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Structural studies of 30Na2O–5SiO2–65[(1 − x)P2O5–xB2O3] glasses by nuclear magnetic resonance, Raman and infrared spectroscopy
Journal of Non-Crystalline Solids 248 (1999) 115±126
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Structural studies of 30Na2O±5SiO2±65[(1 ÿ x)P2O5±xB2O3] glasses by nuclear magnetic resonance, Raman and infrared spectroscopy Hiroshi Yamashita, Keishi Nagata, Hidetake Yoshino, Katsumasa Ono, Takashi Maekawa * Department of Applied Chemistry, Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan Received 25 June 1998; received in revised form 17 March 1999
Abstract The 29 Si, 31 P and 11 B magic angle spinning (MAS) NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses were examined together with Raman and IR data. A correlation of the Raman signal at 1200 cmÿ1 or IR signal at 660 cmÿ1 with the 29 Si NMR around ÿ215 ppm is seen. The 6-coordinated silicon atoms are observed in glasses when x is smaller than 0.250. The 3-coordinated boron atoms begin to appear in glasses with x > 0.500. It is assumed that the phosphate species were made up of six types: O P±é3=2 (P), O P±(ONa)é2=2 (P), (NaO)±P±é3=2 (T), (NaO)2 ±P± é2=2 (T), (NaO)3 ±P±é1=2 (T) and P±é4=2 (T), where é is bridging oxygen and T is either B, P or Si. The distribution curves derived from the deconvolution of the spectra by component signal of each phosphate and borate species agree with those calculated using species with dierent [Na2 O]/[P2 O5 ] ratios when P2 O5 reacts with Na2 O and B2 O3 preferably. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Magic angle spinning (MAS) NMR techniques have been extensively applied to structural studies around network forming elements in various minerals and glasses [1]. The distributions of Si(Qn ) species in silicate glasses, where n is the number of bridging oxygen, have been widely examined [2±8]. In some glasses, the presence of the 6-coordinated silicon atom, denoted hereafter as Si(Q6 ) species, was observed [9±15]. The isotropic chemical shift (r) of the NMR of the Si(Q6 ) species was about ÿ215 ppm, which nearly equals that of a cubic * Corresponding author. Tel.: +81-89 927 9926; fax: +81-89 927 9943; e-mail: [email protected]
SiP2 O7 crystal [9]. Nogami et al. derived a relation between the number of Si(Q6 ) species and ([P2 O5 ] ÿ [SrO])/[SiO2 ] ratio in SrO±P2 O5 ±SiO2 glasses [14]. This was understood by assuming that P2 O5 reacted with SrO preferentially and excess P2 O5 participated in a formation of the Si(Q6 ) species. In a previous paper, a relation between the distribution of the Si(Q6 ) species and the basicity of the glasses was discussed [15]. In a glass of 30Na2 O± 5SiO2 ±65P2 O5 only the Si(Q6 ) species was observed. The presence of the Si(Q6 ) species was seen in glasses where the [P2 O5 ]/[Na2 O] composition ratio was greater than unity. With addition of P2 O5 to sodium silicate glasses, sodium ions attached to the Si(Q3 ) species are expelled by the P(Q3 ) species of P2 O5 to join the formation of the P(Q2 ) species of
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 2 4 1 - 0
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sodium metaphosphate. Thus, the population of the Si(Q4 ) species increased [15]. These results were explained by assuming that P2 O5 was a strong acid compared to SiO2 . Buckermann et al. examined competitive network former eects in Al2 O3 ±B2 O3 ± P2 O5 glasses by the NMR measurement [16]. Generally, B2 O3 is categorized as an acidic oxide in usual alkali borate glasses. Thus, the addition of alkali oxides to B2 O3 converts the 3-coordinated boron atoms to 4-coordinated ones accompanying alkali ions. In the usual alkali phosphate glasses the P(Q4 ) species, where four oxygen atoms attached to the phosphorous atom are all bridging ones seen in crystalline BPO4 , are absent. On the other hand, Dupree et al. showed that the P(Q4 ) species appeared in phosphoalumino silicate glasses [10]. Ducel et al. investigated the structural study of NaPO3 ±Na2 B4 O7 pseudo binary glasses by Raman and IR spectroscopy [17] and 31 P and 11 B NMR [18,19]. The various structural units of borophosphate species were identi®ed. Although the exact distributions of individual phosphate species against compositions could not be made clear, the phosphate species compensated by borate units were distinguished from the 31 P NMR chemical shifts. However, the BPO4 type's cluster or the P(Q4 ) species, the present notation, could not be identi®ed, because the sodium content of the glasses examined were relatively high. In alkali borophosphate glasses of low alkali contents, the P(Q4 ) species can be expected to be formed. In order to see the appearance of the P(Q4 ) species and to see the conditions of the formations of the Si(Q6 ) and P(Q4 ) species, B2 O3 is substituted for P2 O5 in 30Na2 O±5SiO2 ±65P2 O5 glass and the 29 Si, 31 P and 11 B MAS NMR spectra are measured. After the deconvolution of the NMR spectra, the distributions of individual species are compared with those calculated by structural hypotheses based on acid±base concepts. Raman and IR spectra are also measured and are compared with the NMR spectra. 2. Experimental Glasses of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ± xB2 O3 ] (0 6 x 6 1) were prepared. The reagents for
producing glasses were Na2 CO3 , NaPO3 , (NH4 )2 HPO4 , H3 BO3 and SiO2 . The desired quantities of reagents were thoroughly mixed with acetone in a mortar. A small amount of Gd2 O3 (0.05 mol%) was added to batches as a spin relaxation reagent for the 29 Si NMR measurements. After evaporation of acetone, the homogeneously mixed powders were melted in a platinum crucible at 1300°C using an electric furnace and poured into a cold metal plate. The 29 Si, 31 P and 11 B MAS-NMR spectra were obtained by a spectrometer operating at 59.74, 121.7 and 96.47 MHz, respectively. The chemical shift standards for 29 Si, 31 P and 11 B were Si(CH3 )4 , 85% H3 PO3 aqueous solution and BF3 (C2 H5 )2 O, respectively. The ground glass powder was loaded in a zirconia spinner tube and spun at 4±6 kHz. Other experimental details were described in a previous paper [15]. All glasses were also characterized by FT-Raman spectroscopy. The 1064 nm line of a Nd:YAG laser was used as an excitation light source. FT-IR spectra were obtained using a spectrophotometer. X-ray powder patterns were measured with a diractometer. The 29 Si and 31 P NMR spectra were deconvoluted with Gaussian peaks. On the other hand, the deconvolutions of 11 B spectra were obtained by setting values of the isotropic chemical shift, quadrupole coupling constant and quadrupole asymmetric parameters. Preliminary measurements of the 11 B NMR were made on binary sodium and lithium borate glasses and deconvolutions of the spectra were applied. The distribution of 4-coordinated boron atoms in two glass systems could be estimated from the peak areas of the deconvoluted peaks and was in accordance with the reported values [20]. 3. Results 3.1. NMR spectra Fig. 1 represents the selected 29 Si MAS NMR spectra of the examined glasses. The pro®les of the NMR spectra change systematically with x. The NMR spectra of glasses with x 6 0.118 consist of one peak at ÿ215 ppm, that is assigned to the Si(Q6 ) species. The X-ray powder patterns did not show
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
Fig. 1. 29 Si MAS-NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x) P2 O5 ±xB2 O3 ] glasses. The solid and broken lines denote the experimental and deconvolution data, respectively.
crystalline phases in any glasses that contain the Si(Q6 ) species. The signal assigned to the Si(Q4 ) is also observed. When x is greater than 0.250 the signal due to the Si(Q6 ) species disappears and the position of the apparent peak at ÿ115 ppm shifts to higher frequency (lower magnetic ®eld) side. The peak width increases and it seems to consist of one or more components other than the Si(Q4 ) species. The 31 P MAS NMR spectra are shown in Fig. 2. The 31 P NMR spectrum of sodium phosphosilicate glass (x 0) consists of two peaks at ÿ25 and ÿ37 ppm. The former peak is assigned to the P(Q2 ) species. The intensity of the latter peak, which is assigned to the P(Q3 ) species, decreases with an increase in x and these two peaks begin to overlap. The apparent peak position changes gradually to higher frequency and a new peak appears at +5 ppm as a shoulder. Fig. 3 represents the 11 B MAS NMR spectra. NMR spectra of glasses with x 6 0.333 consist of
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Fig. 2. 31 P MAS-NMR spectra of 30Na2 O±5SiO2 ± 65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. These lines are the same as in Fig. 1.
