Nuclear magnetic resonance studies of alkaline earthphosphosilicate and aluminoborosilicate glasses

Hiroshi Yamashita, Hidetake Yoshino, Keishi Nagata, Hyuma Inoue,Takeshi Nakajin, Takashi Maekawa *

Department of Applied Chemistry, Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan

Received 14 June 1999; received in revised form 6 December 1999

Abstract

The 11B, 27Al, 29Si and 31P magic angle spinning (MAS) NMR spectra of MO±P2O5, MO±SiO2±P2O5 and

MO(M02O)±SiO2±Al2O3±B2O3 (M�Mg, Ca, Sr and Ba, M0 � Na) glasses were examined. In binary MO±P2O5 (M�Ca

and Mg) glasses, the distributions of the phosphate sites, P(Qn), can be expressed by a theoretical prediction that P2O5

reacts quantitatively with MO. In the ternary 0.30MO±0.05SiO2±0.65P2O5 glasses, the 6-coordinated silicon sites were

detected, whose population increases in the order of MgO < CaO < SrO;BaO. In xCaO±0.05SiO2±(0:95ÿ x)P2O5

glasses, its population increases with an increase in f �� ��P2O5� ÿ �MO� ÿ �B2O3� ÿ �Na2O��=�SiO2�� and has maximum

at f � 9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three

network-forming oxides such as SiO2, Al2O3 and B2O3 coexist, Al2O3 reacts preferably with MO. The populations of 4-

coordinated boron atoms, N4, are expressed well with r=�1ÿ r�, where r � ��Na2O� ÿ �Al2O3��=��Na2O� ÿ �Al2O3�� �B2O3��. The correlation of the Raman signal at 1210 and 1350 cmÿ1 with the NMR signal of Si(Q6) at )215 ppm is

also seen. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

Recently, various glasses have been developedfor optical uses. In particular alkali ion-free glassesor Al2O3-containing glasses used for liquid crystaldisplays are among glasses of interest. In opticalglasses, the various network-forming oxides, SiO2,B2O3, Al2O3 and P2O5 coexist with many kinds ofnetwork-modifying oxides such as alkaline earthand alkali metal oxides. In order to examine thephysico-chemical properties as well as the struc-

tures of the multi-component glasses, it is impor-tant to see the relative strengths of the networkforming or modifying abilities of the constituentoxides based on studies of the simple binary orternary glasses. One way to examine the structureof the glasses is by spectroscopic determination ofthe distributions of the structure sites of Si-, B-,Al- and P-polyhedra. An acid±base concept mayhelp to understand the structural information ob-tained by these methods. For example, in binarysilicate glasses, the distributions of structural sitessuch as Si(Qn) (n� 0 to 4), where n is the numberof bridging oxygens, have been widely examinedby NMR [1±8] and Raman [9±11] spectroscopyand discussed in terms of acid±base reactions [6±8]. In binary alkali borate glasses, NMR spectra

Journal of Non-Crystalline Solids 270 (2000) 48±59

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* Corresponding author. Tel.: +81-89 927 9926; fax: +81-89

927 9943.

E-mail address: maekawa@en3.ehime-u.ac.jp (T. Maekawa).

0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 0 5 6 - 9

show the 3-coordinated boron atoms are convert-ed to the 4-coordinated ones accompanying thealkali ions as charge compensation. In alkaliphosphate glasses one of the phosphorous±oxygenbonds is P¸O double bond and the P(Q4) struc-tures, where four oxygen atoms attached to thephosphorous atoms are all bridging, are absent.The 27Al NMR spectra in glasses and mineralshave been widely investigated and coordinationnumbers of aluminum ions have been evaluated[3]. Aluminum ions can be present in glasses andminerals as AlO4-type clusters, by joining alkaliions as charge compensation. In other words,Al2O3 is considered to be a strong acid if the re-action with alkali metal oxides is interpreted interms of an acid±base one.

