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Spectroscopic study of molybdenum-containing glass structure
98
Journal of Non-Crystalline Solids 111 (1989) 98-102 North-Holland, Amsterdam
S P E C T R O S C O P I C STUDY O F M O L Y B D E N U M - C O N T A I N I N G GLASS S T R U C T U R E N.M. BOBKOVA, I.L. RAKOV and N.P. SOLOVEI S.M. Kirov Byelorussian Technological Institute, 220630, Minsk, USSR Institute of Radio Engineerin~ 220069, Minsk, USSR Received 27 April 1987 Revised manuscript received 21 April 1989
The structure of molybdenum-containing cadmium-aluminoborate glasses was studied using IR spectroscopy, Raman scattering and electronic paramagnetic resonance spectroscopy, depending on the MoO 3 concentration ranging from 5 to 25 mol% and the CdO/B203 ratio. The measurements have shown that the structural net of the synthesized glasses consists mainly of [BOa], [A104] and [SiO4] groups. [BO4] groups may possibly be present when CdO concentrations range between 10 and 20 mol%. Molybdenum is predominantly four fold coordinated. Six fold coordinated molybdenum may exist in glasses with low CdO and high MoO 3 concentrations. The EPR spectra of the glasses have revealed paramagnetic centers which are due to Mo 5+ ions. Octahedral arrangement around Mo 5+ ions is also suggested, with symmetry varying from axial to rhombic depending on chemical compositions.
1. Introduction Molybdenum-containing glasses possess a variety of specific features which arouse interest in view of their applications. It is known that MoO 3 adds to glass semiconductor properties with an n-type conduction. MoO3-containing glasses are also used for the development of optical and radiation glasses and crystalline glass materials. In addition, molybdenum trioxide very efficiently reduces viscosity and surface tension of glasses and makes them fusible. Many of the properties of molybdenum glasses are, in many respects, determined by the structure, valence and coordination states of the molybdenum ions. There are very few reported studies concerning structural details for molybdenum-containing borate and aluminoborate glasses. No information is available on IR spectroscopic and Raman scattering (RS) studies. Reports about the electron paramagnetic spectra (EPR) are also scanty. The present paper contains the results of studies of the structure of C d O - P b O - M o O 3 - A 1 / O 3SiO2-B203 glass systems as a function of MoO 3 concentrations and of C d O / / B 2 0 3 ratios by the above techniques. The mole fractions of 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
aluminium, silicon and lead oxides were kept constant and amounted to 15, 10 and 5 mol%, respectively.
2. Experimental The glasses were synthesized from H3BO3, A 1 2 0 3 , M o O 3, C d O , P b 3 0 4, and S i O 2 components
in corundum crucibles at 1350 + 10 ° C. Vitrification ranges had been measured earlier and are described in ref. [1]. The spectroscopic measurements were performed on the compositions of two series. In one series the CdO content remained constant, and the concentration of MoO 3 (instead of B203) was varied from 5 to 25 mol%. In the second series the MoO 3 content was constant and the CdO//B203 ratio varied. The IR absorption spectra in the range of 400 to 1700 cm - t were obtained with a UR-20 spectrometer. The Raman scattering (RS) spectra were recorded on a double Spex Romalog spectrometer at a photon count of 3 × 1 0 4 / / m l n . The excitation was produced with an argon laser line at ~ = 514.5
99
N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
9ga
nm. The electron paramagnetic resonance (EPR) absorption spectra were recorded by a RE-1306 radiospectrometer in the 3-cm range at room temperature. The signal intensity was determined as the product of the amplitude of the first derivative of the signal by the squared width of the line.
,5
3. Results The IR spectroscopic study of the glasses (fig. 1) shows intense bands in the range of 1200-1500, 1050-1080, 850-890, and 710-720 cm-1, and less pronounced bands between 510 and 530 cm-1 and between 450 and 470 cm-1.
t~ao
/)~,
iaaa
8ao
coo
Fig. 2. RS spectra of C d O - P b O - M o O 3 - A I 2 0 3 - S i O 2 - B 2 0 3 glasses. Introduction of MoO 3 instead of B203 (mol%) at constant CdO concentration of 10%: 0 (1), 5 (2), 10 (3), 20 (4),
25 (5).