only one symmetric peak assigned to 4-coordinated boron atom and locate at around ÿ2.5 ppm. When x becomes larger than 0.333, the peak position changes to a higher frequency side and 3coordinated boron atoms appear. 3.2. Raman and IR spectra Fig. 4 shows the Raman spectra of the glasses. The peak intensity at 680 cmÿ1 (arrow a) decreases with an increase in x. The peaks at 1200 cmÿ1 (arrow c) and 1350 cmÿ1 (arrow d) decrease until the peaks disappear when x reaches 0.250. The peak at 1160 cmÿ1 (arrow b) is commonly seen in glasses of low x. The peak at 1000 cmÿ1 (arrow f) appears when x becomes greater than 0.500. The intensity at 620 cmÿ1 (arrow e) increases with an increase in B2 O3 content. However, it decreases in
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Fig. 3. 11 B MAS-NMR spectra of 30Na2 O±5SiO2 ± 65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. These lines are the same as in Fig. 1.
Fig. 4. Raman spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ± xB2 O3 ] glasses.
intensity when x becomes 0.8. New peaks at 500 cmÿ1 (arrow i), 780 cmÿ1 (arrow f) and 1100 cmÿ1 (arrow h) appear in B2 O3 rich glasses. Fig. 5 illustrates IR spectra. Absorptions centered on 660 cmÿ1 (arrow a) and 780 cmÿ1 (arrow b), decrease in their intensities with an increase in x. The intensities of a peak at 1350 cmÿ1 (arrow c) shift to lower wave number and decrease with an increase in x. On the other hand, the peak intensity at 700 cmÿ1 (arrow d) increases with an increase in B2 O3 content. The broad peak around 1350 cmÿ1 (arrow e) can be seen in B2 O3 rich glasses.
Na2 O. The acidity of the three acidic oxides increased in the order of SiO2 < B2 O3 < P2 O5 [21]. Thus, one sees that neutralization of the PO4 network by Na2 O proceeds preferably. SiO2 , the weakest acid among the three, does not react with Na2 O when appreciable amounts of P2 O5 and B2 O3 are present. Actually only the Si(Q6 ) and the Si(Q4 ) species are observed in the P2 O5 rich glasses and the Si(Q3 ) and Si(Q2 ) species appear only in glasses with x > 0.300. The acidity of B2 O3 is intermediate among the three. In alkali borosilicate glasses, B2 O3 may react with Na2 O preferentially [22±24]. However, in glasses of low Na2 O content, it is expected that B2 O3 reacts with P2 O5 to form BPO4 type species; thus it behaves as if it were a basic oxide.
4. Discussion 4.1. General consideration The structures of oxide glasses have been interpreted by a concept of acid-base equilibrium. In the present systems, three acidic oxides, P2 O5 , B2 O3 and SiO2 , compete to react with a basic oxide,
4.2. NMR 4.2.1. 29 Si NMR The deconvoluted spectra of the 29 Si NMR are shown in Fig. 1 as broken lines. The distributions
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
Fig. 5. IR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses.
of the structural units derived from the peak area are shown in Fig. 6 as a function of x or r( [P2 O5 ]/([Na2 O] + [B2 O3 ]) 65(1 ÿ x)/(30 + 65x)). The data, which are not shown in Fig. 1, are also plotted. The distribution of the Si(Q6 ) species in 95 [ yNa2 O±(1 ÿ y)P2 O5 ]±5SiO2 ternary glasses [15] are also shown as a function of [P2 O5 ]/ [Na2 O] ((1 ÿ y)/y). In the ternary glasses, the distribution of the Si(Q6 ) species increased rapidly when the [P2 O5 ]/[Na2 O] ratio increased beyond about unity. On the other hand, in the 30Na2 O± 5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glass, the distribution of the Si(Q6 ) species increases abruptly when r, not the [P2 O5 ]/[Na2 O] ratio, increases beyond unity. The distributions of the two glassy systems can be expressed in the same line. If the conditions of the appearance of the Si(Q6 ) species primarily depend on the content of the unreacted P2 O5 , it is expected that B2 O3 reacts preferentially not with Na2 O but with P2 O5 . After the reactions with Na2 O and B2 O3 are completed, excess P2 O5
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Fig. 6. Correlation between glass composition and [Si(Q6 )]/ [Si(Qn )] ratio. The solid and open symbols are by this work and by previous data [15], respectively. Error bar: see text.
may participate in the formation of the Si(Q6 ) species. standard errors, ÿ The normalized s R yiobs ÿ yicalc =Ryiobs , of the deconvolution of the 29 Si NMR were 10% in average. This is due to the low S/N ratio of the observed spectra. The content of Na2 O that was consumed by the formation of Si(Qn ) species with n < 4 can be calculated by 2.5 R(4 ÿ n) ´ [Si(Qn )] and are shown in Fig. 7. The compositional dependences of the chemical shift of the Si(Qn ) species are shown in Fig. 8. The average values of the Si(Qn ) species with n 6 4 are also shown. The average values were evaluated by weighting each peak by its peak area. The chemical shift of the Si(Q4 ) species in P2 O5 rich glasses (ÿ100 ÿ120 ppm) is more negative than those seen in binary alkali silicates glasses (ÿ96 ÿ105 pm) [2]. The positions of individual Si(Qn ) species shift linearly to higher frequency with an increase in x, indicating 29 Si nuclei to be less shielded. This suggests that the electronegative phosphorous atom attached to the silicon tetrahedra is replaced by more electropositive boron
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seen, indicating that the environment around the Si(Q6 ) species does not change much with composition.
Fig. 7. Relationship between B2 O3 content (x) and Na2 O content consumed by the formation of Si(Qn ) species. The experimental errors are based on Fig. 6.
Fig. 8. 29 Si chemical shift for Si(Qn ) species as a function of B2 O3 content (x). The experimental errors of r are 0.5 ppm.
atoms and the number of Si±O±B bonds increase. On the other hand, the composition dependence of the chemical shift of the Si(Q6 ) species cannot be
4.2.2. 31 P NMR Ducel et al. summarized the phosphate species and their notations. In the glasses of zNaPO3 ± (1 ÿ z)Na2 B4 O7 the phosphate species seen in alkali phosphate glasses were dierentiated into two or three sub species [17±19]. The branching species consisted of two types; that is, BP species seen in crystalline P2 O5 (r*ÿ40 ppm) and BB seen in crystalline BPO4 (r*ÿ30 ppm). However, the presence of the BP species could not be con®rmed. Three types of middle species were MP, MB1 (or MB01 ), and MB2 (or MB02 ). The MP species was seen in usual alkali metaphosphate glasses, i.e. the P(Q2 ) species. The MB1 and MB2 were structural units when one boron and one phosphorous atom, or two boron atoms were attached to central phosphate site, respectively. MB01 and MB02 are the structural units in which one or two boron atoms attach to the phosphate unit with two non-bridging oxygen atoms. There were no P@O double bonds in the MB01 and MB02 species by their notation. The reported chemical shifts [17±19] were ÿ20, ÿ10 and ÿ5 ppm for MP, MB1 (or MB01 ) and MB2 (or MB02 ), respectively. There were two end species; i.e., bonded to one phosphorous atom (EP) or one boron atom (EB). Ducel et al. showed that the EP, i.e. P(Q1 ), species could not be formed in B2 O3 rich glasses, because probability of the formation of the P±O±P linking was low [18]. However, the numerical distribution of the each phosphate species against z could not be made. In the present study, we will try to deconvolute the distribution of borate and phosphate species from the NMR spectra. The species used in the present deconvolutions are tabulated in Table 1. Here P[Q4ÿn (nNa)] (n 0, 1, 2 and 3) means the sodium borophosphate unit in which n sodium ions were attached to the PO4 tetrahedron. The notations by Ducel et al. are also shown. Thus, MB01 and MB02 are included into P[Q2 (2Na)] species. The P(Q4 ) are denoted, hereafter, by the P[Q4 (0Na)]. The P(Q2 ) and P(Q3 ) are the structural units usually seen in binary sodium phosphate glasses, so that the same notations are used. In the
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Table 1 The possible formulae and symbols of silicate, phosphate and borate species expected in 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. T:B, P or Si, é: bridging oxygen No 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Unit Si±é6=2 (P) Si±é4=2 (T) (NaO)±Si±é3=2 (T) (NaO)2 ±Si±é2=2 (T) O P±é3=2 (P) O P±(ONa)é2=2 (P) (NaO)±P±é3=2 (T) (NaO)2 ±P±é2=2 (T) (NaO)3 ±P±é1=2 (T) P±é4=2 (B) P±é4=2 (Si) B±é3=2 (B) B±é4=2 (T) Na B±é4=2 (P)
deconvolutions, the following will be taken into consideration. 1. Because of the 30 mol% content of Na2 O, the P(Q4 ) structural unit should be formed by the substitution of B2 O3 in P2 O5 rich glasses. Actually, the spectra could not be well deconvoluted by two Gaussian functions; i.e., assigned to the P(Q2 ) and P(Q3 ) species. We will tentatively assign the third peak, which appears at the position between two peaks of the above two, as a signal due to the P(Q4 ) species (r:ÿ30 ppm, see broken lines in Fig. 2. 2. P@O double bonds should disappear in glasses when the B2 O3 content increases. Thus, the MB01 and MB02 species are preferable in the present glasses rather than MB1 and MB2 species. 3. If the MB01 and MB02 are present, any species that have one non-bridging oxygen and three bridging oxygen atoms can be expected to form. They are further distinguished by the number of boron atoms from one to three. 4. The chemical shifts of 31 P depend primarily on the number of sodium ions attached to the phosphorous atom, so that the notations of the species will be expressed by the number of sodium ions. Fig. 9 is the distribution of each phosphate species against x. Generally the normalized stan-
Symbol of the species This work
Ducel et al.
