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The structure of oxide glasses containing SeO2
Journal of Non-Crystalline Solids 293±295 (2001) 410±415
www.elsevier.com/locate/jnoncrysol
The structure of oxide glasses containing SeO2 Y. Dimitriev a,*, St. Yordanov b, L. Lakov b a
University of Chemical Technology and Metallurgy, `Kliment Ohridski' 8 Blvd., 1756 So®a, Bulgaria b Institute of Metal Science, Bulgarian Academy of Science, So®a 1113, Bulgaria
Abstract The results for glass formation in selenite systems obtained during the last 20 years are reviewed. For all of them the vitri®cation regions are situated near the SeO2 corner, which is the main glass-former. The structure of model compositions is studied to elucidate the role of the dierent building units in the formation of the amorphous network. The IR spectra of binary selenite glasses are compared with those for more complicated compositions. It is proven that it is possible to modify the network of selenite glasses by introducing compatible polyhedra TeO4 ; TeO3 ; VO5 ; BiO6 ; Mo2 O8 or BO3 with SeO3 units. The better glass-forming tendency of compositions between 90 and 50 mol% SeO2 is related to the creation of additional disorder in the SeO3 chains by involving other structural units in them. These new glass-forming units are capable of transforming the structure into layers or three-dimensional random networks with a low atomic mobility. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The idea of synthesizing selenite glasses belongs to Rawson [1] and Stanworth [2] who obtained glasses in the K2 O±SeO2 and SeO2 ±TeO2 ±PbO systems. The main diculty in preparation of glasses of this type is the volatilization from selenite at the liquidus and sublimation of SeO2 at atmospheric pressure at a temperature above 315 °C. Until now, selenite glasses have not been extensively studied. There are few papers [3±10] in which the synthesis and properties of these materials were considered. Besides the volatility, the other main obstacle for practical applications is their hygroscopicity. Nevertheless they are interesting in connection with some general problems of the glassy state. From 1981 to date, we * Corresponding author. Tel.:+359-2 68 11 20; fax: +359-2 68 54 88. E-mail address: [email protected] (Y. Dimitriev).
have investigated the glass formation and the structure of dierent binary and multicomponent selenite systems. Much data have been collected on glass-formation ability, thermal stability and structure. The purpose of this paper is to present a review of the results obtained, and to discuss the problem of network formation in selenite glasses with increasing number of components. The perspectives for new research are pointed out.
2. Experimental procedure Two methods of synthesis of the samples were applied: (i) melting in silica crucibles situated in an autoclave at high pressure; (ii) melting in silica ampoules (volume 2 cm3 ), evacuated at a pressure of P 0:1 Pa [11,12]. The ®rst method is more appropriate for obtaining glasses in large amounts. The gas oxygen pressure during the experiments
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 3 6 - 5
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
411
was maintained at 35 MPa and the maximum temperature was 823 K. The samples were held for 20 min at this temperature and then cooled slowly at the rate of 2±2.5 deg/min. Using the second method it is possible to obtain rapidly cooled specimens. This technique has a preference for obtaining glasses in binary systems in a wider concentration range. 3. Result and discussion 3.1. Glass formation Pure SeO2 was not vitri®ed. The vitri®cation regions for most of the binary systems are too narrow [11]. Stable glasses have been synthesized with MoO3 ; V2 O5 ; B2 O3 , PbO, MgO, La2 O3 ; Sc2 O3 ; Nd2 O3 . On the basis of these initial results were selected a set of three component systems: SeO2 ±TeO2 ±Mn Om ; SeO2 ±V2 O5 ±Mn Om , SeO2 ±MoO3 ±Mn Om and SeO2 ± Bi2 O3 ±Mn Om Mn Om @CuO; MgO; CoO; Sc2 O3 ; Sb2 O3 ; B2 O3 ; Nd2 O3 ; Pr2 O3 ; WO3 . The established boundaries of glass formation are between 10 and 15 mol% of the third component. Widest ranges of glass compositions are obtained in the systems: SeO2 ±TeO2 ±MoO3 ; SeO2 ±TeO2 ±V2 O5 ; SeO2 ±V2 O5 ± MoO3 [12±16]. A special attention is paid to the system SeO2 ±MoO3 ±CuO [15] with high content of CuO (40 mol%) and to SeO2 ±MoO3 ±B2 O3 , where the glass-formation region reaches to pure B2 O3 . The tendency for liquid phase separation was not taken into account in the last system, but these studies are underway. Four component glass-forming systems are combined mainly of the structure-determining oxides, which alone or in combinations with others build up the glass network under the applied special experimental conditions described elsewhere [11,12]. The threedimensional image (Figs. 1±3) of the glass-formation range was obtained in the quaternary systems [17±19]. The area in every tetrahedron representing the vitri®ed compositions is speci®ed by the dotted interfaces above which the crystallization processes take place. Comparing these data it is possible to obtain tentative information for the competitive role of the components. Obviously the
Fig. 1. Glass-formation region in the SeO2 ±TeO2 ±V2 O5 ±MoO3 system.