/100 t/aa
xgao ttao
~
~ao
¢oa e,e-
Fig. 1. IR absorption spectra of C d O - P b O - M o O 3 - A 1 2 0 3 SiO2-B203 glasses. Introduction of MoO 3 instead of BzO 3 (mol%) at constant CdO concentration of 10%: 5 (1), 10 (2), 15 (3), 20 (4), 25 (4). Introduction of CdO instead of B2o3 (mol%) at constant MoO 3 concentration of 5%: 5 (6), 10 (7), 15 (8), 20 (9), 25 (10), 35 (11), and 40 (12).
A change in the chemical glass composition, from 5 to 40 mol% CdO, in particular (fig. 1, curves 6-12), does not practically affect the 1200-1500 cm -1 absorption. The spectra show only some redistribution of the intensity bands with a maximum at 1240-1280 and 1350-139 cm -1. The experimental data indicate that the glasses containing CdO concentrations between 10-20 mol% are characterized by a more intense maximum at 1350-1390 cm -a and less intense maximum at 1240-1280 cm -1. By contrast, for CdO concentration over 25 mol% the 1240-1280 cm -~ maximum intensity increases and that of 1350-1390 cm -a decreases. Besides, the 10-20 mol% CdO glasses show some increase in the absorption intensity of the band with a 1080 c m maximum. Increasing MoO 3 concentrations from 5 to 25 mol% (fig. 1 curves 1-5) produces a marked variation in the IR spectra of the absorption band with
100
N.M. Bobkova et a L / Spectroscopic study of molybdenum-containing glass structure ¢t
A
#
#
Fig. 3. The shape of EPR spectra of Mo5 + in the tested glasses.
a peak at 880 cm-1. In this case, the narrowing of the band relating to the three-fold coordinated boron is observed. At the same time its intensity increases with a decrease of the B203 concentration. This decrease is revealed especially distinctly in the spectra of glasses with 20-25 mol% MOO3. Upon introduction of MoO 3 into glasses, the RS spectra show absorption bands with maxima at 325 and 960 cm -1 (fig. 2), the intensities of which increases with MoO 3 concentration. Fig. 3 presents the two main types of EPR spectra which are observed. In all cases they are a single non-symmetrical line with a g-factor (g = hv/flH) of 1.921 to 1.932. The symbols have their customary meanings with H being the field at which the maximum of an EPR component occors. The width of these lines varies from 40 to 100 Hs. The shapes of the EPR spectra depend on the MoO 3 concentration in glasses and the C d O / B 2 0 ratio. At a low MoO 3 concentration, the spectrum shape shown in fig. 3(a) predominates and, in the opinion of Simon and Nicula [2], it is characteristic of axial symmetry. With increasing MoO 3 concentration the shape of spectra changes and assumes the form shown in fig. 3(b) which is attributed to rhombic symmetry of the paramagnetic site [2]. In so doing, it is to be noted that with the increased of the MoO 3 concentration to 20 mol%, the EPR signal intensity sharply increases. However, with further increase in the MoO 3 content the signal intensity is approximately invariant. The g-factor values decrease from 1.929 at 5 mol% MoO 3 concentration to 1.925 at 25 mol% MoO 3. The spectra of the second glass series also evidence a change in the EPR spectra shape depending on the CdO/B203 ratio. At the low CdO
content (5 mol%) the spectra correspond to the type shown in fig. 3(b). With increasing CdO concentration the spectra successively vary and at the concentration of CdO > 15 mol% assume the form corresponding to that in fig. 3(a). A complicated dependence of the g-factor is observed upon variation of the CdO content in glasses. Its maximum value (1.932) is specific to the glasses with the 15-20 mol% CdO. The g-factor values decrease with either an increase or decrease of the CdO content.