Si(Q6 ) Si(Q4 ) Si(Q3 ) Si(Q2 ) P(Q3 ) P(Q2 ) P[Q3 (1Na)] P[Q2 (2Na)] P[Q1 (3Na)] P[Q4 (0Na)] P[Q4 (4Si)] B(Q3 ) B[Q4 (1Na)] B[Q4 (0Na)]
BP MP ÿ MB01 ; MB02 EB BB CP , CNa
dard errors of the deconvolution of the NMR spectra were within 2.5%. With an increase in x, the P(Q3 ) species decreases and the P(Q2 ) and P[Q4 (0Na)] species increase. The P[Q3 (1Na)] appears in accordance with the decrease in the P(Q2 )
Fig. 9. Experimentally determined P(Qn ) distribution as a function of B2 O3 content (x). The lines are the calculated ones shown in Table 2. When x < 0.269, the solid and broken lines denote f 0 and 1, respectively. Here, f is the fraction of Si(Q6 ) species. Error bar: see text.
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Fig. 10. Relationship between B2 O3 content (x) and chemical shift (a) or spectral width (b). The experimental errors of r are 0.1 ppm.
species. The chemical shifts and spectral width of each species by deconvolution are shown in Fig. 10(a) and (b). Among the structural units with the same n, the chemical shift moves gradually to the high frequency side with an increase in x, indicating that 31 P nuclei become less shielded with an increase in the number of B2 O3 attached to each phosphate group. The width of the deconvoluted spectra with the same n value has a maximum at the intermediate composition. This is due to the presence of the species of dierent numbers of boron atoms attached to the phosphate unit, whose chemical shifts dier according to the number of boron atoms. 4.2.3. 11 B NMR The fraction of 4-coordinated boron atoms remained 1 for the glasses of z < 0.1 in the (1 ÿ z)NaPO3 ±zNa2 B4 O7 system [17±19]. The fraction of 4-coordinated boron atoms decreased abruptly for z close to 0.6 and tended slowly toward 0.47, corresponding with that of vitreous Na2 B4 O7 . Fig. 11 represents the fraction of 4-coordinated boron atoms obtained by deconvolutions in the present glasses. The normalized standard errors of the deconvolution of the NMR spectra was 5.5% on average. In the (1 ÿ z)
NaPO3 ±zNa2 B4 O7 glasses examined by Ducel et al., r values were less than unity in all z (0 6 z 6 1). Thus, the P[Q4 (0Na)] species might not be seen. In the present study, it is to be noted that
Fig. 11. Experimentally determined B(Qn ) distribution as a function of B2 O3 content (x). The lines are the calculated ones shown in Table 2. Error bar: see text.
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
the population of 4-coordinated boron atoms remains almost 100% even when x increases up to 0.5. However, the peak width of the glass with x 0.500 is somewhat broader and the deconvolution could not succeed with a single peak of quadrupole symmetric factor equal to 1. The presence of the two types of 4-coordinated boron atoms are expected in the glass. This corresponds to the appearance of the P[Q4 ÿ n (nNa)] (n ¹ 0) species in addition to the P[Q4 (0Na)]. The peak position shifts to higher frequency, indicating that 11 B nuclei become less shielded. This is due to the changes of charge compensating group from phosphate (n 0) to electropositive sodium (n P 1). The fraction of 4-coordinated boron atoms in a glass of x 1 coincides fairly well with the value reported in sodium borosilicate glasses [22± 24]. 4.3. Raman and IR spectra As seen in a previous paper, the Raman peaks at 1200 and 1350 cmÿ1 are related to the Si(Q6 ) species [15]. Actually, the appearance of this peak is correlated with the presence of the peak at ÿ215 ppm in the 29 Si NMR spectra. Unfortunately, exact assignment of this signal cannot be made. These Raman peaks decrease in intensity until the peaks disappear when x reaches 0.250. Lee et al. examined Raman spectra of K2 O±Al2 O3 ±P2 O5 glasses and assigned the peak at 1346 cmÿ1 to the P@O stretching mode [25]. The decrease in its intensity should be a consequence of the disruption of the P(Q2 ) species according to the formation of the P[Q4ÿn (n Na)] species. The Raman peak at 1160 cmÿ1 is the symmetric stretching mode of the terminal oxygen, (the P(Q2 ) species), on each tetrahedron. In accordance with 31 P NMR its intensity decreases with an increase in x. The intensity of the other peak at 680 cmÿ1 is due to the (POPsym ) stretching vibration of the P(Q2 ) species [8] and its intensity decreases with an increase in B2 O3 content. The intensity of the shoulder at 770 cmÿ1 , that is due to the BO4 stretching in borate species [17], decreases with an increase in B2 O3 content when x becomes greater than 0.5. On the other hand, the intensity of a new peak at 780 cmÿ1 , assigned to the BO3
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stretching vibration, increases with an increase in B2 O3 content with x > 0.5 [26]. At the same time, the weak bands from 1300 to 1500 cmÿ1 , which are associated with the vibration of 3-coordinated boron atoms [17,27] appear clearly in glasses with x > 0.800. Among the various peaks in IR spectra, the absorption at 660 cmÿ1 is most striking. Although the exact assignment could not be made, this absorption is due to the Si(Q6 ) species [10]. Its intensity decreases with an increase in x. The peak at 1350 cmÿ1 arises from a P@O stretching vibration [11]. This absorption disappears with an increase in x, whose behavior is in accordance with the Raman peak at the same wave number. The absorption at 780 cmÿ1 is due to the bending motion of Si±O±P, P±O±P and O± P±O [11], whose intensity decreases also with an increase in x. Thus, Raman and IR spectra can be used to understand the NMR spectra of each element. 4.4. Numerical estimation of the phosphate and borate species In order to see the quantitative estimation of the distributions of the various species represented by the deconvolution of the NMR spectra, we will calculate the distribution by a structural consideration based on the acid-base concept. The numerical equations for the distribution of each species are tabulated in Table 2. Six regions are distinguished in the calculation. (i) 0 6 x < 0:269: P2 O5 > Na2 O B2 O3 . The species present in the glass with x 0 are the P[Q4 (0Na)], P(Q3 ) and P(Q2 ), and all silicon species are the Si(Q6 ). The following reactions should occur in this region: P2 O5 Pf Q3 g B2 O3 ! 2BPO4 fBQ4 0Na; PQ4 0Nag
1
and P2 O5 fP Q3 g Na2 O ! 2NaPO3 fP Q2 g: 2 Added B2 O3 (65x mol) is used to form the P[Q4 (0Na)] species. All Na2 O (30 mol) reacts
0 130x ÿ 35 ÿ g 30 ÿ g 30 ÿ g 30 ÿ g 30 ÿ g 0 0 0 0 130x ÿ 100 ÿ g 230 ÿ 260x + g
1 35 ÿ 65x + g 35 ÿ 65x + g 0 0 0
P[Q1 (3Na)]
0 130x ÿ 35 ÿ g 30 ÿ g 100 ÿ 130x + g 0 0
0 0 0 65x ÿ 35 ÿ g 165 ÿ 195x + g 0
P(Q2 )
30 65 ÿ 130x 0 0 0 0 35 ÿ 130x ÿ 5f 0 0 0 0 0
P(Q3 ) P[Q4 (0Na)] + P[Q4 (4Si)]
65x + 5f 35 ÿ 65x + g 35 ÿ 65x + g 0 0 0
Symbol x
