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Glass formation and structure in the system MoO3–Bi2O3–Fe2O3
Journal of Non-Crystalline Solids 231 (1998) 227±233
Glass formation and structure in the system MoO3±Bi2O3±Fe2O3 R. Iordanova a, Y. Dimitriev a b
b,*
, V. Dimitrov b, S. Kassabov a, D. Klissurski
a
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, So®a 1113, Bulgaria University of Chemical Technology and Metallurgy, Blvd. `Kliment Ohridski' 8, So®a 1756, Bulgaria Received 8 September 1997; received in revised form 23 February 1998
Abstract The glass formation region in the MoO3 ±Bi2 O3 ±Fe2 O3 system was investigated by the roller-quenching method. Glasses melted at temperatures <1400°C were obtained in the MoO3 -rich compositions. Analysis of the glasses was made by X-ray diraction, dierential thermal analysis (DTA), infra-red spectroscopy (IR), and M ossbauer spectroscopy. According to the DTA data, the glass transformation temperature, Tg , for the dierent compositions varies between 360°C and 440°C and the crystallization temperature, Tx , is in the range 390±470°C. Structural model for the glasses are suggested on the basis of IR and M ossbauer spectral data. In the region of MoO3 rich compositions, the network forming units MoO6 are connected by Mo±O±Mo bridging bonds. The presence of Me2 O3 (Me Bi, Fe) leads to transformation of MoO6 to MoO4 . Thus, in a wide region of compositions the glass network has a scheelite-like structure containing isolated MoO4 structural units which are surrounded by MeO6 groups. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The oxides MoO3 , Bi2 O3 and Fe2 O3 , are known as un-conventional network formers. The ®rst MoO3 glass was obtained by Sarjeant and Roy using a rapid quenching technique [1]. The ®rst glasses based on Bi2 O3 and Fe2 O3 without the participation of a conventional glass former were synthesized by Dumbaugh [2,3] and Kantor et al. [4], respectively. More recently, a series of binary and multicomponent un-conventional glasses have been prepared using MoO3 , Bi2 O3 and Fe2 O3 [5±9].
* Corresponding author. Tel.: +359-2 625 4234; fax: +359-2 685 488.
0022-3093/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 4 5 6 - 6
The purpose of the present work is to study the glass-forming ability of these oxides and their role in the formation of a glass network. 2. Experimental procedure All batches were prepared using MoO3 , Bi2 O3 and Fe2 O3 (reagent grade) as starting materials. The batches, homogenized in an agate mill, were melted for 10 min in air within the temperature range of 900±1300°C in corundum crucibles. Owing to crystallization of the melts, glasses were obtained at cooling rates between 104 and 105 K/s using a roller-quenching technique. Fragmented 1±3 mm ¯at pieces, about 50±100 lm thick, were produced. The amorphous state of the samples was established by X-ray phase analysis (Dron-3,
228
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
diractometer Co Ka radiation) and dierential thermal analysis (DTA) (Paulic Paulic Erdey derivatograph). Infra-red (IR) spectra of the glasses were recorded on a double-beam spectrophotometer (Specord M80) within the range of 1300±300 cmÿ1 . The samples were spectrally investigated both by duplicate determination in nujol mulls and by the KBr disk technique. The recording accuracy of the absorption maxima was 3 cmÿ1 for the glass and 1.5 cmÿ1 for the crystalline samples. To compare the IR spectra of an amorphous and a crystalline Bi3 (FeO4 )(MoO4 )2 phase, a crystalline compound was synthesized by a solid phase reaction between Bi2 O3 , Fe2 O3 and MoO3 at 850°C ossbauer for 15 h. The room-temperature 57 Fe M absorption spectra were recorded with powdered samples on a constant acceleration spectrometer in a standard transmission geometry. A c-ray source, 57 Co/Cr, of about 25 mCi activity was used. The spectrometer was calibrated using a sodium nitroprusside (SNP) reference sample and all observed isomer shifts (IS) are given against this standard. The 512-channel spectra were computer processed and resolved to single Lorentzian lines by a non-linear least-squares minimization
procedure. Chi-square values were utilized to check the quality of the curve-®tting. 3. Results Fig. 1 shows the glass-formation region in the MoO3 ±Bi2 O3 ±Fe2 O3 system. Stable glasses containing no crystalline inclusions are obtained in the region of samples with the largest MoO3 content. The structures of the three-component samples were analysed for compositions at the intersections A, B and C, with a constant Bi2 O3 content (10, 20, 30 mol%) and Fe2 O3 content increasing at the expense of MoO3 . The DTA data (Fig. 2) of the samples from the system have endothermic peaks preceding crystallization, from which the parameters of the glassy state can be determined [10±12]. The in¯ection point of the ®rst endothermic peak corresponds to the glass transformation temperature, Tg , which ranges from 360°C to 440°C, whereas the endothermic minimum is denoted as the softening point, Ts , which varies between 380°C and 450°C. The exothermic eects corresponding to the crys-
Fig. 1. Glass formation range and positions of the investigated compositions in the MoO3 ±Bi2 O3 ±Fe2 O3 system.
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
229
Fig. 3. IR spectra of crystalline and vitreous Bi3 (FeO4 )(MoO4 )2 phases.
