in each spectrum. In order to observe the unique crystallization behavior of the silver borotellurite glasses, we have undertaken a study of temperature dependence of the Raman spectra. However, due to the limitations of the experimental setup for the temperature study, we were unable to reach the melt temperature of these glasses. Thus, the samples remained in a crystalline state; but the spectra at temperatures closer to the glass transition temperature were found to have a profile similar to that of the spectra at 25°C, except with an increase in the intensity of the peak at 780 cm21.

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FIG. 9 Temperature variation study of Raman spectra of 0.30Ag2Oz0.14MoO3z0.56TeO2 glass (a) from glass to molten state and (b) from molten state to glass.

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FIG. 10 Temperature variation study of Raman spectra of 0.30Ag2Oz0.14P2O5z0.56TeO2 glass (a) from glass to molten state and (b) from molten state to glass.

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CONCLUSIONS The presence of the various structural groups such as TeO4, TeO3, WO4, WO6, MoO4, MoO6, PO4, BO3, and BO4 polyhedrons were identified from detailed analysis of the Raman spectra. The Raman intensity ratio indicating the fraction of tp units is found to vary systematically with the composition. The temperature variation study of Raman spectra suggests the unique crystallization behavior of tellurite glasses. It is concluded that the TeO3 tp units in these glasses increases continuously with increase in temperature. The relative amounts of the TeO3 tp units compared with the TeO4 tbp units are much larger in melts than in the corresponding glasses. This behavior contradicts that of other glass-forming systems. ACKNOWLEDGMENTS The authors wish to thank Dr. Shen for enabling us to do the study of Raman spectra in his laboratory. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

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