0 ÿ 0.269 0.269 ÿ 0.5 0.5 ÿ 0.538 0.538 ÿ 0.769 0.769 ÿ 0.846 >0.846
P[Q3 (1Na)]
P[Q2 (2Na)]
B[Q4 (0Na)]
B[Q4 (1Na)]
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126 Table 2 The numerical equations of the distribution of each species. Here, f and g mean the fraction of Si(Q6 ) species and Na2 O content consumed by the formation of Si(Qn ) species, respectively. The g value is calculated from the straight line in Fig. 7. These equations are only shown by a numerator. The denominator of P(Qn ) and B(Qn ) distributions are 65(1 ÿ x) and 65x, respectively
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with P2 O5 (65 ÿ 65x mol) to form the P(Q2 ) species. For example, the fraction of the P(Q3 ) species is shown by P Q3 f65 1 ÿ x ÿ 65x ÿ 30 ÿ 5f g =65 1 ÿ x 35 ÿ 130x ÿ 5f =65 1 ÿ x: 3 Here, f means the fraction of Si(Q6 ) species. When x 0.269, f becomes zero and there are no P(Q3 ) remaining. (ii) 0.269 < x < 0.500 : [Na2 O] + [B2 O3 ] > [P2 O5 ] > [B2 O3 ]. Mainly the following reactions will occur. B2 O3 reacts with the P(Q2 ) species and forms the P[Q3 (1Na)] species such as 2NaPO3 fP Q2 g B2 O3 ! 2NaPBO3 fPQ3 1Nag:
4
A part of Na2 O (g) begins to be used for the formation of the Si(Q3 ) species. Although the formation of B[Q4 (1Na)] species can be expected, it is not clealy seen in 11 B NMR spectra. When x 0.500 the P(Q2 ) species disappeared and there remained two species, i.e., P[Q3 (1Na)] species and P[Q4 (0Na)] species. (iii) 0.500 < x < 0.538 : [B2 O3 ] > [P2 O5 ] > [Na2 O]. It is hard to show the reaction that occurs in this range. Experimentally it is expected that the P[Q4 (0Na)] species convert into the P[Q3 (1Na)] species. (iv) 0.538 < x < 0.769 : 1/2[Na2 O] < [P2 O5 ] < [Na2 O]. The P[Q3 (1Na)] and the P[Q2 (2Na)] species are present. (v) 0.769 < x < 0.846 : 1/3[Na2 O] < [P2 O5 ] < 1/2[Na2 O]. The P[Q2 (2Na)] and the P[Q1 (3Na)] species are present. The (vi) 0.846 < x : 1/3[Na2 O] > [P2 O5 ]. P[Q1 (3Na)] species and borate units are present. From region (iv) the P[Q2 (2Na)] and P[Q1 (3Na)] species are formed stepwise. Na2 O is consumed also by the formation of the Si(Q3 ) and Si(Q2 ) species, so that the true boundaries in (iv), (v) and (vi) depend on the contents of Na2 O that react with the SiO2 and dier a little from those cited above. The calculation in region (vi) cannot be made, because the content of P2 O5 is low and comparison of the experimental distribution with
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the calculation is dicult. The lines seen in the distribution curves of phosphate units (Fig. 9) are the calculated ones. Generally the calculations give satisfactory results, although disagreement is seen in the B2 O3 rich ranges. The standard deviation 2 1=2 ÿ P Qn cal for the calcula R P Qn obs i i =n tions was within 2.5% for each P(Qn ) distributions with x < 0.8. The numerical equations of the distributions of 4-coordinated boron atoms can be estimated by counting Na2 O content that is used for the formation of the P[Q4ÿn (nNa)] species (n 0, 1, 2 and 3) shown in Fig. 7. It is assumed that all Na2 O other than those used in the formation of the Si(Q3 ) and Si(Q2 ) species are used to form 4-coordinated boron atoms. The estimated fractions of 4-coordinated boron atoms are shown in Fig. 11 by a solid line. The standard deviation for the calculations was 8.5% for x P 0.5. The calculations seem to give somewhat larger values than the experimental distribution of 4-coordinated boron atoms. At present, it is considered that the two sets of data coincide well. 5. Conclusions The 29 Si, 31 P and 11 B MAS NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses were examined. The 6-coordinated silicon atoms were observed in glasses when x was smaller than 0.250, which corresponded to [P2 O5 ]/ ([Na2 O] + [B2 O3 ]) P 1.0. The distributions of each phosphate and borate species were interpreted by a model in which P2 O5 reacts with Na2 O and B2 O3 preferably. The distributions could be expressed by considering that the [Na2 O]/[P2 O5 ] ratio determines primarily the chemical shift of the phosphate units. The 3-coordinated boron atoms began to appear in the glass of x > 0.500. The Raman signal at 1200 cmÿ1 and IR signal at 660 cmÿ1 , assigned to the Si(Q6 ) species, were also con®rmed. Acknowledgements The authors wish to thank the Advanced Instrumentation Center for Chemical Analysis,
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Ehime University, for the MAS-NMR measurements. This work was supported in part by a Grant-in-Aid for Scienti®c Research (No. 09450327) from the Ministry of Education, Science and Culture, Japan.
References [1] G. Engelhardt, D. Michel, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, New York, 1987. [2] H. Maekawa, T. Maekawa, K. Kawamura, T. Yokokawa, J. Non-Cryst. Solids 127 (1991) 53. [3] P. Zhang, C. Dunlap, P. Florian, P.J. Grandinetti, I. Farnan, J.F. Stebbins, J. Non-Cryst. Solids 204 (1996) 294. [4] P. Losso, B. Schnabel, C. J ager, U. Sternberg, D. Stachel, D.O. Smith, J. Non-Cryst. Solids 143 (1992) 265. [5] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Non-Cryst. Solids 116 (1990) 39. [6] R.K. Sato, R.J. Kirkpatrick, R.K. Brow, J. Non-Cryst. Solids 143 (1992) 257. [7] R.K. Brow, D.R. Tallant, J.J. Hudgens, S.W. Martin, A.D. Irwin, J. Non-Cryst. Solids 177 (1994) 221. [8] R.K. Brow, D.R. Tallant, S.T. Myers, C.C. Phifer, J. NonCryst. Solids 191 (1995) 45. [9] R. Dupree, D. Holland, M.G. Mortuza, Nature 328 (1987) 416. [10] R. Dupree, D. Holland, M.G. Mortuza, J.A. Collins, M.W.G. Lockyer, J. Non-Cryst. Solids 112 (1989) 111. [11] M.W.G. Lockyer, D. Holland, R. Dupree, Phys. Chem. Glasses 36 (1995) 22. [12] T. Sekiya, N. Mochida, A. Ohtsuka, K. Uchida, J. Ceram. Soc. Jpn. 96 (1988) 571. [13] S.-P. Szu, L.C. Klein, M. Greenblatt, J. Non-Cryst. Solids 143 (1992) 21. [14] M. Nogami, K. Miyamura, Y. Kawasaki, Y. Abe, J. NonCryst. Solids 211 (1997) 208. [15] H. Yamashita, H. Yoshino, K. Nagata, I. Yamaguchi, M. Ookawa, T. Maekawa, J. Ceram. Soc. Jpn. 106 (1998) 539. [16] W.A. Buckermann, W. M uller-Warmuth, C. Mundus, J. Non-Cryst. Solids 208 (1996) 217. [17] J.F. Ducel, J.J. Videau, M. Couzi, Phys. Chem. Glasses 34 (1993) 212. [18] J.F. Ducel, J.J. Videau, K.S. Suh, J. Senegas, Phys. Chem. Glasses 35 (1994) 10. [19] J.J. Videau, J.F. Ducel, K.S. Suh, J. Senegas, J. Alloys Compounds 188 (1992) 157. [20] J. Zhong, P.J. Bray, J. Non-Cryst. Solids 111 (1989) 67. [21] T. Yokokawa, S. Kohsaka, J. Chem. Eng. Data 24 (1979) 167. [22] G. El-Damrawi, W. M uller-Warmuth, H. Doweidar, I.A. Gohar, J. Non-Cryst. Solids 146 (1992) 137. [23] J.W. MacKenzie, A. Bhatnagar, D. Bain, S. Bhowmik, C. Parameswar, K. Budhwani, S.A. Feller, M.L. Royle, S.W. Martin, J. Non-Cryst. Solids 177 (1994) 269.