Fig. 2. DTA curves of the glasses melted at 900±1300°C in air.
tallization temperature, Tx , are in the range 390± 470°C. After the exothermic peaks due to crystallization, the DTA curves exhibits endothermic eects associated with the melting. The low-temperature endoeects at about 660°C are associated with the appearance of a liquid phase when the eutectic temperature is reached, whereas the endothermic eects at about 750°C are probably due to a liquidus temperature at which the samples are completely melted. Fig. 3 shows the IR spectra of the crystalline phase Bi3 (FeO4 )(MoO4 )2 synthesized by us and of a glass whose composition corresponds to the
same phase. There is good agreement between our spectral data on the crystalline phase and those available in the literature [13]. The IR bands in the spectrum of glassy Bi3 (FeO4 )(MoO4 )2 are broadened, their intensity decreasing simultaneously with their number as a result of the formation of a disordered aperiodic structure. The spectrum of the amorphous sample is characterized by a shoulder at 910 and two broad absorption bands centered at 800 and 500 cmÿ1 . Fig. 4(A)±(C) show the IR spectra of the other glass samples. The spectrum of the MoO3 -rich glass (sample 1) has shoulders at 940 and 560 cmÿ1 and absorption bands at 820 and 780 cmÿ1 . With an increasing Me2 O3 content (samples 2±5), a broad absorption band centered at 800 cmÿ1 appears and becomes predominant in all glass spectra. The new band is distinguished at 480 cmÿ1 . Further increase in the Me2 O3 content (samples 6±10) leads to a shift of the shoulder at 940±920 cmÿ1 . Along with the band at 800 cmÿ1 , a broad band is present in the range of 580±400 cmÿ1 . The M ossbauer spectra of crystalline and glassy Bi3 (FeO4 )(MoO4 )2 phases are shown in Fig. 5(a) and (b). 4. Discussion 4.1. IR spectroscopy studies Comparison of the glass spectra (Figs. 3 and 4) with the spectrum of the crystalline product is
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Fig. 5. M ossbauer spectra of a Bi3 (FeO4 )(MoO4 )2 phase: (a) crystalline; (b) glass. The obtained normalized residual deviations (in standard error units) of resultant ®ts from the experimental data are plotted in ®gures under the spectra. The zero of the velocity scale in the ®gures is related to the source. Fig. 4. IR spectra of glasses on intersects A, B and C: 1 90MoO3 5Bi2 O3 5Fe2 O3 ; 2 ± 80MoO3 10Bi2 O3 10Fe2 O3 ; 3 70MoO3 10Bi2 O3 20Fe2 O3 ; 4 ± 60MoO3 10Bi2 O3 30Fe2 O3 ; 5 70MoO3 20Bi2 O3 10Fe2 O3 ; 6 ± 60MoO3 20Bi2 O3 20Fe2 O3 ; 7 50MoO3 20Bi2 O3 30Fe2 O3 ; 8 ± 60MoO3 30Bi2 O3 10Fe2 O3 ; 9 50MoO3 30Bi2 O3 20Fe2 O3 ; 10 ± 40MoO3 30Bi2 O3 30Fe2 O3 .
± ± ± ± ±
made in accordance with the concept of independent vibration in glasses, discussed in the reviews by Tarte [14] and Condrate [15]. This model assumes that the vibrations of the characteristic
groups of atoms in the glass network are independent of the vibrations of other groups. According to Jeitschko et al. [13], the structure of the crystalline phase Bi3 (FeO4 )(MoO4 )2 is the scheelite-type ABO4 , in which the iron and molybdenum atoms are in tetrahedral cationic sites. The fundamental frequencies for isolated MoO4 groups according to Ref. [16] are m1 894, m2 407, m3 833 and m4 318 cmÿ1 . The characteristic vibrations of FeO4 groups in pure Fe2 O3 and ferrite compounds range from 660 to
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
625 cmÿ1 , while for FeO6 they are at 580±550 and 470 cmÿ1 [17]. The spectral range around 480 cmÿ1 is typical for the normal vibrations of BiO6 groups [9,18]. On the basis of the foregoing, the bands between 880 and 760 cmÿ1 in the IR spectrum of the Bi3 (FeO4 )(MoO4 )2 crystalline phases (Fig. 3) are assumed to be the result of activation of m1 vibrations and elimination of the degeneracy of m3 vibrations of the Mo±O bonds in distorted MoO4 groups probably possessing C2v point symmetry. This spectral result is consistent with the crystal data reported in Ref. [13], according to which two Mo±O (0.1738 and 0.1751 nm) and two longer (0.1779 and 0.1825 nm) Mo±O distances are obtained. The bands in the 570±450 cmÿ1 region are assigned to vibrations of FeO4 [17] and BiO6 groups [9,18]. The shift of bands due to FeO4 groups to lower frequencies is ascribed, to the fact that in the scheelite structure Bi3 (FeO4 )(MoO4 )2 [19] the iron atoms occupy the tetrahedral sites of the molybdenum atoms, as a result of which the FeO4 tetrahedra are distorted. As can be seen (Fig. 3), the IR-spectrum of Bi3 (FeO4 )(MoO4 )2 glass is similar to that of the crystal. This similarity means that the broad absorption band at 800 cmÿ1 in the IR-spectra of the glassy sample may be attributed to the triply degenerate m3 vibration of isolated MoO4 , while the band at 910 cmÿ1 is due to the m1 vibration of the same groups. The similarity of the spectra of the three-component glasses with largest Bi2 O3 content (Fig. 4, compositions 6±10) and those of the amorphous and crystalline Bi3 (FeO4 )(MoO4 )2 samples is an indication of structural similarity. In the spectra of the glasses with the largest MoO3 content (Fig. 4, compositions 1±5) the shoulder at 920 cmÿ1 is shifted to 940 cmÿ1 . According to the IR spectral analysis of pure MoO3 [20] and IR spectral data on multicomponent V2 O5 ±MoO3 ±Bi2 O3 [7] glasses, this shoulder is probably due to the vibration of a short Mo@O bond of deformed MoO6 groups. The band at 820 cmÿ1 in the spectrum of the MoO3 -rich glass (Fig. 4 sample 1) is assigned to the antisymmetric stretching vibration of a Mo±short Olong ±Mo bridge associated with MoO6 octahedra having a Mo@O bond [21]. Taking into account the infrared spectroscopy study of the
231