4. Discussion According to ref. [3] the B - O - B bonds are characterized by absorption bands within 1200 to 1500 cm -1 and 930 and 1100 cm -1 ranges. The intense 1200-1500 cm -1 band of the glasses may be attributed to the absorption band of b o r o n oxygen groups [BO3] , in which boron is three-fold coordinated [4]. The maximum at 1240-1280 and 710 cm -1 is due to formation of [BO3] groups, which are either isolated or included in meta- and pyroborates containing boroxol rings. The [BOa] groups are characterized by the band between 930 and 1100 cm -1 [4,5]. In the same range SiO2-containing glasses exhibit stretching vibrations of S i - O - S i bonds in the framework silicates and in the lamellar or chain intermediates [3]. Weakly-bound oxygen is known to be required for making boron four-fold coordinated. In our study additional oxygen is introduced with cadmium and lead oxides. The experimental data show that an increase of CdO content from 5 to 40 mol% (fig. 1, curves 6-12) does not actually affect the absorption between 1200 and 1500 cm-1. It is most likely that in the glasses boron is threefold coordinated. However, somewhat increased intensity of the bands with maxima at 1080 cm-1 in the compositions with CdO concentrations from 10 to 20 mol% suggests that some boron becomes four-fold coordinated. The 1080 cm -1 maximum in the IR spectra seems to result from superposition of the bands of [ B O ] 4 , [SiO4] tetrahedra and is attributed to the presence of boron-siliconoxygen groups with a high degree of polymerization indicative of their framework structure.
N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
The observed redistribution of intensities of the absorption bands with a maximum at 1350-1390 and 1240-1280 cm -1 (fig. 1, curves 6-12) can be accounted for by a different role of Cd 2+ ions in the glass structure. The behaviour of Cd 2÷ in these glasses seems to be similar to that of Pb 2÷ in borate glasses [6]. In the compositions with 5 to 10% CdO (curves 6, 7) the structural net is presumably represented by solitary borate and molybdate groupings (electron spectroscopy reveals liquid-phase separation in this range). In the compositions with CdO concentration above 25 mol% (fig. 1, curves 11, 12) the Cd 2÷ ions may be considered as components of the glass framework linking various glass groupings by O - C d - O bonds. This linkage results in a greater proportion of solitary [BO3] groups in the glass structure. An absorption band with a maximum at 710-720 cm -1 is attributed to the presence of A I - O bonds in [A1On] tetrahedra [3]. The studies of glasses of the same system with different A1203 contents from 5 to 15 mol% support the above results [7]. The absorption band in the long-wave spectrum of these glasses with a maximum at 450-470 cm-1 may be due to the deformation vibrations of S i - O - S i bonds, while the weak absorption band at 510-530 cm -1 may be ascribed to vibrations of C d - O and P b - O bonds [3]. As the MoO 3 concentration in glasses increases the absorption band intensity with a maximum at 880 cm-~ also increases. This band is indicative of the presence of M o - O bonds in glasses characteristic of the tetrahedral [MoOn] groups. The 960 and 325 cm -a absorption bands of RS spectra also suggest the presence of [MoOn] groups [3,8] where Mo is four-fold coordinated (fig. 2). However, because of its specific structure, molybdenum can easily change its coordinations [9]. The X-ray phase analysis of crystallization products has shown that in the samples with higher MoO 3 contents and with low CdO concentrations, along with cadmium molybdates there are polymolybdates of the RO(MoO3) n type composed of tetrahedral and octahedral molybdenum-oxygen polyhedra [9]. We suggest that the structural net also includes the above polyhedral. It is most likely that in these glasses molybdenum is mainly
101
four-fold coordinated, but in the compositions with higher MoO 3 contents and low CdO concentrations of 5 to 10 mol%, six-fold coordinated molybdenum is expected to exist. It is noteworthy that as the concentration of MoO 3 instead of B203 increases, the IR spectral components of the samples relating to the threefold coordinated boron (fig. 1, curves 1-5) narrow. Its intensity increases although the B203 concentration decreases. This increase may possibly be attributed to the formation of the molybdate component in the glass and transition of some CdO into it from the borate component. The latter is isolated with subsequent formation of a boronenriched phase. Such a structural change is especially distinct in IR spectra (fig. 1, curves 4, 5) and electron microscopic pictures of glasses containing MoO 3 in concentrations above 20 mol%. The existence of CdO and MoO 3 in one phase is supported by X-ray phase analysis of the crystallization products with CdMoO 4 formed as one of the crystalline phases. The spectrum line shape, as well as the g-factor values ranging from 1.921 to 1.932 show that the EPR spectra obtained are associated with Mo 5+ ions in these glasses. The two EPR signal type given in fig. 3 are evidence that, depending on the MoO 3 concentration and C d O / B 2 0 n ratio, the Mo 5+ ions in the glasses can be located in the sites with axial or rhombic symmetry [2]. The studies show that in compositions with low MoO 3 contents the arrangement around Mo 5 + ions correspond to axial or rhombic symmetry depending on the CdO/B203 ratio. As the MoO 3 concentration increases, the spectral shape successively changes and for glasses with MoO 3 concentration over 20 mol% it becomes rhombic. Changes in the symmetry of Mo 5+ surroundings are also observed for the glasses in series 2. The compositions with low CdO concentrations exhibit rhombic symmetry, whereas axial symmetry is observed with higher CdO contents. The compositions with 15-25 mol% CdO show the smallest distortion of symmetry around Mo 5+ ions which may probably be ascribed to four-fold coordinated boron in the glass structure. As CdO concentration increases or decreases, the distortion becomes more pronounced.