In the multi-component glasses, the determi-nation of the distributions of each structural sitebecomes di�cult, because competitive reactionswith network-modifying oxides occur among net-work-forming oxides. Dupree et al. [12] showedthat P(Q4) appeared in phosphoaluminosilicateglasses. Ducel et al. investigated the structuralstudy of NaPO3±Na2B4O7 pseudo-binary glassesby Raman and IR spectroscopy [13] and 31P and11B NMR [14,15] and identi®ed the various struc-tural sites of borate±phosphate connections. Al-though the exact distributions of individual P(Qn)against composition could not be made clear, thephosphate sites compensated by borate sites weredistinguished from the 31P NMR chemical shifts.In the addition of P2O5 to sodium silicate glasses,sodium ions attached to Si(Q3) were expelled byP(Q3) of P2O5 to join the formation of P(Q2) of thesodium metaphosphate. Thus, the population ofSi(Q4) increased [16]. These results are explainedby the fact that P2O5 is a strong acid compared toSiO2. In some phosphate glasses, the presence ofthe 6-coordinated silicon atom, denoted as Si(Q6),was observed [12,16±22]. Nogami et al. [21] de-rived a relation between the number of Si(Q6) andthe ��P2O5� ÿ �SrO��=�SiO2� ratio, where [A] meansmole of A, in SrO±P2O5±SiO2 glasses. This wasunderstood as a reaction of P2O5 with SrO pref-erentially and excess P2O5 participated in the for-mation of Si(Q6). In a previous paper [16], arelation between the distribution of Si(Q6) and thebasicity of the glasses was discussed. In a glass of

0.30Na2O±0.05SiO2±0.65P2O5, only the Si(Q6) wasobserved. The presence of Si(Q6) was seen inglasses where the [P2O5]/[Na2O] ratio was greaterthan unity. The 29Si, 31P and 11B MAS-NMRspectra of 0.30Na2O±0.05SiO2±0.65 {(1ÿ x)P2O5±xB2O3} glasses were examined [22]. Si(Q6) wereobserved in glasses when x was smaller than 0.250,which corresponded to �P2O5�=��Na2O� � �B2O3��P 1:0. The distributions of each phosphate andborate site were interpreted by a model in whichP2O5 reacts with Na2O and B2O3 preferably.

This report concerns mainly the NMR investi-gation of multi-component glasses containing al-kaline earth metal oxides. The di�erence in thereactivity among coexisting network-forming ox-ides with alkaline earth oxides will be compared.The relationship between the formation of Si(Q6)and the composition will be discussed in detail,because the acid±base concept may be applied tothe interpretation of the presence of this structuralsite in glasses. We will ®rst examine binary alkalineearth phosphate glasses and alkaline earth phos-phosilicate glasses. NMR measurements will beextended to alkaline earth aluminoborosilicateglasses of similar composition used in liquid crys-tal display glasses. The di�erence between the re-activity of Al2O3 and B2O3 with basic oxides whenthey coexist in glasses will be clari®ed. The workwill be focused on the determination of the relativeacid strength of four oxides, P2O5, B2O3, SiO2 andAl2O3 when they coexist in the glasses.

2. Experimental

The reagents for producing glasses wereNa2CO3, NaH2PO4, (NH4)2HPO4, H3BO3, SiO2,Al2O3, MgO, CaCO3, SrCO3 and BaO. The de-sired quantities of reagents in a mole ratio werethoroughly mixed with acetone in a mortar. Asmall amount of Gd2O3 (0.05 mol%) was added tobatches as a spin relaxation reagent for the 29SiNMR measurements. After evaporation of ace-tone the homogeneously mixed powders weremelted in a platinum crucible at 1200±1600°C us-ing an electric furnace and poured into a coldmetal plate. SiP2O7 was prepared by a sol±gelmethod. A mixture of PO(C2H5O)3, Si(CH3O)4,

H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59 49

H2O, C2H5OH and 0.1 M HCl, whose weightswere 7.2, 3.0, 7.1, 9.1 and 0.1 g, respectively, wereused for the starting solution. The hydrolysis re-quired 3 h at 0°C. The solution then was put intoan oven and dried at 110°C for 3 days. The re-sulting powders were heated to 1200°C and cooledto room temperature. BPO4 powder was obtainedfrom a commercial source that is usually used as asecondary standard reagent for 11B NMR. The11B, 27Al, 29Si and 31P MAS-NMR spectra wererecorded on a JEOL JNM-CMX 300 spectrome-ter. The chemical shift standards for 11B, 27Al, 29Siand 31P were BF3(C2H5)2O, 1 M Al(NO3)3 aque-ous solution, Si(CH3)4 and 85% H3PO3 aqueoussolution, respectively. The ground glass powderswere loaded in a zirconia spinner tube and spun at4±6 kHz. Other experimental details were de-scribed in the previous papers [16,22]. A part of theglasses was also characterized by FT-Ramanspectroscopy using an FT-IR spectrometer with aRaman attachment. The 1064 nm line of aNd:YAG laser was used as an excitation lightsource. X-ray powder patterns were recorded on adi�ractometer. The 29Si and 31P NMR spectrawere deconvoluted with Gaussian peaks. Settingvalues of the isotropic chemical shift, quadruplecoupling constant and quadruple asymmetric pa-rameters made the deconvolutions of 11B spectra.