phases with scheelite-type structure [22] and the scheelite-like structure of V2 O5 ±MoO3 ±Bi2 O3 glasses [7], the band at 780 cmÿ1 (Fig. 4 sample 1) is related to the vibration of isolated MoO4 groups. Such groups means that addition of 5 mol% Bi2 O3 and 5 mol% Fe2 O3 to MoO3 results in the cleavage of the Mo±O±Mo bridge and part of the MoO6 groups are transformed into MoO4 groups. With further increase in Me2 O3 amount these groups become dominant over a range of concentrations. The interpretation of the bands in the 580± 400 cmÿ1 range is complicated. In view of the structure and IR spectrum of the Bi3 (FeO4 )(MoO4 )2 crystalline phase, these bands in the glass spectra (Figs. 3 and 4) are assigned to vibrations of FeO4 groups. On the other hand, vibrational frequencies of FeO6 groups present in pure Fe2 O3 and its compounds, are, as was pointed out above, at 565 and 470 cmÿ1 . Hence, the presence of FeO6 in the glasses cannot be excluded. Simultaneously, taking into account the structure of the crystalline and the vitreous Bi2 O3 containing phases and their IR spectra [18,23,24], the vibrations of Bi±O bonds of distorted BiO6 octahedra should also be within this range. On this basis we assume that in our samples the Bi3 ion is incorporated into the network as distorted BiO6 groups. 4.2. M ossbauer spectroscopy studies The M ossbauer spectrum of crystalline Bi3 (FeO4 )(MoO4 )2 (Fig. 5(a)) has been ®tted to a single Lorentzian doublet. The analysis indicates that iron occupies only one type of crystallographic site in the compound. The observed isomer shift IS 0.54 mm/s (relative to SNP), 1 and the quadrupole splitting, QS 1.04 mm/s, for the doublet correspond to paramagnetic high-spin Fe3 ions in tetrahedral oxygen coordination [25]. The larger QS indicates distorted tetrahedra. The obtained M ossbauer parameters for this crystalline
1 Subtract 0.26 mm/s to obtain this value with respect to metallic iron.
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compound are nearly the same as those reported previously [13]. The spectrum of the glassy sample with the same composition (Fig. 5(b)) is more complex. This complexity is indicative of the distribution of quadrupole splittings and isomer shifts due to the presence of more than one non-identical crystallographic iron site. The spectral data are ®tted to two overlapping symmetric Fe3 Lorentzian doublets (the four larger lines) and one symmetric Fe2 Lorentzian doublet. Fe3 may occupy both octahedral (Oh) and tetrahedral (Td) sites in glasses [26,27]. For the iron ions with the same valence, the larger their coordination number, the larger their IS. On the other hand, QS of Fe3 (Td), which has a lower symmetry, is larger than in the case of Fe3 (Oh). We have used both IS and QS values as criteria for analysis of the hyper®ne parameters obtained from the spectrum. The doublet with a larger QS 1.18 mm/s and a smaller IS 0.58 mm/s (two outer Lorentzian lines in the central region of the spectrum) has been assigned to the paramagnetic high-spin Fe3 (Td). The doublet assigned to paramagnetic high-spin Fe3 (Oh) (two inner lines) has IS 0.60 mm/s and QS 0.76 mm/s. The observed line widths (full width at half maximum) of 0.45 mm/s for Fe3 (Td) and 0.40 mm/s for Fe3 (Oh) are comparable to those of other glasses [27]. The Fe2 ions have usually larger IS and QS than do Fe3 ions. The third doublet (two smaller lines) with IS 1.29 mm/s, QS 1.88 mm/s and a line width of 0.52 mm/s correspond to paramagnetic high-spin Fe2 in octahedral oxygen coordination [27]. The line width is 0.52 mm/s for Fe2 and this line broadening can be explained by a distribution of bond lengths and angles between Fe2 and oxygen in the polyhedra. The relative distribution of the iron ions in different sites is estimated directly from the absorption area of the corresponding doublets assuming the same recoil-free fraction for the ions. The Fe2 (Oh) fraction is 11%. The Fe3 (Td) and Fe3 (Oh) fractions are 42% and 47%, respectively. The observed larger QS and line widths for iron ions indicate that the polyhedral distortion and the
number of non-identical sites are larger in the glass sample than in the crystal sample. The isomer shifts for Fe3 (Td) and Fe3 (Oh) in the glass samples are larger than the isomer shift for Fe3 (Td) in the crystal. This corresponds perhaps to an increase in ionic character of the Fe3 ±O bond in the glass. The presence of FeO6 groups in the glass network, which was assumed in the interpretation of IR spectra, was con®rmed by the M ossbauer spectral analysis. The presence of two dierently coordinated iron ions in mixed valence glass samples makes the glass structure complicated. 4.3. Structural model Analysis of the IR spectra shows that in the range of compositions rich in MoO3 (Fig. 4 compositions 1±5) the structural units of the amorphous network are MoO6 groups connected by Mo±O±Mo bridging bonds. The introduction of small amounts of Bi2 O3 and Fe2 O3 (10±15 mol%) results in a rapid transformation of MoO6 into MoO4 groups which become determining over a wide concentration range. According to the theory of Zachariasen [28], the existence of tetrahedra connected by bridging bonds is a necessary condition of glass formation. However, in the case under consideration the spectral characteristic of MoO4 is typical of an isolated group. Hence, the question arises how the network is formed and how far the concept about bridging and non-bridging bonds is valid. On the basis of the spectral data we assumed that the MoO4 groups are surrounded by another type of MeO6 complexes. These complexes with relatively longer bonds (vibrations below 600 cmÿ1 ) play, perhaps, the role of modifying units. They break the Mo± O±Mo bridge and form Me±long Oshort ±Mo weaker bonds which are not typical bridging bonds. In this way, depolymerization of the molybdate network and deterioration of the glass formation occur. The presence of tetrahedrally coordinated Fe3 ions (42% of total Fe content) at the substitutional sites of Mo6 ions (by analogy with the crystalline structure) should stimulate the formation of a more stable amorphous network.