102
N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
The authors of ref. [2] dealing with N a 2 0 B 2 0 3 - M o O 3 glasses show that coordination of Mo 5+ in the glass may be inferred from the symmetry of the EPR spectra line of the ion. If the spectral line is symmetric, which corresponds to cubic symmetry around the paramagnetic center, then Mo5+ ions become vitrifiers. In our case the spectra line is not highly symmetric. So, it may be supposed that Mo 5÷ ions are in octahedral coordination in the tested glasses. As MoO 3 concentration increases to 20 mol%, the EPR signal intensity drastically increases which we suggest is due to an increase of the density of paramagnetic centers. In the compositions with MoO 3 > 20 mol% the intensity changes insignificantly. Such a change of the EPR signal intensity in this range is probably associated with the liquid phase separation and formation of two immiscible phases. It seems possible that in this range molybdenum forms its independent component in the glass structure, hindering the formation of new paramagnetic centers. In doing so one cannot deny the probability of a change in the valence state of molybdenum.
5. C o n c l u s i o n s
The following conclusions can be drawn from the studies of CdO-PbO-MoO3-ml203-SiO 2B20 glass system: - The structural net of the glasses is composed of [BO3], [A104] and [SiO4] groups. In the compositions with CdO concentrations of 10 to 20 mol%, some boron becomes four-fold coordinated.
- Molybdenum (Mo 6+) is mainly four-fold coordinated in the glasses. In the range of compositions with higher MoO 3 contents and also with low CdO concentrations along with four-fold coordinated molybdenum, there may be six-fold coordinated Mo. - The glasses contain both Mo 6÷ and Mo 5÷. There are two types of lines in the EPR spectra which correspond to different symmetry around the paramagnetic centers. - The concentrations of paramagnetic centers depend on MoO 3 concentrations and structural properties of the glasses. Liquid-phase separation and onset of two immiscible phase hinder generation of new paramagnetic centers.
References
[1] Z.N. Shalimo, I.L. Rakov, N.P. Solovei and A.P. Molochko, Glass, Glass ceram. Silic. 12 (1983) 8. [2] S. Simon and AI. Nicula. J. Non-Cryst. Solids 57 (1983) 23. [3] I.N. Plyusnina, IR Spectra of Minerals (Moscow State Univ. Press, Moscow, 1977): IR Spectra of Silicates (Moscow State Univ. Press, Moscow, 1967). [4] T.A. Sidorov, Opt. Spectr. 18 (1965) 384. [5] V.A. Kolesova, Phys. Chem. Glasses 12 (1986) 4. [6] M. Naboru and J. Hiroshi, Bull. Just. Chem. Res. Kyoto Univ. 59 (1981) 196. [7] N.M. Bobkova, I.L. Rakov and M.P. Solovei, ZhPS 38 (1983) 1016. [8] S.S. Sheik, G. Aruldhas and H.D. Bist, J. Sol. St. Chem. 48 (1983) 77. [9] M.A. Porai-Koshits and L.U. Atovmyan, Kristal and Stereochemistry of Coordinational Molybdenum Compounds (Nauka, Moscow, 1974).