3. Results

3.1. MgO±P2O5 and CaO±P2O5 binary glasses

The 31P MAS-NMR spectra of xCaO±�1ÿ x�P2O5 and binary glasses are broad andasymmetric. The spectra are shown in Fig. 1(a).They resemble those seen in binary alkali phos-phate glasses [23]. The intensity of the spectraaround )35 ppm decreases with an increase in x.Thus, the broad peak consists of at least two sig-nals due to P(Q2) and P(Q3) [16]. The apparentpeak position moves to lower magnetic ®eld, i.e.,higher frequency, with an increase in x. Fig. 1(b)shows the Raman spectra of xCaO±�1ÿ x�P2O5

glasses. Two Raman signals are observed from1400 to 1000 cmÿ1. The intensity at 1300 cmÿ1

decreased, whereas signals at 1160 cmÿ1 increased

in intensity with an increase in x. The NMRspectra of xMgO±�1ÿ x�P2O5 glasses are shown inFig. 1(c). The spectra are similar to those observedin CaO±P2O5 glasses but locate on higher magnetic

Fig. 1. 31P MAS-NMR ((a) and (c)) and Raman (b) spectra of

xMO±(1ÿ x)P2O5 glasses (M�Ca ((a) and (b)), Mg (c)).

50 H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59

®eld side than those of CaO±P2O5 glasses. Here,the intensity around )40 ppm also decreases withan increase in x. The spectrum becomes symmetricwhen x � 0:5.

3.2. xCaO±0.05SiO2±(0:95ÿ x)P2O5 glasses

Fig. 2(a) represents the 29Si NMR spectra ofcalcium phosphosilicate glasses. The NMR spectraconsist of the three peaks around )118, )215 and)165 ppm. The former two are assigned to Si(Q4)and Si(Q6), respectively [16]. The X-ray powderpatterns did not show crystalline phases in anyglasses that contain Si(Q6). The origin of a weaksignal at )165 ppm will be discussed later. Thepositions of NMR of Si(Q4) of the present glassesdo not change with x but are located on highermagnetic ®eld side than that of sodium phospho-silicate glasses [12]. With an increase in x thepopulation of Si(Q6) increases and has maximumwhen x reaches about 0.25. When x � 0:45 thesignal due to Si(Q6) disappears and spectra consistof only the Si(Q4).

The 31P NMR spectra of calcium phosphosili-cate glasses are shown in Fig. 2(b). The NMRspectra consist of the two apparent peaks around)10 and )35 ppm. The chemical shift of the formerpeak is assigned to P(Q1), that is equal to that seenin a-Ca2P2O7 [24]. As a whole the spectral shapechanges systematically with x, which resemblesthose observed in binary CaO±P2O5 glasses.However, due to the inclusion of 5% SiO2, theminor di�erence of the spectra is seen. Thus, inaddition to the P(Q2) and P(Q3), a weak signal dueto P(Q4) seems to overlap in the very broad peak.

Fig. 2(c) shows the Raman spectra of the cal-cium phosphosilicate glasses. The relative peakintensity at 1350 cmÿ1 (peak a) decreases and thepeak disappears when x reaches 0.45. The peakintensity at 1300 cmÿ1 (peak b) decreases also withan increase in x but remains when x exceeds 0.45.The composition dependence of the peak intensityat 1210 cmÿ1 (peak c) resembles that of peak a.The peak at 1160 cmÿ1 (peak d) is commonly seenin all glasses and its presence is clearly seen in theglasses of high x value. A shoulder at 1100 cmÿ1

(peak e) and peak at 1010 cmÿ1 (peak f) appear

Fig. 2. 29Si (a) and 31P (b) MAS-NMR and Raman (c) spectra

of xCaO±0.05SiO2±(0:95ÿ x)P2O5 glasses.