best glass-former is the SeO2 at applied experimental conditions. 3.2. Glass structure The IR spectroscopy is chosen as the main structural method because it is very sensitive to detect some structural units with characteristic vibrational modes and to recognize the structural transformations with composition. The concept of separate vibrations of speci®c groups of atoms in the network is applied [20]. The IR spectra are interpreted on the basis of structural and spectral data collected from the literature on selenite [21], vanadate [22,23], tellurite [24,25] and molybdate [26,27] compounds. Three model systems SeO2 ±TeO2 ; SeO2 ±MoO3 and SeO2 ± V2 O5 with wide glass-formation ranges have been selected in order to obtain reliable structural information. It could be useful for characterization of the network of more complicated compositions.
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Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
Fig. 2. Glass-formation region in the SeO2 ±V2 O5 ±MoO3 ± Bi2 O3 system.
Fig. 3. Glass-formation region in the SeO2 ±V2 O5 ±MoO3 ±B2 O3 system.
A comparison of the spectra with variation of the composition in the SeO2 ±TeO2 system shows [17] that the stretching vibration band at ms 640 cm 1 typical for TeO4 units is shifted to ms 670 cm 1 which corresponds to the vibration mode of TeO3 isolated groups. This means that a destruction of three-dimensional network takes place. It is accompanied by the formation of SeO3 units with an isolated Se@O bond m 880 cm 1 that are transformed into chains through Se±O±Se bridges. The coordination number of Se against oxygen from the ®rst coordination shell of X-ray radial distribution function (RDF) has been calculated. By this way it was con®rmed that SeO3 units are formed in the amorphous structure [28]. The peak is a complex one and may be reat 3.22±3.30 A lated with Se±Te and Se±Se distances, which are in the SeO2 crystal while it is at 3.6 A for 3.16 A the vitreous TeO2 [29]. The structure of SeO2 ±V2 O5 glasses with increasing of the SeO2 content is characterized by a modi®cation of the network from layers to chains [21]. The VO5 unit participates in all concentration ranges without a direct attack on the isolated V@O bond because its vibration frequency shifts from 1020 to 980 cm 1 only. This conclusion is made in accordance with the spectral data and corresponding models proposed previously for the transformation of the vanadate glass structure with introduction of modi®ers or glass-formers [22]. When SeO2 content increases isolated SeO3 groups (ms 860±800 cm 1 and md 720± 1 700 cm ) are associated into chains. This is proven by the appearance of isolated Se@O bond m 900±880 cm 1 that has been observed in the chains of pure SeO2 . For SeO2 ±MoO3 glasses [15] at low SeO2 content the dominant bands are at about 990 cm 1 that involve the Mo@O stretching modes from MoO6 distorted polyhedra. The band at 840 cm 1 is attributed to the vibrations of Mo±O±Mo bridging bonds. By analogy with glasses in MoO3 ±TeO2 system [26] the appearance of a band at 960±940 cm 1 is connected with the vibrations of Mo2 O8 group that is formed as a result of a destruction of MoO6 octahedra network. The spectra of glasses rich in SeO2 show a new band at
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
413
920±890 cm 1 . Their assignment is the same as for the infrared spectra of the SeO2 ±TeO2 and SeO2 ±V2 O5 systems. It is dicult to make more precise conclusions for the structural units in the central part of the model systems by the IR spectra as some overlapping of the bands takes place. In Figs. 4 and 5 are selected the IR absorption spectra of multi-component glasses. They are not well resolved but nevertheless it is possible to recognize several characteristic bands. They are assigned taking into consideration the spectral data of the model binary glasses. It is reasonable to suppose the existence of the following structural units:
Fig. 5. Infrared spectra of glasses in SeO2 ±TeO2 ±V2 O5 ±MoO3 system.