Journal of Non-Crystalline Solids 111 (1989) 98-102 North-Holland, Amsterdam
S P E C T R O S C O P I C STUDY O F M O L Y B D E N U M - C O N T A I N I N G GLASS S T R U C T U R E N.M. BOBKOVA, I.L. RAKOV and N.P. SOLOVEI S.M. Kirov Byelorussian Technological Institute, 220630, Minsk, USSR Institute of Radio Engineerin~ 220069, Minsk, USSR Received 27 April 1987 Revised manuscript received 21 April 1989
The structure of molybdenum-containing cadmium-aluminoborate glasses was studied using IR spectroscopy, Raman scattering and electronic paramagnetic resonance spectroscopy, depending on the MoO 3 concentration ranging from 5 to 25 mol% and the CdO/B203 ratio. The measurements have shown that the structural net of the synthesized glasses consists mainly of [BOa], [A104] and [SiO4] groups. [BO4] groups may possibly be present when CdO concentrations range between 10 and 20 mol%. Molybdenum is predominantly four fold coordinated. Six fold coordinated molybdenum may exist in glasses with low CdO and high MoO 3 concentrations. The EPR spectra of the glasses have revealed paramagnetic centers which are due to Mo 5+ ions. Octahedral arrangement around Mo 5+ ions is also suggested, with symmetry varying from axial to rhombic depending on chemical compositions.
1. Introduction Molybdenum-containing glasses possess a variety of specific features which arouse interest in view of their applications. It is known that MoO 3 adds to glass semiconductor properties with an n-type conduction. MoO3-containing glasses are also used for the development of optical and radiation glasses and crystalline glass materials. In addition, molybdenum trioxide very efficiently reduces viscosity and surface tension of glasses and makes them fusible. Many of the properties of molybdenum glasses are, in many respects, determined by the structure, valence and coordination states of the molybdenum ions. There are very few reported studies concerning structural details for molybdenum-containing borate and aluminoborate glasses. No information is available on IR spectroscopic and Raman scattering (RS) studies. Reports about the electron paramagnetic spectra (EPR) are also scanty. The present paper contains the results of studies of the structure of C d O - P b O - M o O 3 - A 1 / O 3SiO2-B203 glass systems as a function of MoO 3 concentrations and of C d O / / B 2 0 3 ratios by the above techniques. The mole fractions of 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
aluminium, silicon and lead oxides were kept constant and amounted to 15, 10 and 5 mol%, respectively.
2. Experimental The glasses were synthesized from H3BO3, A 1 2 0 3 , M o O 3, C d O , P b 3 0 4, and S i O 2 components
in corundum crucibles at 1350 + 10 ° C. Vitrification ranges had been measured earlier and are described in ref. [1]. The spectroscopic measurements were performed on the compositions of two series. In one series the CdO content remained constant, and the concentration of MoO 3 (instead of B203) was varied from 5 to 25 mol%. In the second series the MoO 3 content was constant and the CdO//B203 ratio varied. The IR absorption spectra in the range of 400 to 1700 cm - t were obtained with a UR-20 spectrometer. The Raman scattering (RS) spectra were recorded on a double Spex Romalog spectrometer at a photon count of 3 × 1 0 4 / / m l n . The excitation was produced with an argon laser line at ~ = 514.5
99
N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
9ga
nm. The electron paramagnetic resonance (EPR) absorption spectra were recorded by a RE-1306 radiospectrometer in the 3-cm range at room temperature. The signal intensity was determined as the product of the amplitude of the first derivative of the signal by the squared width of the line.
,5
3. Results The IR spectroscopic study of the glasses (fig. 1) shows intense bands in the range of 1200-1500, 1050-1080, 850-890, and 710-720 cm-1, and less pronounced bands between 510 and 530 cm-1 and between 450 and 470 cm-1.
t~ao
/)~,
iaaa
8ao
coo
Fig. 2. RS spectra of C d O - P b O - M o O 3 - A I 2 0 3 - S i O 2 - B 2 0 3 glasses. Introduction of MoO 3 instead of B203 (mol%) at constant CdO concentration of 10%: 0 (1), 5 (2), 10 (3), 20 (4),
25 (5).