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5. Conclusions The glass formation region is located near the MoO3 side. Hence, this oxide is the glass-former in the system. On the basis of the IR and M ossbauer spectra, the existence of several structural units: MoO6 , MoO4 , FeO6 , FeO4 and BiO6 is proved. The simultaneous presence of Bi2 O3 and Fe2 O3 in the compositions leads to transformation of the MoO6 groups to MoO4 groups. Thus, the scheelite structure of isolated MoO4 units prevails over a wide range of compositions. Acknowledgements This research work was supported by the Bulgarian Ministry of Science and Education, Contract MY-X 13, 1996. References [1] [2] [3] [4]
P. Sarjeant, R. Roy, J. Am. Ceram. Soc. 50 (1967) 500. W. Dumbaugh, Phys. Chem. Glasses 19 (6) (1978) 191. W. Dumbaugh, Phys. Chem. Glasses 27 (3) (1986) 119. P. Kantor, A. Revcolevschi, R. Collongues, J. Mater. Sci. 8 (1973) 1359. [5] R. Gossink, Ph.D. Thesis, Eindhoven Technical University, 1971. [6] R. Iordanova, Y. Dimitriev, V. Dimitrov, D. Klissurski, J. Non-Crystallogn. Solids 167 (1994) 74. [7] R. Iordanova, V. Dimitrov, Y. Dimitriev, D. Klissurski, J. Non-Cryst. Solids 180 (1994) 58.
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[8] A. Bishay, C. Maghrabi, Phys. Chem. Glasses 10 (1969) 1. [9] Y. Dimitriev, Ch. Petkov, T. Stoilova, G. Gochev, J. NonCryst. Solids 112 (1989) 120. [10] J. Roberdo, Verres Refract. 21 (1967) 539. [11] J. Bednaric, J. Neely, Phys. Chem. Glasses 23 (1982) 204. [12] W. Shi, P. James, J. Mater. Sci. 28 (1993) 469. [13] W. Jeitschko, W. Sleight, W. McClellan, J. Weiher, Acta Crystallogr. B 32 (1976) 1163. [14] P. Tarte, Spectrochim. Acta 18 (1962) 467. [15] R. Condrate, J. Non-Cryst. Solids 84 (1986) 26. [16] G. Clark, W. Doyle, Spectrochim. Acta 22 (1966) 1441. [17] K. Marinaga, Y. Suginohara, Y. Yanagase, J. Jpn. Inst. Met. 40 (1976) 775. [18] R. Betsch, W. White, Spectrochim. Acta 34A (1978) 505. [19] T. Notermann, G. Keulks, O. Krilov, J. Catal. 39 (1975) 286. [20] C. Barraclough, J. Lewis, R. Nyholm, J. Chem. Soc. 3552 (1959) 13. [21] I. Beattie, T. Gilson, J. Chem. Soc. A (1969) 2322. [22] P. Tarte, M. Liegeois-Duyckaerts, Spectrochim. Acta 28A (1972) 2029. [23] F. Miyaji, T. Yoko, W. White, J. Non-Cryst. Solids 126 (1990) 170. [24] Y. Dimitriev, V. Mihailova, in: A. Duran, F. Navarro (Eds.), Proceedings of the International Congress on Glass, Madrid, vol. 3, Madrid, 1992, p. 293. [25] C. Kurkjian, E. Sigety, Phys. Chem. Glasses 9 (1968) 73; J. Danon, in: V. Goldanski, R. Herber (Eds.), Chemical Applications of M ossbauer Spectroscopy, Academic Press, New York, 1968; R. Ingalls et al., in: G. Shenoy, F. Wagner (Eds.), M ossbauer Isomer Shifts, Ch. 7, NorthHolland, Amsterdam, 1978. [26] R. Levy, C. Lupis, P. Flinn, Phys. Chem. Glasses 17 (1976) 94. [27] C. Kurkjian, J. Non-Cryst. Solids 3 (1970) 157; M. Dyar, Amer. Mineral. 70 (1985) 304; T. Nishida, J. Non-Cryst. Solids 177 (1994) 257. [28] W. Zachariasen, J. Am. Ceram. Soc. 54 (1932) 3841.