Fig. 4. Infrared spectra of glasses in SeO2 ±V2 O5 ±MoO3 ±Bi2 O3 system.
VO5 with V@O (m 1000±960 cm 1 ; TeO4 m 650±640 cm 1 ; TeO3 m 680±670cm 1 ; SeO3 m 840±815 cm 1 ; m 700 cm 1 ; Mo2 O8 m 960± 900 cm 1 ; m 520±500 cm 1 . The neutron diraction studies [30,31] of the binary SeO2 ±MoO3 glasses and of more complicated chemically stable compositions show that the ®rst RDF maximum is due to Se±O distances in SeO3 units, but it is slightly shorter than in the model RDF curve for pure SeO2 . The XPS spectra of the selected chemically stable compositions show [19] that the O1s binding energy is centered around 531.5 eV and it is characteristic of oxide materials. This is an im-
414
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
portant additional information on the connection of the atoms in the studied selenite glasses. It is assumed that introduction of dierent Mn Om components does not stimulate the formation of non-bridging bonds, as there is no shift of the single peak toward lower energy. The Se 3d peaks are centered at 59.4 eV which means that the selenite glasses contain only Se4 and evidence for Mo6 mainly is also con®rmed (BE 233.0 eV). 3.3. Topological models The present data for the preparation of stable homogeneous glasses at slow cooling rate con®rmed the good miscibility between dierent kind of polyhedra that stimulate the disorder in the network to be realized too easily. The consequence is a better glass-forming tendency. Actually different topological models are possible to build taking into account the alternative way of bonding of several glass-forming units together. The variety of structural arrangement makes the selenite glasses a suitable object for solving some fundamental problems of the glass structure. It is important to understand the creation of the middle range order and the mutual connectivity of dierent kind of polyhedra. The question concerning the role of the non-bridging bonds remains unsolved yet. That is why it is interesting to synthesize new selenite glasses with introduction of ionic modi®ers.