/100 t/aa
xgao ttao
~
~ao
¢oa e,e-
Fig. 1. IR absorption spectra of C d O - P b O - M o O 3 - A 1 2 0 3 SiO2-B203 glasses. Introduction of MoO 3 instead of BzO 3 (mol%) at constant CdO concentration of 10%: 5 (1), 10 (2), 15 (3), 20 (4), 25 (4). Introduction of CdO instead of B2o3 (mol%) at constant MoO 3 concentration of 5%: 5 (6), 10 (7), 15 (8), 20 (9), 25 (10), 35 (11), and 40 (12).
A change in the chemical glass composition, from 5 to 40 mol% CdO, in particular (fig. 1, curves 6-12), does not practically affect the 1200-1500 cm -1 absorption. The spectra show only some redistribution of the intensity bands with a maximum at 1240-1280 and 1350-139 cm -1. The experimental data indicate that the glasses containing CdO concentrations between 10-20 mol% are characterized by a more intense maximum at 1350-1390 cm -a and less intense maximum at 1240-1280 cm -1. By contrast, for CdO concentration over 25 mol% the 1240-1280 cm -~ maximum intensity increases and that of 1350-1390 cm -a decreases. Besides, the 10-20 mol% CdO glasses show some increase in the absorption intensity of the band with a 1080 c m maximum. Increasing MoO 3 concentrations from 5 to 25 mol% (fig. 1 curves 1-5) produces a marked variation in the IR spectra of the absorption band with
100
N.M. Bobkova et a L / Spectroscopic study of molybdenum-containing glass structure ¢t
A
#
#
Fig. 3. The shape of EPR spectra of Mo5 + in the tested glasses.
a peak at 880 cm-1. In this case, the narrowing of the band relating to the three-fold coordinated boron is observed. At the same time its intensity increases with a decrease of the B203 concentration. This decrease is revealed especially distinctly in the spectra of glasses with 20-25 mol% MOO3. Upon introduction of MoO 3 into glasses, the RS spectra show absorption bands with maxima at 325 and 960 cm -1 (fig. 2), the intensities of which increases with MoO 3 concentration. Fig. 3 presents the two main types of EPR spectra which are observed. In all cases they are a single non-symmetrical line with a g-factor (g = hv/flH) of 1.921 to 1.932. The symbols have their customary meanings with H being the field at which the maximum of an EPR component occors. The width of these lines varies from 40 to 100 Hs. The shapes of the EPR spectra depend on the MoO 3 concentration in glasses and the C d O / B 2 0 ratio. At a low MoO 3 concentration, the spectrum shape shown in fig. 3(a) predominates and, in the opinion of Simon and Nicula [2], it is characteristic of axial symmetry. With increasing MoO 3 concentration the shape of spectra changes and assumes the form shown in fig. 3(b) which is attributed to rhombic symmetry of the paramagnetic site [2]. In so doing, it is to be noted that with the increased of the MoO 3 concentration to 20 mol%, the EPR signal intensity sharply increases. However, with further increase in the MoO 3 content the signal intensity is approximately invariant. The g-factor values decrease from 1.929 at 5 mol% MoO 3 concentration to 1.925 at 25 mol% MoO 3. The spectra of the second glass series also evidence a change in the EPR spectra shape depending on the CdO/B203 ratio. At the low CdO
content (5 mol%) the spectra correspond to the type shown in fig. 3(b). With increasing CdO concentration the spectra successively vary and at the concentration of CdO > 15 mol% assume the form corresponding to that in fig. 3(a). A complicated dependence of the g-factor is observed upon variation of the CdO content in glasses. Its maximum value (1.932) is specific to the glasses with the 15-20 mol% CdO. The g-factor values decrease with either an increase or decrease of the CdO content.