Glass formation and structure in the system MoO3±Bi2O3±Fe2O3 R. Iordanova a, Y. Dimitriev a b
b,*
, V. Dimitrov b, S. Kassabov a, D. Klissurski
a
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, So®a 1113, Bulgaria University of Chemical Technology and Metallurgy, Blvd. `Kliment Ohridski' 8, So®a 1756, Bulgaria Received 8 September 1997; received in revised form 23 February 1998
Abstract The glass formation region in the MoO3 ±Bi2 O3 ±Fe2 O3 system was investigated by the roller-quenching method. Glasses melted at temperatures <1400°C were obtained in the MoO3 -rich compositions. Analysis of the glasses was made by X-ray diraction, dierential thermal analysis (DTA), infra-red spectroscopy (IR), and M ossbauer spectroscopy. According to the DTA data, the glass transformation temperature, Tg , for the dierent compositions varies between 360°C and 440°C and the crystallization temperature, Tx , is in the range 390±470°C. Structural model for the glasses are suggested on the basis of IR and M ossbauer spectral data. In the region of MoO3 rich compositions, the network forming units MoO6 are connected by Mo±O±Mo bridging bonds. The presence of Me2 O3 (Me Bi, Fe) leads to transformation of MoO6 to MoO4 . Thus, in a wide region of compositions the glass network has a scheelite-like structure containing isolated MoO4 structural units which are surrounded by MeO6 groups. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The oxides MoO3 , Bi2 O3 and Fe2 O3 , are known as un-conventional network formers. The ®rst MoO3 glass was obtained by Sarjeant and Roy using a rapid quenching technique [1]. The ®rst glasses based on Bi2 O3 and Fe2 O3 without the participation of a conventional glass former were synthesized by Dumbaugh [2,3] and Kantor et al. [4], respectively. More recently, a series of binary and multicomponent un-conventional glasses have been prepared using MoO3 , Bi2 O3 and Fe2 O3 [5±9].
* Corresponding author. Tel.: +359-2 625 4234; fax: +359-2 685 488.
0022-3093/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 4 5 6 - 6
The purpose of the present work is to study the glass-forming ability of these oxides and their role in the formation of a glass network. 2. Experimental procedure All batches were prepared using MoO3 , Bi2 O3 and Fe2 O3 (reagent grade) as starting materials. The batches, homogenized in an agate mill, were melted for 10 min in air within the temperature range of 900±1300°C in corundum crucibles. Owing to crystallization of the melts, glasses were obtained at cooling rates between 104 and 105 K/s using a roller-quenching technique. Fragmented 1±3 mm ¯at pieces, about 50±100 lm thick, were produced. The amorphous state of the samples was established by X-ray phase analysis (Dron-3,
228
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
diractometer Co Ka radiation) and dierential thermal analysis (DTA) (Paulic Paulic Erdey derivatograph). Infra-red (IR) spectra of the glasses were recorded on a double-beam spectrophotometer (Specord M80) within the range of 1300±300 cmÿ1 . The samples were spectrally investigated both by duplicate determination in nujol mulls and by the KBr disk technique. The recording accuracy of the absorption maxima was 3 cmÿ1 for the glass and 1.5 cmÿ1 for the crystalline samples. To compare the IR spectra of an amorphous and a crystalline Bi3 (FeO4 )(MoO4 )2 phase, a crystalline compound was synthesized by a solid phase reaction between Bi2 O3 , Fe2 O3 and MoO3 at 850°C ossbauer for 15 h. The room-temperature 57 Fe M absorption spectra were recorded with powdered samples on a constant acceleration spectrometer in a standard transmission geometry. A c-ray source, 57 Co/Cr, of about 25 mCi activity was used. The spectrometer was calibrated using a sodium nitroprusside (SNP) reference sample and all observed isomer shifts (IS) are given against this standard. The 512-channel spectra were computer processed and resolved to single Lorentzian lines by a non-linear least-squares minimization
procedure. Chi-square values were utilized to check the quality of the curve-®tting. 3. Results Fig. 1 shows the glass-formation region in the MoO3 ±Bi2 O3 ±Fe2 O3 system. Stable glasses containing no crystalline inclusions are obtained in the region of samples with the largest MoO3 content. The structures of the three-component samples were analysed for compositions at the intersections A, B and C, with a constant Bi2 O3 content (10, 20, 30 mol%) and Fe2 O3 content increasing at the expense of MoO3 . The DTA data (Fig. 2) of the samples from the system have endothermic peaks preceding crystallization, from which the parameters of the glassy state can be determined [10±12]. The in¯ection point of the ®rst endothermic peak corresponds to the glass transformation temperature, Tg , which ranges from 360°C to 440°C, whereas the endothermic minimum is denoted as the softening point, Ts , which varies between 380°C and 450°C. The exothermic eects corresponding to the crys-
Fig. 1. Glass formation range and positions of the investigated compositions in the MoO3 ±Bi2 O3 ±Fe2 O3 system.
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
229
Fig. 3. IR spectra of crystalline and vitreous Bi3 (FeO4 )(MoO4 )2 phases.