second network-former a modi®cation of the structure results as a change from chains to layers or to a more complex three-dimensional network. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
4. Conclusions The present investigations indicate that stable homogeneous glasses are obtained with high content of SeO2 in combination with other non-traditional network formers: V2 O5 ; TeO2 ; MoO3 ; Bi2 O3 . According to IR spectra the independent SeO3 pyramids with ms 860±810 cm 1 and md 720±710 cm 1 participate in the network when the SeO2 concentration is low. As the SeO2 content increases SeO3 groups become associated into chains which contain isolated Se@O bonds with a vibration frequency at 900±880 cm 1 . It is proven the good compatibility of dierent structural units with SeO3 polyhedra. Depending on the
[18] [19] [20] [21] [22] [23] [24] [25]
H. Rawson, Phys. Chem. Glasses 1 (1960) 170. I. Stanworth, J. Soc. Glass Technol. 36 (1952) 217. C. Sunandana, A. Bhatnagar, J. Phys. C 17 (1984) 467. C. Angell, Solid State Ionics 18&19 (1986) 72. T. Minami, J. Non-Cryst. Solids 56 (1983) 15. G. Govindaraj, N. Baskaran, K. Shahi, P. Monoravi, Solid State Ionics 75 (1995) 45. G. Govindaraj, N. Baskaran, Mater. Sci. Eng. B 25 (1994) 135. G. Govindaraj, N. Satyanarayana, A. Karthikeayan, J. Mater. Sci. Lett. 9 (1990) 1123. A. Karthikeayan, G. Govindaraj, N. Satyanarayana, Mater. Sci. Eng. B 13 (1992) 295. N. Satyanarayana, G. Govindaraj, A. Karthikeayan, J. Non-Cryst. Solids 136 (1991) 219. L. Lakov, Y. Dimitriev, Phys. Chem. Glasses 22 (1981) 69. L. Lakov, Y. Dimitriev, in: Proceedings of the International Symposium on Glass Problems, Turkey, 1, 1996, p. 487. L. Lakov, Y. Dimitriev, Phys. Chem. Glasses 23 (1982) 76. St. Jordanov, L. Lakov, K. Toncheva, Y. Dimitriev, in: Proceedings of the `Electronic, Optoelectronic and Magnetic Thin Films', Varna, Bulgaria, 1994, p. 415. Y. Dimitriev, St. Yordanov, L. Lakov, J. Non-Cryst. Solids 192&193 (1995) 179. St. Yordanov, L. Lakov, Y. Dimitriev, in: Proceedings of the 12th Conference On Glass and Ceramics, So®a, Bulgaria, 1997, p. 161. Y. Dimitriev, I. Ivanova, V. Dimitrov, L. Lakov, J. Mater. Sci. 21 (1986) 142. St. Yordanov, L. Lakov, Y. Dimitriev, K. Toncheva, in: Proceedings of the `Electronic, Optoelectronic and Magnetic Thin Films', Varna, Bulgaria, 1994, p. 569. St. Yordanov, V. Krastev, L. Lakov, Y. Dimitriev, in: Proceedings of the 12th Confereence On Glass and Ceramics, So®a, Bulgaria, 1997, p. 331. P. Tarte, in: J. Prins (Ed.), Physics of Non-crystalline Solids, North Holland, Amsterdam, 1984, p. 549. Y. Dimitriev, V. Dimitrov, L. Lakov, Phys. Chem. Glasses 29 (1988) 45. Y. Dimitriev, V. Dimitrov, M. Arnaudov, D. Topalov, J. Non-Cryst. Solids 57 (1983) 147. R. Iordanova, Y. Dimitriev, V. Dimitrov, S. Kassabov, D. Klisurski, J. Non-Cryst. Solids 204 (1996) 141. M. Arnaudov, V. Dimitrov, Y. Dimitriev, L. Markova, Mater. Res. Bull. 17 (1982) 1121. Y. Dimitriev, V. Dimitrov, M. Arnaudov, J. Mater. Sci. 18 (1983) 1353.
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415 [26] J. Bart, F. Caariati, A. Sgamellotti, Inorg. Chim. Acta 36 (1979) 105. [27] Y. Dimitriev, J. Bart, V. Dimitrov, M. Arnaudov, Z. Anorg. Allg. Chem. 479 (1981) 229. [28] Y. Dimitriev, V. Dimitrov, L. Lakov, J. Kovachev, Glastechn. Ber. 56K (1983) 881. [29] Y. Dimitriev, V. Dimitrov, E. Gatev, E. Kashchieva, H. Petkov, J. Non-Cryst. Solids 96 (1987) 937.