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[24] S.W. Martin, J.W. MacKenzie, A. Bhatnagar, S. Bhowmik, S.A. Feller, M.L. Royle, Phys. Chem. Glasses 36 (1995) 82. [25] S.W. Lee, J.H. Lee, Phys. Chem. Glasses 36 (1995) 127.
[26] W.L. Konijnendijk, J.M. Stevels, J. Non-Cryst. Solids 18 (1975) 307. [27] B.C. Bunker, D.R. Tallant, R.J. Kirkpatric, G.L. Turner, Phys. Chem. Glasses 31 (1990) 30.
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Structural studies of 30Na2O±5SiO2±65[(1 ÿ x)P2O5±xB2O3] glasses by nuclear magnetic resonance, Raman and infrared spectroscopy Hiroshi Yamashita, Keishi Nagata, Hidetake Yoshino, Katsumasa Ono, Takashi Maekawa * Department of Applied Chemistry, Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan Received 25 June 1998; received in revised form 17 March 1999
Abstract The 29 Si, 31 P and 11 B magic angle spinning (MAS) NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses were examined together with Raman and IR data. A correlation of the Raman signal at 1200 cmÿ1 or IR signal at 660 cmÿ1 with the 29 Si NMR around ÿ215 ppm is seen. The 6-coordinated silicon atoms are observed in glasses when x is smaller than 0.250. The 3-coordinated boron atoms begin to appear in glasses with x > 0.500. It is assumed that the phosphate species were made up of six types: O P±é3=2 (P), O P±(ONa)é2=2 (P), (NaO)±P±é3=2 (T), (NaO)2 ±P± é2=2 (T), (NaO)3 ±P±é1=2 (T) and P±é4=2 (T), where é is bridging oxygen and T is either B, P or Si. The distribution curves derived from the deconvolution of the spectra by component signal of each phosphate and borate species agree with those calculated using species with dierent [Na2 O]/[P2 O5 ] ratios when P2 O5 reacts with Na2 O and B2 O3 preferably. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Magic angle spinning (MAS) NMR techniques have been extensively applied to structural studies around network forming elements in various minerals and glasses [1]. The distributions of Si(Qn ) species in silicate glasses, where n is the number of bridging oxygen, have been widely examined [2±8]. In some glasses, the presence of the 6-coordinated silicon atom, denoted hereafter as Si(Q6 ) species, was observed [9±15]. The isotropic chemical shift (r) of the NMR of the Si(Q6 ) species was about ÿ215 ppm, which nearly equals that of a cubic * Corresponding author. Tel.: +81-89 927 9926; fax: +81-89 927 9943; e-mail: [email protected]
SiP2 O7 crystal [9]. Nogami et al. derived a relation between the number of Si(Q6 ) species and ([P2 O5 ] ÿ [SrO])/[SiO2 ] ratio in SrO±P2 O5 ±SiO2 glasses [14]. This was understood by assuming that P2 O5 reacted with SrO preferentially and excess P2 O5 participated in a formation of the Si(Q6 ) species. In a previous paper, a relation between the distribution of the Si(Q6 ) species and the basicity of the glasses was discussed [15]. In a glass of 30Na2 O± 5SiO2 ±65P2 O5 only the Si(Q6 ) species was observed. The presence of the Si(Q6 ) species was seen in glasses where the [P2 O5 ]/[Na2 O] composition ratio was greater than unity. With addition of P2 O5 to sodium silicate glasses, sodium ions attached to the Si(Q3 ) species are expelled by the P(Q3 ) species of P2 O5 to join the formation of the P(Q2 ) species of
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 2 4 1 - 0
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sodium metaphosphate. Thus, the population of the Si(Q4 ) species increased [15]. These results were explained by assuming that P2 O5 was a strong acid compared to SiO2 . Buckermann et al. examined competitive network former eects in Al2 O3 ±B2 O3 ± P2 O5 glasses by the NMR measurement [16]. Generally, B2 O3 is categorized as an acidic oxide in usual alkali borate glasses. Thus, the addition of alkali oxides to B2 O3 converts the 3-coordinated boron atoms to 4-coordinated ones accompanying alkali ions. In the usual alkali phosphate glasses the P(Q4 ) species, where four oxygen atoms attached to the phosphorous atom are all bridging ones seen in crystalline BPO4 , are absent. On the other hand, Dupree et al. showed that the P(Q4 ) species appeared in phosphoalumino silicate glasses [10]. Ducel et al. investigated the structural study of NaPO3 ±Na2 B4 O7 pseudo binary glasses by Raman and IR spectroscopy [17] and 31 P and 11 B NMR [18,19]. The various structural units of borophosphate species were identi®ed. Although the exact distributions of individual phosphate species against compositions could not be made clear, the phosphate species compensated by borate units were distinguished from the 31 P NMR chemical shifts. However, the BPO4 type's cluster or the P(Q4 ) species, the present notation, could not be identi®ed, because the sodium content of the glasses examined were relatively high. In alkali borophosphate glasses of low alkali contents, the P(Q4 ) species can be expected to be formed. In order to see the appearance of the P(Q4 ) species and to see the conditions of the formations of the Si(Q6 ) and P(Q4 ) species, B2 O3 is substituted for P2 O5 in 30Na2 O±5SiO2 ±65P2 O5 glass and the 29 Si, 31 P and 11 B MAS NMR spectra are measured. After the deconvolution of the NMR spectra, the distributions of individual species are compared with those calculated by structural hypotheses based on acid±base concepts. Raman and IR spectra are also measured and are compared with the NMR spectra. 2. Experimental Glasses of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ± xB2 O3 ] (0 6 x 6 1) were prepared. The reagents for
producing glasses were Na2 CO3 , NaPO3 , (NH4 )2 HPO4 , H3 BO3 and SiO2 . The desired quantities of reagents were thoroughly mixed with acetone in a mortar. A small amount of Gd2 O3 (0.05 mol%) was added to batches as a spin relaxation reagent for the 29 Si NMR measurements. After evaporation of acetone, the homogeneously mixed powders were melted in a platinum crucible at 1300°C using an electric furnace and poured into a cold metal plate. The 29 Si, 31 P and 11 B MAS-NMR spectra were obtained by a spectrometer operating at 59.74, 121.7 and 96.47 MHz, respectively. The chemical shift standards for 29 Si, 31 P and 11 B were Si(CH3 )4 , 85% H3 PO3 aqueous solution and BF3 (C2 H5 )2 O, respectively. The ground glass powder was loaded in a zirconia spinner tube and spun at 4±6 kHz. Other experimental details were described in a previous paper [15]. All glasses were also characterized by FT-Raman spectroscopy. The 1064 nm line of a Nd:YAG laser was used as an excitation light source. FT-IR spectra were obtained using a spectrophotometer. X-ray powder patterns were measured with a diractometer. The 29 Si and 31 P NMR spectra were deconvoluted with Gaussian peaks. On the other hand, the deconvolutions of 11 B spectra were obtained by setting values of the isotropic chemical shift, quadrupole coupling constant and quadrupole asymmetric parameters. Preliminary measurements of the 11 B NMR were made on binary sodium and lithium borate glasses and deconvolutions of the spectra were applied. The distribution of 4-coordinated boron atoms in two glass systems could be estimated from the peak areas of the deconvoluted peaks and was in accordance with the reported values [20]. 3. Results 3.1. NMR spectra Fig. 1 represents the selected 29 Si MAS NMR spectra of the examined glasses. The pro®les of the NMR spectra change systematically with x. The NMR spectra of glasses with x 6 0.118 consist of one peak at ÿ215 ppm, that is assigned to the Si(Q6 ) species. The X-ray powder patterns did not show
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
Fig. 1. 29 Si MAS-NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x) P2 O5 ±xB2 O3 ] glasses. The solid and broken lines denote the experimental and deconvolution data, respectively.