Fig. 2. DTA curves of the glasses melted at 900±1300°C in air.
tallization temperature, Tx , are in the range 390± 470°C. After the exothermic peaks due to crystallization, the DTA curves exhibits endothermic eects associated with the melting. The low-temperature endoeects at about 660°C are associated with the appearance of a liquid phase when the eutectic temperature is reached, whereas the endothermic eects at about 750°C are probably due to a liquidus temperature at which the samples are completely melted. Fig. 3 shows the IR spectra of the crystalline phase Bi3 (FeO4 )(MoO4 )2 synthesized by us and of a glass whose composition corresponds to the
same phase. There is good agreement between our spectral data on the crystalline phase and those available in the literature [13]. The IR bands in the spectrum of glassy Bi3 (FeO4 )(MoO4 )2 are broadened, their intensity decreasing simultaneously with their number as a result of the formation of a disordered aperiodic structure. The spectrum of the amorphous sample is characterized by a shoulder at 910 and two broad absorption bands centered at 800 and 500 cmÿ1 . Fig. 4(A)±(C) show the IR spectra of the other glass samples. The spectrum of the MoO3 -rich glass (sample 1) has shoulders at 940 and 560 cmÿ1 and absorption bands at 820 and 780 cmÿ1 . With an increasing Me2 O3 content (samples 2±5), a broad absorption band centered at 800 cmÿ1 appears and becomes predominant in all glass spectra. The new band is distinguished at 480 cmÿ1 . Further increase in the Me2 O3 content (samples 6±10) leads to a shift of the shoulder at 940±920 cmÿ1 . Along with the band at 800 cmÿ1 , a broad band is present in the range of 580±400 cmÿ1 . The M ossbauer spectra of crystalline and glassy Bi3 (FeO4 )(MoO4 )2 phases are shown in Fig. 5(a) and (b). 4. Discussion 4.1. IR spectroscopy studies Comparison of the glass spectra (Figs. 3 and 4) with the spectrum of the crystalline product is
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Fig. 5. M ossbauer spectra of a Bi3 (FeO4 )(MoO4 )2 phase: (a) crystalline; (b) glass. The obtained normalized residual deviations (in standard error units) of resultant ®ts from the experimental data are plotted in ®gures under the spectra. The zero of the velocity scale in the ®gures is related to the source. Fig. 4. IR spectra of glasses on intersects A, B and C: 1 90MoO3 5Bi2 O3 5Fe2 O3 ; 2 ± 80MoO3 10Bi2 O3 10Fe2 O3 ; 3 70MoO3 10Bi2 O3 20Fe2 O3 ; 4 ± 60MoO3 10Bi2 O3 30Fe2 O3 ; 5 70MoO3 20Bi2 O3 10Fe2 O3 ; 6 ± 60MoO3 20Bi2 O3 20Fe2 O3 ; 7 50MoO3 20Bi2 O3 30Fe2 O3 ; 8 ± 60MoO3 30Bi2 O3 10Fe2 O3 ; 9 50MoO3 30Bi2 O3 20Fe2 O3 ; 10 ± 40MoO3 30Bi2 O3 30Fe2 O3 .
± ± ± ± ±
made in accordance with the concept of independent vibration in glasses, discussed in the reviews by Tarte [14] and Condrate [15]. This model assumes that the vibrations of the characteristic
groups of atoms in the glass network are independent of the vibrations of other groups. According to Jeitschko et al. [13], the structure of the crystalline phase Bi3 (FeO4 )(MoO4 )2 is the scheelite-type ABO4 , in which the iron and molybdenum atoms are in tetrahedral cationic sites. The fundamental frequencies for isolated MoO4 groups according to Ref. [16] are m1 894, m2 407, m3 833 and m4 318 cmÿ1 . The characteristic vibrations of FeO4 groups in pure Fe2 O3 and ferrite compounds range from 660 to
R. Iordanova et al. / Journal of Non-Crystalline Solids 231 (1998) 227±233
625 cmÿ1 , while for FeO6 they are at 580±550 and 470 cmÿ1 [17]. The spectral range around 480 cmÿ1 is typical for the normal vibrations of BiO6 groups [9,18]. On the basis of the foregoing, the bands between 880 and 760 cmÿ1 in the IR spectrum of the Bi3 (FeO4 )(MoO4 )2 crystalline phases (Fig. 3) are assumed to be the result of activation of m1 vibrations and elimination of the degeneracy of m3 vibrations of the Mo±O bonds in distorted MoO4 groups probably possessing C2v point symmetry. This spectral result is consistent with the crystal data reported in Ref. [13], according to which two Mo±O (0.1738 and 0.1751 nm) and two longer (0.1779 and 0.1825 nm) Mo±O distances are obtained. The bands in the 570±450 cmÿ1 region are assigned to vibrations of FeO4 [17] and BiO6 groups [9,18]. The shift of bands due to FeO4 groups to lower frequencies is ascribed, to the fact that in the scheelite structure Bi3 (FeO4 )(MoO4 )2 [19] the iron atoms occupy the tetrahedral sites of the molybdenum atoms, as a result of which the FeO4 tetrahedra are distorted. As can be seen (Fig. 3), the IR-spectrum of Bi3 (FeO4 )(MoO4 )2 glass is similar to that of the crystal. This similarity means that the broad absorption band at 800 cmÿ1 in the IR-spectra of the glassy sample may be attributed to the triply degenerate m3 vibration of isolated MoO4 , while the band at 910 cmÿ1 is due to the m1 vibration of the same groups. The similarity of the spectra of the three-component glasses with largest Bi2 O3 content (Fig. 4, compositions 6±10) and those of the amorphous and crystalline Bi3 (FeO4 )(MoO4 )2 samples is an indication of structural similarity. In the spectra of the glasses with the largest MoO3 content (Fig. 4, compositions 1±5) the shoulder at 920 cmÿ1 is shifted to 940 cmÿ1 . According to the IR spectral analysis of pure MoO3 [20] and IR spectral data on multicomponent V2 O5 ±MoO3 ±Bi2 O3 [7] glasses, this shoulder is probably due to the vibration of a short Mo@O bond of deformed MoO6 groups. The band at 820 cmÿ1 in the spectrum of the MoO3 -rich glass (Fig. 4 sample 1) is assigned to the antisymmetric stretching vibration of a Mo±short Olong ±Mo bridge associated with MoO6 octahedra having a Mo@O bond [21]. Taking into account the infrared spectroscopy study of the