415
[30] S. Neov, I. Gerasimova, St. Yordanov, L. Lakov, P. Mikula, P. Lukas, Y. Dimitriev, Phys. Chem. Glasses 40 (1999) 111. [31] S. Neov, St. Yordanov, R. Bellissent, M. Bionducci, Y. Dimitriev, L. Lakov, in: Proceedings of the 13th Conference On Glass and Ceramics, So®a, Bulgaria, 1999, p. 146.
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The structure of oxide glasses containing SeO2 Y. Dimitriev a,*, St. Yordanov b, L. Lakov b a
University of Chemical Technology and Metallurgy, `Kliment Ohridski' 8 Blvd., 1756 So®a, Bulgaria b Institute of Metal Science, Bulgarian Academy of Science, So®a 1113, Bulgaria
Abstract The results for glass formation in selenite systems obtained during the last 20 years are reviewed. For all of them the vitri®cation regions are situated near the SeO2 corner, which is the main glass-former. The structure of model compositions is studied to elucidate the role of the dierent building units in the formation of the amorphous network. The IR spectra of binary selenite glasses are compared with those for more complicated compositions. It is proven that it is possible to modify the network of selenite glasses by introducing compatible polyhedra TeO4 ; TeO3 ; VO5 ; BiO6 ; Mo2 O8 or BO3 with SeO3 units. The better glass-forming tendency of compositions between 90 and 50 mol% SeO2 is related to the creation of additional disorder in the SeO3 chains by involving other structural units in them. These new glass-forming units are capable of transforming the structure into layers or three-dimensional random networks with a low atomic mobility. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The idea of synthesizing selenite glasses belongs to Rawson [1] and Stanworth [2] who obtained glasses in the K2 O±SeO2 and SeO2 ±TeO2 ±PbO systems. The main diculty in preparation of glasses of this type is the volatilization from selenite at the liquidus and sublimation of SeO2 at atmospheric pressure at a temperature above 315 °C. Until now, selenite glasses have not been extensively studied. There are few papers [3±10] in which the synthesis and properties of these materials were considered. Besides the volatility, the other main obstacle for practical applications is their hygroscopicity. Nevertheless they are interesting in connection with some general problems of the glassy state. From 1981 to date, we * Corresponding author. Tel.:+359-2 68 11 20; fax: +359-2 68 54 88. E-mail address: [email protected] (Y. Dimitriev).
have investigated the glass formation and the structure of dierent binary and multicomponent selenite systems. Much data have been collected on glass-formation ability, thermal stability and structure. The purpose of this paper is to present a review of the results obtained, and to discuss the problem of network formation in selenite glasses with increasing number of components. The perspectives for new research are pointed out.
2. Experimental procedure Two methods of synthesis of the samples were applied: (i) melting in silica crucibles situated in an autoclave at high pressure; (ii) melting in silica ampoules (volume 2 cm3 ), evacuated at a pressure of P 0:1 Pa [11,12]. The ®rst method is more appropriate for obtaining glasses in large amounts. The gas oxygen pressure during the experiments
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 3 6 - 5
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
411
was maintained at 35 MPa and the maximum temperature was 823 K. The samples were held for 20 min at this temperature and then cooled slowly at the rate of 2±2.5 deg/min. Using the second method it is possible to obtain rapidly cooled specimens. This technique has a preference for obtaining glasses in binary systems in a wider concentration range. 3. Result and discussion 3.1. Glass formation Pure SeO2 was not vitri®ed. The vitri®cation regions for most of the binary systems are too narrow [11]. Stable glasses have been synthesized with MoO3 ; V2 O5 ; B2 O3 , PbO, MgO, La2 O3 ; Sc2 O3 ; Nd2 O3 . On the basis of these initial results were selected a set of three component systems: SeO2 ±TeO2 ±Mn Om ; SeO2 ±V2 O5 ±Mn Om , SeO2 ±MoO3 ±Mn Om and SeO2 ± Bi2 O3 ±Mn Om Mn Om @CuO; MgO; CoO; Sc2 O3 ; Sb2 O3 ; B2 O3 ; Nd2 O3 ; Pr2 O3 ; WO3 . The established boundaries of glass formation are between 10 and 15 mol% of the third component. Widest ranges of glass compositions are obtained in the systems: SeO2 ±TeO2 ±MoO3 ; SeO2 ±TeO2 ±V2 O5 ; SeO2 ±V2 O5 ± MoO3 [12±16]. A special attention is paid to the system SeO2 ±MoO3 ±CuO [15] with high content of CuO (40 mol%) and to SeO2 ±MoO3 ±B2 O3 , where the glass-formation region reaches to pure B2 O3 . The tendency for liquid phase separation was not taken into account in the last system, but these studies are underway. Four component glass-forming systems are combined mainly of the structure-determining oxides, which alone or in combinations with others build up the glass network under the applied special experimental conditions described elsewhere [11,12]. The threedimensional image (Figs. 1±3) of the glass-formation range was obtained in the quaternary systems [17±19]. The area in every tetrahedron representing the vitri®ed compositions is speci®ed by the dotted interfaces above which the crystallization processes take place. Comparing these data it is possible to obtain tentative information for the competitive role of the components. Obviously the