crystalline phases in any glasses that contain the Si(Q6 ) species. The signal assigned to the Si(Q4 ) is also observed. When x is greater than 0.250 the signal due to the Si(Q6 ) species disappears and the position of the apparent peak at ÿ115 ppm shifts to higher frequency (lower magnetic ®eld) side. The peak width increases and it seems to consist of one or more components other than the Si(Q4 ) species. The 31 P MAS NMR spectra are shown in Fig. 2. The 31 P NMR spectrum of sodium phosphosilicate glass (x 0) consists of two peaks at ÿ25 and ÿ37 ppm. The former peak is assigned to the P(Q2 ) species. The intensity of the latter peak, which is assigned to the P(Q3 ) species, decreases with an increase in x and these two peaks begin to overlap. The apparent peak position changes gradually to higher frequency and a new peak appears at +5 ppm as a shoulder. Fig. 3 represents the 11 B MAS NMR spectra. NMR spectra of glasses with x 6 0.333 consist of
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Fig. 2. 31 P MAS-NMR spectra of 30Na2 O±5SiO2 ± 65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. These lines are the same as in Fig. 1.
only one symmetric peak assigned to 4-coordinated boron atom and locate at around ÿ2.5 ppm. When x becomes larger than 0.333, the peak position changes to a higher frequency side and 3coordinated boron atoms appear. 3.2. Raman and IR spectra Fig. 4 shows the Raman spectra of the glasses. The peak intensity at 680 cmÿ1 (arrow a) decreases with an increase in x. The peaks at 1200 cmÿ1 (arrow c) and 1350 cmÿ1 (arrow d) decrease until the peaks disappear when x reaches 0.250. The peak at 1160 cmÿ1 (arrow b) is commonly seen in glasses of low x. The peak at 1000 cmÿ1 (arrow f) appears when x becomes greater than 0.500. The intensity at 620 cmÿ1 (arrow e) increases with an increase in B2 O3 content. However, it decreases in
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Fig. 3. 11 B MAS-NMR spectra of 30Na2 O±5SiO2 ± 65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. These lines are the same as in Fig. 1.
Fig. 4. Raman spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ± xB2 O3 ] glasses.
intensity when x becomes 0.8. New peaks at 500 cmÿ1 (arrow i), 780 cmÿ1 (arrow f) and 1100 cmÿ1 (arrow h) appear in B2 O3 rich glasses. Fig. 5 illustrates IR spectra. Absorptions centered on 660 cmÿ1 (arrow a) and 780 cmÿ1 (arrow b), decrease in their intensities with an increase in x. The intensities of a peak at 1350 cmÿ1 (arrow c) shift to lower wave number and decrease with an increase in x. On the other hand, the peak intensity at 700 cmÿ1 (arrow d) increases with an increase in B2 O3 content. The broad peak around 1350 cmÿ1 (arrow e) can be seen in B2 O3 rich glasses.
Na2 O. The acidity of the three acidic oxides increased in the order of SiO2 < B2 O3 < P2 O5 [21]. Thus, one sees that neutralization of the PO4 network by Na2 O proceeds preferably. SiO2 , the weakest acid among the three, does not react with Na2 O when appreciable amounts of P2 O5 and B2 O3 are present. Actually only the Si(Q6 ) and the Si(Q4 ) species are observed in the P2 O5 rich glasses and the Si(Q3 ) and Si(Q2 ) species appear only in glasses with x > 0.300. The acidity of B2 O3 is intermediate among the three. In alkali borosilicate glasses, B2 O3 may react with Na2 O preferentially [22±24]. However, in glasses of low Na2 O content, it is expected that B2 O3 reacts with P2 O5 to form BPO4 type species; thus it behaves as if it were a basic oxide.
4. Discussion 4.1. General consideration The structures of oxide glasses have been interpreted by a concept of acid-base equilibrium. In the present systems, three acidic oxides, P2 O5 , B2 O3 and SiO2 , compete to react with a basic oxide,
4.2. NMR 4.2.1. 29 Si NMR The deconvoluted spectra of the 29 Si NMR are shown in Fig. 1 as broken lines. The distributions
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Fig. 5. IR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses.
of the structural units derived from the peak area are shown in Fig. 6 as a function of x or r( [P2 O5 ]/([Na2 O] + [B2 O3 ]) 65(1 ÿ x)/(30 + 65x)). The data, which are not shown in Fig. 1, are also plotted. The distribution of the Si(Q6 ) species in 95 [ yNa2 O±(1 ÿ y)P2 O5 ]±5SiO2 ternary glasses [15] are also shown as a function of [P2 O5 ]/ [Na2 O] ((1 ÿ y)/y). In the ternary glasses, the distribution of the Si(Q6 ) species increased rapidly when the [P2 O5 ]/[Na2 O] ratio increased beyond about unity. On the other hand, in the 30Na2 O± 5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glass, the distribution of the Si(Q6 ) species increases abruptly when r, not the [P2 O5 ]/[Na2 O] ratio, increases beyond unity. The distributions of the two glassy systems can be expressed in the same line. If the conditions of the appearance of the Si(Q6 ) species primarily depend on the content of the unreacted P2 O5 , it is expected that B2 O3 reacts preferentially not with Na2 O but with P2 O5 . After the reactions with Na2 O and B2 O3 are completed, excess P2 O5
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Fig. 6. Correlation between glass composition and [Si(Q6 )]/ [Si(Qn )] ratio. The solid and open symbols are by this work and by previous data [15], respectively. Error bar: see text.
may participate in the formation of the Si(Q6 ) species. standard errors, ÿ The normalized s R yiobs ÿ yicalc =Ryiobs , of the deconvolution of the 29 Si NMR were 10% in average. This is due to the low S/N ratio of the observed spectra. The content of Na2 O that was consumed by the formation of Si(Qn ) species with n < 4 can be calculated by 2.5 R(4 ÿ n) ´ [Si(Qn )] and are shown in Fig. 7. The compositional dependences of the chemical shift of the Si(Qn ) species are shown in Fig. 8. The average values of the Si(Qn ) species with n 6 4 are also shown. The average values were evaluated by weighting each peak by its peak area. The chemical shift of the Si(Q4 ) species in P2 O5 rich glasses (ÿ100 ÿ120 ppm) is more negative than those seen in binary alkali silicates glasses (ÿ96 ÿ105 pm) [2]. The positions of individual Si(Qn ) species shift linearly to higher frequency with an increase in x, indicating 29 Si nuclei to be less shielded. This suggests that the electronegative phosphorous atom attached to the silicon tetrahedra is replaced by more electropositive boron
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seen, indicating that the environment around the Si(Q6 ) species does not change much with composition.
Fig. 7. Relationship between B2 O3 content (x) and Na2 O content consumed by the formation of Si(Qn ) species. The experimental errors are based on Fig. 6.