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phases with scheelite-type structure [22] and the scheelite-like structure of V2 O5 ±MoO3 ±Bi2 O3 glasses [7], the band at 780 cmÿ1 (Fig. 4 sample 1) is related to the vibration of isolated MoO4 groups. Such groups means that addition of 5 mol% Bi2 O3 and 5 mol% Fe2 O3 to MoO3 results in the cleavage of the Mo±O±Mo bridge and part of the MoO6 groups are transformed into MoO4 groups. With further increase in Me2 O3 amount these groups become dominant over a range of concentrations. The interpretation of the bands in the 580± 400 cmÿ1 range is complicated. In view of the structure and IR spectrum of the Bi3 (FeO4 )(MoO4 )2 crystalline phase, these bands in the glass spectra (Figs. 3 and 4) are assigned to vibrations of FeO4 groups. On the other hand, vibrational frequencies of FeO6 groups present in pure Fe2 O3 and its compounds, are, as was pointed out above, at 565 and 470 cmÿ1 . Hence, the presence of FeO6 in the glasses cannot be excluded. Simultaneously, taking into account the structure of the crystalline and the vitreous Bi2 O3 containing phases and their IR spectra [18,23,24], the vibrations of Bi±O bonds of distorted BiO6 octahedra should also be within this range. On this basis we assume that in our samples the Bi3 ion is incorporated into the network as distorted BiO6 groups. 4.2. M ossbauer spectroscopy studies The M ossbauer spectrum of crystalline Bi3 (FeO4 )(MoO4 )2 (Fig. 5(a)) has been ®tted to a single Lorentzian doublet. The analysis indicates that iron occupies only one type of crystallographic site in the compound. The observed isomer shift IS 0.54 mm/s (relative to SNP), 1 and the quadrupole splitting, QS 1.04 mm/s, for the doublet correspond to paramagnetic high-spin Fe3 ions in tetrahedral oxygen coordination [25]. The larger QS indicates distorted tetrahedra. The obtained M ossbauer parameters for this crystalline
1 Subtract 0.26 mm/s to obtain this value with respect to metallic iron.
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compound are nearly the same as those reported previously [13]. The spectrum of the glassy sample with the same composition (Fig. 5(b)) is more complex. This complexity is indicative of the distribution of quadrupole splittings and isomer shifts due to the presence of more than one non-identical crystallographic iron site. The spectral data are ®tted to two overlapping symmetric Fe3 Lorentzian doublets (the four larger lines) and one symmetric Fe2 Lorentzian doublet. Fe3 may occupy both octahedral (Oh) and tetrahedral (Td) sites in glasses [26,27]. For the iron ions with the same valence, the larger their coordination number, the larger their IS. On the other hand, QS of Fe3 (Td), which has a lower symmetry, is larger than in the case of Fe3 (Oh). We have used both IS and QS values as criteria for analysis of the hyper®ne parameters obtained from the spectrum. The doublet with a larger QS 1.18 mm/s and a smaller IS 0.58 mm/s (two outer Lorentzian lines in the central region of the spectrum) has been assigned to the paramagnetic high-spin Fe3 (Td). The doublet assigned to paramagnetic high-spin Fe3 (Oh) (two inner lines) has IS 0.60 mm/s and QS 0.76 mm/s. The observed line widths (full width at half maximum) of 0.45 mm/s for Fe3 (Td) and 0.40 mm/s for Fe3 (Oh) are comparable to those of other glasses [27]. The Fe2 ions have usually larger IS and QS than do Fe3 ions. The third doublet (two smaller lines) with IS 1.29 mm/s, QS 1.88 mm/s and a line width of 0.52 mm/s correspond to paramagnetic high-spin Fe2 in octahedral oxygen coordination [27]. The line width is 0.52 mm/s for Fe2 and this line broadening can be explained by a distribution of bond lengths and angles between Fe2 and oxygen in the polyhedra. The relative distribution of the iron ions in different sites is estimated directly from the absorption area of the corresponding doublets assuming the same recoil-free fraction for the ions. The Fe2 (Oh) fraction is 11%. The Fe3 (Td) and Fe3 (Oh) fractions are 42% and 47%, respectively. The observed larger QS and line widths for iron ions indicate that the polyhedral distortion and the
number of non-identical sites are larger in the glass sample than in the crystal sample. The isomer shifts for Fe3 (Td) and Fe3 (Oh) in the glass samples are larger than the isomer shift for Fe3 (Td) in the crystal. This corresponds perhaps to an increase in ionic character of the Fe3 ±O bond in the glass. The presence of FeO6 groups in the glass network, which was assumed in the interpretation of IR spectra, was con®rmed by the M ossbauer spectral analysis. The presence of two dierently coordinated iron ions in mixed valence glass samples makes the glass structure complicated. 4.3. Structural model Analysis of the IR spectra shows that in the range of compositions rich in MoO3 (Fig. 4 compositions 1±5) the structural units of the amorphous network are MoO6 groups connected by Mo±O±Mo bridging bonds. The introduction of small amounts of Bi2 O3 and Fe2 O3 (10±15 mol%) results in a rapid transformation of MoO6 into MoO4 groups which become determining over a wide concentration range. According to the theory of Zachariasen [28], the existence of tetrahedra connected by bridging bonds is a necessary condition of glass formation. However, in the case under consideration the spectral characteristic of MoO4 is typical of an isolated group. Hence, the question arises how the network is formed and how far the concept about bridging and non-bridging bonds is valid. On the basis of the spectral data we assumed that the MoO4 groups are surrounded by another type of MeO6 complexes. These complexes with relatively longer bonds (vibrations below 600 cmÿ1 ) play, perhaps, the role of modifying units. They break the Mo± O±Mo bridge and form Me±long Oshort ±Mo weaker bonds which are not typical bridging bonds. In this way, depolymerization of the molybdate network and deterioration of the glass formation occur. The presence of tetrahedrally coordinated Fe3 ions (42% of total Fe content) at the substitutional sites of Mo6 ions (by analogy with the crystalline structure) should stimulate the formation of a more stable amorphous network.