Fig. 1. Glass-formation region in the SeO2 ±TeO2 ±V2 O5 ±MoO3 system.
best glass-former is the SeO2 at applied experimental conditions. 3.2. Glass structure The IR spectroscopy is chosen as the main structural method because it is very sensitive to detect some structural units with characteristic vibrational modes and to recognize the structural transformations with composition. The concept of separate vibrations of speci®c groups of atoms in the network is applied [20]. The IR spectra are interpreted on the basis of structural and spectral data collected from the literature on selenite [21], vanadate [22,23], tellurite [24,25] and molybdate [26,27] compounds. Three model systems SeO2 ±TeO2 ; SeO2 ±MoO3 and SeO2 ± V2 O5 with wide glass-formation ranges have been selected in order to obtain reliable structural information. It could be useful for characterization of the network of more complicated compositions.
412
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
Fig. 2. Glass-formation region in the SeO2 ±V2 O5 ±MoO3 ± Bi2 O3 system.
Fig. 3. Glass-formation region in the SeO2 ±V2 O5 ±MoO3 ±B2 O3 system.
A comparison of the spectra with variation of the composition in the SeO2 ±TeO2 system shows [17] that the stretching vibration band at ms 640 cm 1 typical for TeO4 units is shifted to ms 670 cm 1 which corresponds to the vibration mode of TeO3 isolated groups. This means that a destruction of three-dimensional network takes place. It is accompanied by the formation of SeO3 units with an isolated Se@O bond m 880 cm 1 that are transformed into chains through Se±O±Se bridges. The coordination number of Se against oxygen from the ®rst coordination shell of X-ray radial distribution function (RDF) has been calculated. By this way it was con®rmed that SeO3 units are formed in the amorphous structure [28]. The peak is a complex one and may be reat 3.22±3.30 A lated with Se±Te and Se±Se distances, which are in the SeO2 crystal while it is at 3.6 A for 3.16 A the vitreous TeO2 [29]. The structure of SeO2 ±V2 O5 glasses with increasing of the SeO2 content is characterized by a modi®cation of the network from layers to chains [21]. The VO5 unit participates in all concentration ranges without a direct attack on the isolated V@O bond because its vibration frequency shifts from 1020 to 980 cm 1 only. This conclusion is made in accordance with the spectral data and corresponding models proposed previously for the transformation of the vanadate glass structure with introduction of modi®ers or glass-formers [22]. When SeO2 content increases isolated SeO3 groups (ms 860±800 cm 1 and md 720± 1 700 cm ) are associated into chains. This is proven by the appearance of isolated Se@O bond m 900±880 cm 1 that has been observed in the chains of pure SeO2 . For SeO2 ±MoO3 glasses [15] at low SeO2 content the dominant bands are at about 990 cm 1 that involve the Mo@O stretching modes from MoO6 distorted polyhedra. The band at 840 cm 1 is attributed to the vibrations of Mo±O±Mo bridging bonds. By analogy with glasses in MoO3 ±TeO2 system [26] the appearance of a band at 960±940 cm 1 is connected with the vibrations of Mo2 O8 group that is formed as a result of a destruction of MoO6 octahedra network. The spectra of glasses rich in SeO2 show a new band at
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
413
920±890 cm 1 . Their assignment is the same as for the infrared spectra of the SeO2 ±TeO2 and SeO2 ±V2 O5 systems. It is dicult to make more precise conclusions for the structural units in the central part of the model systems by the IR spectra as some overlapping of the bands takes place. In Figs. 4 and 5 are selected the IR absorption spectra of multi-component glasses. They are not well resolved but nevertheless it is possible to recognize several characteristic bands. They are assigned taking into consideration the spectral data of the model binary glasses. It is reasonable to suppose the existence of the following structural units:
Fig. 5. Infrared spectra of glasses in SeO2 ±TeO2 ±V2 O5 ±MoO3 system.