4. Discussion According to ref. [3] the B - O - B bonds are characterized by absorption bands within 1200 to 1500 cm -1 and 930 and 1100 cm -1 ranges. The intense 1200-1500 cm -1 band of the glasses may be attributed to the absorption band of b o r o n oxygen groups [BO3] , in which boron is three-fold coordinated [4]. The maximum at 1240-1280 and 710 cm -1 is due to formation of [BO3] groups, which are either isolated or included in meta- and pyroborates containing boroxol rings. The [BOa] groups are characterized by the band between 930 and 1100 cm -1 [4,5]. In the same range SiO2-containing glasses exhibit stretching vibrations of S i - O - S i bonds in the framework silicates and in the lamellar or chain intermediates [3]. Weakly-bound oxygen is known to be required for making boron four-fold coordinated. In our study additional oxygen is introduced with cadmium and lead oxides. The experimental data show that an increase of CdO content from 5 to 40 mol% (fig. 1, curves 6-12) does not actually affect the absorption between 1200 and 1500 cm-1. It is most likely that in the glasses boron is threefold coordinated. However, somewhat increased intensity of the bands with maxima at 1080 cm-1 in the compositions with CdO concentrations from 10 to 20 mol% suggests that some boron becomes four-fold coordinated. The 1080 cm -1 maximum in the IR spectra seems to result from superposition of the bands of [ B O ] 4 , [SiO4] tetrahedra and is attributed to the presence of boron-siliconoxygen groups with a high degree of polymerization indicative of their framework structure.
N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
The observed redistribution of intensities of the absorption bands with a maximum at 1350-1390 and 1240-1280 cm -1 (fig. 1, curves 6-12) can be accounted for by a different role of Cd 2+ ions in the glass structure. The behaviour of Cd 2÷ in these glasses seems to be similar to that of Pb 2÷ in borate glasses [6]. In the compositions with 5 to 10% CdO (curves 6, 7) the structural net is presumably represented by solitary borate and molybdate groupings (electron spectroscopy reveals liquid-phase separation in this range). In the compositions with CdO concentration above 25 mol% (fig. 1, curves 11, 12) the Cd 2÷ ions may be considered as components of the glass framework linking various glass groupings by O - C d - O bonds. This linkage results in a greater proportion of solitary [BO3] groups in the glass structure. An absorption band with a maximum at 710-720 cm -1 is attributed to the presence of A I - O bonds in [A1On] tetrahedra [3]. The studies of glasses of the same system with different A1203 contents from 5 to 15 mol% support the above results [7]. The absorption band in the long-wave spectrum of these glasses with a maximum at 450-470 cm-1 may be due to the deformation vibrations of S i - O - S i bonds, while the weak absorption band at 510-530 cm -1 may be ascribed to vibrations of C d - O and P b - O bonds [3]. As the MoO 3 concentration in glasses increases the absorption band intensity with a maximum at 880 cm-~ also increases. This band is indicative of the presence of M o - O bonds in glasses characteristic of the tetrahedral [MoOn] groups. The 960 and 325 cm -a absorption bands of RS spectra also suggest the presence of [MoOn] groups [3,8] where Mo is four-fold coordinated (fig. 2). However, because of its specific structure, molybdenum can easily change its coordinations [9]. The X-ray phase analysis of crystallization products has shown that in the samples with higher MoO 3 contents and with low CdO concentrations, along with cadmium molybdates there are polymolybdates of the RO(MoO3) n type composed of tetrahedral and octahedral molybdenum-oxygen polyhedra [9]. We suggest that the structural net also includes the above polyhedral. It is most likely that in these glasses molybdenum is mainly
101
four-fold coordinated, but in the compositions with higher MoO 3 contents and low CdO concentrations of 5 to 10 mol%, six-fold coordinated molybdenum is expected to exist. It is noteworthy that as the concentration of MoO 3 instead of B203 increases, the IR spectral components of the samples relating to the threefold coordinated boron (fig. 1, curves 1-5) narrow. Its intensity increases although the B203 concentration decreases. This increase may possibly be attributed to the formation of the molybdate component in the glass and transition of some CdO into it from the borate component. The latter is isolated with subsequent formation of a boronenriched phase. Such a structural change is especially distinct in IR spectra (fig. 1, curves 4, 5) and electron microscopic pictures of glasses containing MoO 3 in concentrations above 20 mol%. The existence of CdO and MoO 3 in one phase is supported by X-ray phase analysis of the crystallization products with CdMoO 4 formed as one of the crystalline phases. The spectrum line shape, as well as the g-factor values ranging from 1.921 to 1.932 show that the EPR spectra obtained are associated with Mo 5+ ions in these glasses. The two EPR signal type given in fig. 3 are evidence that, depending on the MoO 3 concentration and C d O / B 2 0 n ratio, the Mo 5+ ions in the glasses can be located in the sites with axial or rhombic symmetry [2]. The studies show that in compositions with low MoO 3 contents the arrangement around Mo 5 + ions correspond to axial or rhombic symmetry depending on the CdO/B203 ratio. As the MoO 3 concentration increases, the spectral shape successively changes and for glasses with MoO 3 concentration over 20 mol% it becomes rhombic. Changes in the symmetry of Mo 5+ surroundings are also observed for the glasses in series 2. The compositions with low CdO concentrations exhibit rhombic symmetry, whereas axial symmetry is observed with higher CdO contents. The compositions with 15-25 mol% CdO show the smallest distortion of symmetry around Mo 5+ ions which may probably be ascribed to four-fold coordinated boron in the glass structure. As CdO concentration increases or decreases, the distortion becomes more pronounced.