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5. Conclusions The glass formation region is located near the MoO3 side. Hence, this oxide is the glass-former in the system. On the basis of the IR and M ossbauer spectra, the existence of several structural units: MoO6 , MoO4 , FeO6 , FeO4 and BiO6 is proved. The simultaneous presence of Bi2 O3 and Fe2 O3 in the compositions leads to transformation of the MoO6 groups to MoO4 groups. Thus, the scheelite structure of isolated MoO4 units prevails over a wide range of compositions. Acknowledgements This research work was supported by the Bulgarian Ministry of Science and Education, Contract MY-X 13, 1996. References [1] [2] [3] [4]
P. Sarjeant, R. Roy, J. Am. Ceram. Soc. 50 (1967) 500. W. Dumbaugh, Phys. Chem. Glasses 19 (6) (1978) 191. W. Dumbaugh, Phys. Chem. Glasses 27 (3) (1986) 119. P. Kantor, A. Revcolevschi, R. Collongues, J. Mater. Sci. 8 (1973) 1359. [5] R. Gossink, Ph.D. Thesis, Eindhoven Technical University, 1971. [6] R. Iordanova, Y. Dimitriev, V. Dimitrov, D. Klissurski, J. Non-Crystallogn. Solids 167 (1994) 74. [7] R. Iordanova, V. Dimitrov, Y. Dimitriev, D. Klissurski, J. Non-Cryst. Solids 180 (1994) 58.
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[8] A. Bishay, C. Maghrabi, Phys. Chem. Glasses 10 (1969) 1. [9] Y. Dimitriev, Ch. Petkov, T. Stoilova, G. Gochev, J. NonCryst. Solids 112 (1989) 120. [10] J. Roberdo, Verres Refract. 21 (1967) 539. [11] J. Bednaric, J. Neely, Phys. Chem. Glasses 23 (1982) 204. [12] W. Shi, P. James, J. Mater. Sci. 28 (1993) 469. [13] W. Jeitschko, W. Sleight, W. McClellan, J. Weiher, Acta Crystallogr. B 32 (1976) 1163. [14] P. Tarte, Spectrochim. Acta 18 (1962) 467. [15] R. Condrate, J. Non-Cryst. Solids 84 (1986) 26. [16] G. Clark, W. Doyle, Spectrochim. Acta 22 (1966) 1441. [17] K. Marinaga, Y. Suginohara, Y. Yanagase, J. Jpn. Inst. Met. 40 (1976) 775. [18] R. Betsch, W. White, Spectrochim. Acta 34A (1978) 505. [19] T. Notermann, G. Keulks, O. Krilov, J. Catal. 39 (1975) 286. [20] C. Barraclough, J. Lewis, R. Nyholm, J. Chem. Soc. 3552 (1959) 13. [21] I. Beattie, T. Gilson, J. Chem. Soc. A (1969) 2322. [22] P. Tarte, M. Liegeois-Duyckaerts, Spectrochim. Acta 28A (1972) 2029. [23] F. Miyaji, T. Yoko, W. White, J. Non-Cryst. Solids 126 (1990) 170. [24] Y. Dimitriev, V. Mihailova, in: A. Duran, F. Navarro (Eds.), Proceedings of the International Congress on Glass, Madrid, vol. 3, Madrid, 1992, p. 293. [25] C. Kurkjian, E. Sigety, Phys. Chem. Glasses 9 (1968) 73; J. Danon, in: V. Goldanski, R. Herber (Eds.), Chemical Applications of M ossbauer Spectroscopy, Academic Press, New York, 1968; R. Ingalls et al., in: G. Shenoy, F. Wagner (Eds.), M ossbauer Isomer Shifts, Ch. 7, NorthHolland, Amsterdam, 1978. [26] R. Levy, C. Lupis, P. Flinn, Phys. Chem. Glasses 17 (1976) 94. [27] C. Kurkjian, J. Non-Cryst. Solids 3 (1970) 157; M. Dyar, Amer. Mineral. 70 (1985) 304; T. Nishida, J. Non-Cryst. Solids 177 (1994) 257. [28] W. Zachariasen, J. Am. Ceram. Soc. 54 (1932) 3841.