Fig. 4. Infrared spectra of glasses in SeO2 ±V2 O5 ±MoO3 ±Bi2 O3 system.
VO5 with V@O (m 1000±960 cm 1 ; TeO4 m 650±640 cm 1 ; TeO3 m 680±670cm 1 ; SeO3 m 840±815 cm 1 ; m 700 cm 1 ; Mo2 O8 m 960± 900 cm 1 ; m 520±500 cm 1 . The neutron diraction studies [30,31] of the binary SeO2 ±MoO3 glasses and of more complicated chemically stable compositions show that the ®rst RDF maximum is due to Se±O distances in SeO3 units, but it is slightly shorter than in the model RDF curve for pure SeO2 . The XPS spectra of the selected chemically stable compositions show [19] that the O1s binding energy is centered around 531.5 eV and it is characteristic of oxide materials. This is an im-
414
Y. Dimitriev et al. / Journal of Non-Crystalline Solids 293±295 (2001) 410±415
portant additional information on the connection of the atoms in the studied selenite glasses. It is assumed that introduction of dierent Mn Om components does not stimulate the formation of non-bridging bonds, as there is no shift of the single peak toward lower energy. The Se 3d peaks are centered at 59.4 eV which means that the selenite glasses contain only Se4 and evidence for Mo6 mainly is also con®rmed (BE 233.0 eV). 3.3. Topological models The present data for the preparation of stable homogeneous glasses at slow cooling rate con®rmed the good miscibility between dierent kind of polyhedra that stimulate the disorder in the network to be realized too easily. The consequence is a better glass-forming tendency. Actually different topological models are possible to build taking into account the alternative way of bonding of several glass-forming units together. The variety of structural arrangement makes the selenite glasses a suitable object for solving some fundamental problems of the glass structure. It is important to understand the creation of the middle range order and the mutual connectivity of dierent kind of polyhedra. The question concerning the role of the non-bridging bonds remains unsolved yet. That is why it is interesting to synthesize new selenite glasses with introduction of ionic modi®ers.
second network-former a modi®cation of the structure results as a change from chains to layers or to a more complex three-dimensional network. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
4. Conclusions The present investigations indicate that stable homogeneous glasses are obtained with high content of SeO2 in combination with other non-traditional network formers: V2 O5 ; TeO2 ; MoO3 ; Bi2 O3 . According to IR spectra the independent SeO3 pyramids with ms 860±810 cm 1 and md 720±710 cm 1 participate in the network when the SeO2 concentration is low. As the SeO2 content increases SeO3 groups become associated into chains which contain isolated Se@O bonds with a vibration frequency at 900±880 cm 1 . It is proven the good compatibility of dierent structural units with SeO3 polyhedra. Depending on the
[18] [19] [20] [21] [22] [23] [24] [25]
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