Fig. 8. 29 Si chemical shift for Si(Qn ) species as a function of B2 O3 content (x). The experimental errors of r are 0.5 ppm.
atoms and the number of Si±O±B bonds increase. On the other hand, the composition dependence of the chemical shift of the Si(Q6 ) species cannot be
4.2.2. 31 P NMR Ducel et al. summarized the phosphate species and their notations. In the glasses of zNaPO3 ± (1 ÿ z)Na2 B4 O7 the phosphate species seen in alkali phosphate glasses were dierentiated into two or three sub species [17±19]. The branching species consisted of two types; that is, BP species seen in crystalline P2 O5 (r*ÿ40 ppm) and BB seen in crystalline BPO4 (r*ÿ30 ppm). However, the presence of the BP species could not be con®rmed. Three types of middle species were MP, MB1 (or MB01 ), and MB2 (or MB02 ). The MP species was seen in usual alkali metaphosphate glasses, i.e. the P(Q2 ) species. The MB1 and MB2 were structural units when one boron and one phosphorous atom, or two boron atoms were attached to central phosphate site, respectively. MB01 and MB02 are the structural units in which one or two boron atoms attach to the phosphate unit with two non-bridging oxygen atoms. There were no P@O double bonds in the MB01 and MB02 species by their notation. The reported chemical shifts [17±19] were ÿ20, ÿ10 and ÿ5 ppm for MP, MB1 (or MB01 ) and MB2 (or MB02 ), respectively. There were two end species; i.e., bonded to one phosphorous atom (EP) or one boron atom (EB). Ducel et al. showed that the EP, i.e. P(Q1 ), species could not be formed in B2 O3 rich glasses, because probability of the formation of the P±O±P linking was low [18]. However, the numerical distribution of the each phosphate species against z could not be made. In the present study, we will try to deconvolute the distribution of borate and phosphate species from the NMR spectra. The species used in the present deconvolutions are tabulated in Table 1. Here P[Q4ÿn (nNa)] (n 0, 1, 2 and 3) means the sodium borophosphate unit in which n sodium ions were attached to the PO4 tetrahedron. The notations by Ducel et al. are also shown. Thus, MB01 and MB02 are included into P[Q2 (2Na)] species. The P(Q4 ) are denoted, hereafter, by the P[Q4 (0Na)]. The P(Q2 ) and P(Q3 ) are the structural units usually seen in binary sodium phosphate glasses, so that the same notations are used. In the
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Table 1 The possible formulae and symbols of silicate, phosphate and borate species expected in 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses. T:B, P or Si, é: bridging oxygen No 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Unit Si±é6=2 (P) Si±é4=2 (T) (NaO)±Si±é3=2 (T) (NaO)2 ±Si±é2=2 (T) O P±é3=2 (P) O P±(ONa)é2=2 (P) (NaO)±P±é3=2 (T) (NaO)2 ±P±é2=2 (T) (NaO)3 ±P±é1=2 (T) P±é4=2 (B) P±é4=2 (Si) B±é3=2 (B) B±é4=2 (T) Na B±é4=2 (P)
deconvolutions, the following will be taken into consideration. 1. Because of the 30 mol% content of Na2 O, the P(Q4 ) structural unit should be formed by the substitution of B2 O3 in P2 O5 rich glasses. Actually, the spectra could not be well deconvoluted by two Gaussian functions; i.e., assigned to the P(Q2 ) and P(Q3 ) species. We will tentatively assign the third peak, which appears at the position between two peaks of the above two, as a signal due to the P(Q4 ) species (r:ÿ30 ppm, see broken lines in Fig. 2. 2. P@O double bonds should disappear in glasses when the B2 O3 content increases. Thus, the MB01 and MB02 species are preferable in the present glasses rather than MB1 and MB2 species. 3. If the MB01 and MB02 are present, any species that have one non-bridging oxygen and three bridging oxygen atoms can be expected to form. They are further distinguished by the number of boron atoms from one to three. 4. The chemical shifts of 31 P depend primarily on the number of sodium ions attached to the phosphorous atom, so that the notations of the species will be expressed by the number of sodium ions. Fig. 9 is the distribution of each phosphate species against x. Generally the normalized stan-
Symbol of the species This work
Ducel et al.
Si(Q6 ) Si(Q4 ) Si(Q3 ) Si(Q2 ) P(Q3 ) P(Q2 ) P[Q3 (1Na)] P[Q2 (2Na)] P[Q1 (3Na)] P[Q4 (0Na)] P[Q4 (4Si)] B(Q3 ) B[Q4 (1Na)] B[Q4 (0Na)]
BP MP ÿ MB01 ; MB02 EB BB CP , CNa
dard errors of the deconvolution of the NMR spectra were within 2.5%. With an increase in x, the P(Q3 ) species decreases and the P(Q2 ) and P[Q4 (0Na)] species increase. The P[Q3 (1Na)] appears in accordance with the decrease in the P(Q2 )
Fig. 9. Experimentally determined P(Qn ) distribution as a function of B2 O3 content (x). The lines are the calculated ones shown in Table 2. When x < 0.269, the solid and broken lines denote f 0 and 1, respectively. Here, f is the fraction of Si(Q6 ) species. Error bar: see text.
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Fig. 10. Relationship between B2 O3 content (x) and chemical shift (a) or spectral width (b). The experimental errors of r are 0.1 ppm.
species. The chemical shifts and spectral width of each species by deconvolution are shown in Fig. 10(a) and (b). Among the structural units with the same n, the chemical shift moves gradually to the high frequency side with an increase in x, indicating that 31 P nuclei become less shielded with an increase in the number of B2 O3 attached to each phosphate group. The width of the deconvoluted spectra with the same n value has a maximum at the intermediate composition. This is due to the presence of the species of dierent numbers of boron atoms attached to the phosphate unit, whose chemical shifts dier according to the number of boron atoms. 4.2.3. 11 B NMR The fraction of 4-coordinated boron atoms remained 1 for the glasses of z < 0.1 in the (1 ÿ z)NaPO3 ±zNa2 B4 O7 system [17±19]. The fraction of 4-coordinated boron atoms decreased abruptly for z close to 0.6 and tended slowly toward 0.47, corresponding with that of vitreous Na2 B4 O7 . Fig. 11 represents the fraction of 4-coordinated boron atoms obtained by deconvolutions in the present glasses. The normalized standard errors of the deconvolution of the NMR spectra was 5.5% on average. In the (1 ÿ z)
NaPO3 ±zNa2 B4 O7 glasses examined by Ducel et al., r values were less than unity in all z (0 6 z 6 1). Thus, the P[Q4 (0Na)] species might not be seen. In the present study, it is to be noted that
Fig. 11. Experimentally determined B(Qn ) distribution as a function of B2 O3 content (x). The lines are the calculated ones shown in Table 2. Error bar: see text.