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N.M. Bobkova et al. / Spectroscopic study of molybdenum-containing glass structure
The authors of ref. [2] dealing with N a 2 0 B 2 0 3 - M o O 3 glasses show that coordination of Mo 5+ in the glass may be inferred from the symmetry of the EPR spectra line of the ion. If the spectral line is symmetric, which corresponds to cubic symmetry around the paramagnetic center, then Mo5+ ions become vitrifiers. In our case the spectra line is not highly symmetric. So, it may be supposed that Mo 5÷ ions are in octahedral coordination in the tested glasses. As MoO 3 concentration increases to 20 mol%, the EPR signal intensity drastically increases which we suggest is due to an increase of the density of paramagnetic centers. In the compositions with MoO 3 > 20 mol% the intensity changes insignificantly. Such a change of the EPR signal intensity in this range is probably associated with the liquid phase separation and formation of two immiscible phases. It seems possible that in this range molybdenum forms its independent component in the glass structure, hindering the formation of new paramagnetic centers. In doing so one cannot deny the probability of a change in the valence state of molybdenum.
5. C o n c l u s i o n s
The following conclusions can be drawn from the studies of CdO-PbO-MoO3-ml203-SiO 2B20 glass system: - The structural net of the glasses is composed of [BO3], [A104] and [SiO4] groups. In the compositions with CdO concentrations of 10 to 20 mol%, some boron becomes four-fold coordinated.
- Molybdenum (Mo 6+) is mainly four-fold coordinated in the glasses. In the range of compositions with higher MoO 3 contents and also with low CdO concentrations along with four-fold coordinated molybdenum, there may be six-fold coordinated Mo. - The glasses contain both Mo 6÷ and Mo 5÷. There are two types of lines in the EPR spectra which correspond to different symmetry around the paramagnetic centers. - The concentrations of paramagnetic centers depend on MoO 3 concentrations and structural properties of the glasses. Liquid-phase separation and onset of two immiscible phase hinder generation of new paramagnetic centers.
References
[1] Z.N. Shalimo, I.L. Rakov, N.P. Solovei and A.P. Molochko, Glass, Glass ceram. Silic. 12 (1983) 8. [2] S. Simon and AI. Nicula. J. Non-Cryst. Solids 57 (1983) 23. [3] I.N. Plyusnina, IR Spectra of Minerals (Moscow State Univ. Press, Moscow, 1977): IR Spectra of Silicates (Moscow State Univ. Press, Moscow, 1967). [4] T.A. Sidorov, Opt. Spectr. 18 (1965) 384. [5] V.A. Kolesova, Phys. Chem. Glasses 12 (1986) 4. [6] M. Naboru and J. Hiroshi, Bull. Just. Chem. Res. Kyoto Univ. 59 (1981) 196. [7] N.M. Bobkova, I.L. Rakov and M.P. Solovei, ZhPS 38 (1983) 1016. [8] S.S. Sheik, G. Aruldhas and H.D. Bist, J. Sol. St. Chem. 48 (1983) 77. [9] M.A. Porai-Koshits and L.U. Atovmyan, Kristal and Stereochemistry of Coordinational Molybdenum Compounds (Nauka, Moscow, 1974).