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
the population of 4-coordinated boron atoms remains almost 100% even when x increases up to 0.5. However, the peak width of the glass with x 0.500 is somewhat broader and the deconvolution could not succeed with a single peak of quadrupole symmetric factor equal to 1. The presence of the two types of 4-coordinated boron atoms are expected in the glass. This corresponds to the appearance of the P[Q4 ÿ n (nNa)] (n ¹ 0) species in addition to the P[Q4 (0Na)]. The peak position shifts to higher frequency, indicating that 11 B nuclei become less shielded. This is due to the changes of charge compensating group from phosphate (n 0) to electropositive sodium (n P 1). The fraction of 4-coordinated boron atoms in a glass of x 1 coincides fairly well with the value reported in sodium borosilicate glasses [22± 24]. 4.3. Raman and IR spectra As seen in a previous paper, the Raman peaks at 1200 and 1350 cmÿ1 are related to the Si(Q6 ) species [15]. Actually, the appearance of this peak is correlated with the presence of the peak at ÿ215 ppm in the 29 Si NMR spectra. Unfortunately, exact assignment of this signal cannot be made. These Raman peaks decrease in intensity until the peaks disappear when x reaches 0.250. Lee et al. examined Raman spectra of K2 O±Al2 O3 ±P2 O5 glasses and assigned the peak at 1346 cmÿ1 to the P@O stretching mode [25]. The decrease in its intensity should be a consequence of the disruption of the P(Q2 ) species according to the formation of the P[Q4ÿn (n Na)] species. The Raman peak at 1160 cmÿ1 is the symmetric stretching mode of the terminal oxygen, (the P(Q2 ) species), on each tetrahedron. In accordance with 31 P NMR its intensity decreases with an increase in x. The intensity of the other peak at 680 cmÿ1 is due to the (POPsym ) stretching vibration of the P(Q2 ) species [8] and its intensity decreases with an increase in B2 O3 content. The intensity of the shoulder at 770 cmÿ1 , that is due to the BO4 stretching in borate species [17], decreases with an increase in B2 O3 content when x becomes greater than 0.5. On the other hand, the intensity of a new peak at 780 cmÿ1 , assigned to the BO3
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stretching vibration, increases with an increase in B2 O3 content with x > 0.5 [26]. At the same time, the weak bands from 1300 to 1500 cmÿ1 , which are associated with the vibration of 3-coordinated boron atoms [17,27] appear clearly in glasses with x > 0.800. Among the various peaks in IR spectra, the absorption at 660 cmÿ1 is most striking. Although the exact assignment could not be made, this absorption is due to the Si(Q6 ) species [10]. Its intensity decreases with an increase in x. The peak at 1350 cmÿ1 arises from a P@O stretching vibration [11]. This absorption disappears with an increase in x, whose behavior is in accordance with the Raman peak at the same wave number. The absorption at 780 cmÿ1 is due to the bending motion of Si±O±P, P±O±P and O± P±O [11], whose intensity decreases also with an increase in x. Thus, Raman and IR spectra can be used to understand the NMR spectra of each element. 4.4. Numerical estimation of the phosphate and borate species In order to see the quantitative estimation of the distributions of the various species represented by the deconvolution of the NMR spectra, we will calculate the distribution by a structural consideration based on the acid-base concept. The numerical equations for the distribution of each species are tabulated in Table 2. Six regions are distinguished in the calculation. (i) 0 6 x < 0:269: P2 O5 > Na2 O B2 O3 . The species present in the glass with x 0 are the P[Q4 (0Na)], P(Q3 ) and P(Q2 ), and all silicon species are the Si(Q6 ). The following reactions should occur in this region: P2 O5 Pf Q3 g B2 O3 ! 2BPO4 fBQ4 0Na; PQ4 0Nag
1
and P2 O5 fP Q3 g Na2 O ! 2NaPO3 fP Q2 g: 2 Added B2 O3 (65x mol) is used to form the P[Q4 (0Na)] species. All Na2 O (30 mol) reacts
0 130x ÿ 35 ÿ g 30 ÿ g 30 ÿ g 30 ÿ g 30 ÿ g 0 0 0 0 130x ÿ 100 ÿ g 230 ÿ 260x + g
1 35 ÿ 65x + g 35 ÿ 65x + g 0 0 0
P[Q1 (3Na)]
0 130x ÿ 35 ÿ g 30 ÿ g 100 ÿ 130x + g 0 0
0 0 0 65x ÿ 35 ÿ g 165 ÿ 195x + g 0
P(Q2 )
30 65 ÿ 130x 0 0 0 0 35 ÿ 130x ÿ 5f 0 0 0 0 0
P(Q3 ) P[Q4 (0Na)] + P[Q4 (4Si)]
65x + 5f 35 ÿ 65x + g 35 ÿ 65x + g 0 0 0
Symbol x
0 ÿ 0.269 0.269 ÿ 0.5 0.5 ÿ 0.538 0.538 ÿ 0.769 0.769 ÿ 0.846 >0.846
P[Q3 (1Na)]
P[Q2 (2Na)]
B[Q4 (0Na)]
B[Q4 (1Na)]
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126 Table 2 The numerical equations of the distribution of each species. Here, f and g mean the fraction of Si(Q6 ) species and Na2 O content consumed by the formation of Si(Qn ) species, respectively. The g value is calculated from the straight line in Fig. 7. These equations are only shown by a numerator. The denominator of P(Qn ) and B(Qn ) distributions are 65(1 ÿ x) and 65x, respectively
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with P2 O5 (65 ÿ 65x mol) to form the P(Q2 ) species. For example, the fraction of the P(Q3 ) species is shown by P Q3 f65 1 ÿ x ÿ 65x ÿ 30 ÿ 5f g =65 1 ÿ x 35 ÿ 130x ÿ 5f =65 1 ÿ x: 3 Here, f means the fraction of Si(Q6 ) species. When x 0.269, f becomes zero and there are no P(Q3 ) remaining. (ii) 0.269 < x < 0.500 : [Na2 O] + [B2 O3 ] > [P2 O5 ] > [B2 O3 ]. Mainly the following reactions will occur. B2 O3 reacts with the P(Q2 ) species and forms the P[Q3 (1Na)] species such as 2NaPO3 fP Q2 g B2 O3 ! 2NaPBO3 fPQ3 1Nag:
4
A part of Na2 O (g) begins to be used for the formation of the Si(Q3 ) species. Although the formation of B[Q4 (1Na)] species can be expected, it is not clealy seen in 11 B NMR spectra. When x 0.500 the P(Q2 ) species disappeared and there remained two species, i.e., P[Q3 (1Na)] species and P[Q4 (0Na)] species. (iii) 0.500 < x < 0.538 : [B2 O3 ] > [P2 O5 ] > [Na2 O]. It is hard to show the reaction that occurs in this range. Experimentally it is expected that the P[Q4 (0Na)] species convert into the P[Q3 (1Na)] species. (iv) 0.538 < x < 0.769 : 1/2[Na2 O] < [P2 O5 ] < [Na2 O]. The P[Q3 (1Na)] and the P[Q2 (2Na)] species are present. (v) 0.769 < x < 0.846 : 1/3[Na2 O] < [P2 O5 ] < 1/2[Na2 O]. The P[Q2 (2Na)] and the P[Q1 (3Na)] species are present. The (vi) 0.846 < x : 1/3[Na2 O] > [P2 O5 ]. P[Q1 (3Na)] species and borate units are present. From region (iv) the P[Q2 (2Na)] and P[Q1 (3Na)] species are formed stepwise. Na2 O is consumed also by the formation of the Si(Q3 ) and Si(Q2 ) species, so that the true boundaries in (iv), (v) and (vi) depend on the contents of Na2 O that react with the SiO2 and dier a little from those cited above. The calculation in region (vi) cannot be made, because the content of P2 O5 is low and comparison of the experimental distribution with
H. Yamashita et al. / Journal of Non-Crystalline Solids 248 (1999) 115±126
the calculation is dicult. The lines seen in the distribution curves of phosphate units (Fig. 9) are the calculated ones. Generally the calculations give satisfactory results, although disagreement is seen in the B2 O3 rich ranges. The standard deviation 2 1=2 ÿ P Qn cal for the calcula R P Qn obs i i =n tions was within 2.5% for each P(Qn ) distributions with x < 0.8. The numerical equations of the distributions of 4-coordinated boron atoms can be estimated by counting Na2 O content that is used for the formation of the P[Q4ÿn (nNa)] species (n 0, 1, 2 and 3) shown in Fig. 7. It is assumed that all Na2 O other than those used in the formation of the Si(Q3 ) and Si(Q2 ) species are used to form 4-coordinated boron atoms. The estimated fractions of 4-coordinated boron atoms are shown in Fig. 11 by a solid line. The standard deviation for the calculations was 8.5% for x P 0.5. The calculations seem to give somewhat larger values than the experimental distribution of 4-coordinated boron atoms. At present, it is considered that the two sets of data coincide well. 5. Conclusions The 29 Si, 31 P and 11 B MAS NMR spectra of 30Na2 O±5SiO2 ±65[(1 ÿ x)P2 O5 ±xB2 O3 ] glasses were examined. The 6-coordinated silicon atoms were observed in glasses when x was smaller than 0.250, which corresponded to [P2 O5 ]/ ([Na2 O] + [B2 O3 ]) P 1.0. The distributions of each phosphate and borate species were interpreted by a model in which P2 O5 reacts with Na2 O and B2 O3 preferably. The distributions could be expressed by considering that the [Na2 O]/[P2 O5 ] ratio determines primarily the chemical shift of the phosphate units. The 3-coordinated boron atoms began to appear in the glass of x > 0.500. The Raman signal at 1200 cmÿ1 and IR signal at 660 cmÿ1 , assigned to the Si(Q6 ) species, were also con®rmed. Acknowledgements The authors wish to thank the Advanced Instrumentation Center for Chemical Analysis,
125
Ehime University, for the MAS-NMR measurements. This work was supported in part by a Grant-in-Aid for Scienti®c Research (No. 09450327) from the Ministry of Education, Science and Culture, Japan.
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