H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59 51

when x becomes greater than 0.45. Below 800 cmÿ1

appreciable changes of the pro®les cannot be seen.

3.3. 0.30MO±0.05SiO2±0.65P2O5 glasses (M�Mg, Ca, Sr and Ba)

Fig. 3(a) represents the 29Si NMR spectra of the0.30MO±0.05SiO2±0.65P2O5 glasses. In all the

glasses, two structural sites, Si(Q4) and Si(Q6),were clearly observed. Here an additional weakpeak is also seen at )165 ppm. Among the MO thefraction of Si(Q6) increases in the order ofMgO < CaO < SrO;BaO. Note that in the0.30Na2O±0.05SiO2±0.65P2O5 glass the silicatestructural sites are all Si(Q6) [16]. The 31P NMRspectra of the 0.30MO±0.05SiO2±0.65P2O5 glassesare shown in Fig. 3(b). These spectra are asym-metric and have a shoulder, so that one sees theapparent peak consists of several components. Theapparent peak position moves to higher magnetic®eld in the order of MgO > CaO > SrO > BaO.

3.4. MO(M 02O)±SiO2±Al2O3±B2O3 (M�Ca, Sr

and Ba, M 0 � Na) glasses

The compositions of the glasses of this series arebased on the composition of alkali-free glass (codeAN635) by Asahi Glass [25]. Fig. 4(a) shows the29Si NMR spectra of 0.188MO±0.673SiO2±0.077Al2O3±0.062B2O3 glasses. These spectra arevery broad and the peak position moves slightly tohigher magnetic ®eld side in the order of CaO >SrO > BaO > Na2O. Fig. 4(b) represents the27Al NMR spectra of the glasses. These spectraconsist of a peak around 50 ppm. This peak isassigned to Al(Q4) [26]. The peak positions do notdi�er among glasses examined. However, all thespectra have a bulge on the higher magnetic ®eldside.

The 11B NMR spectra of the glasses are shownin Fig. 4(c). The 11B NMR spectra consist of twosignals. One is a sharp signal around 0 ppm andthe other is the broad one from )20 to +20 ppm.The former is originated from the 4-coordinatedboron and the latter from the 3-coordinated boronatom [22]. Thus, the fraction of 3-coordinatedboron atoms decreases in the order of CaO > SrO> BaO. When M0 � Na, all boron atoms seem topresent as 4-coordinated ones. Fig. 5(a) representsthe 29Si NMR spectra of 0.188Na2O±0.673SiO2±(0:139ÿ x)Al2O3±xB2O3 glasses. Here it is aimedat seeing the relative reactivity of Al2O3 and B2O3

with Na2O. With a decrease in the B2O3 content,the peak position moves to lower magnetic ®eldand the peak width decreases. Fig. 5(b) representsthe 27Al NMR spectra of the glasses. These spectra

Fig. 3. 29Si (a) and 31P (b) MAS-NMR spectra of 0.30MO±

0.05SiO2±0.65P2O5 glasses (M�Mg, Ca, Sr and Ba).

52 H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59

Fig. 5. 29Si (a), 27Al (b) and 11B (c) MAS-NMR spectra of

0.188Na2O±0.673SiO2±(0:139ÿ x)Al2O3±xB2O3 glasses.

Fig. 4. 29Si (a), 27Al (b) and 11B (c) MAS-NMR spectra

of 0.188MO(M02O)±0.673SiO2±0.077Al2O3±0.062B2O3 glasses

(M�Ca, Sr and Ba, M0 � Na�.

H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59 53

consist of a peak around 50 ppm. As seen abovethis peak is assigned to Al(Q4) [26]. As seen in Fig.4(b), all the spectra have also a bulge at highermagnetic ®eld. The 11B NMR spectra of the glassesare shown in Fig. 5(c). The fraction of 3-coordi-nated boron atoms increased slightly with an ad-dition of Al2O3. When the content of Na2O isreduced, the increase in the population of 3-coor-dinated boron atoms becomes large (Fig. 6).

4. Discussion

4.1. Binary alkaline earth phosphate glasses

In pure P2O5 glasses the structural site is onlythe P(Q3). By addition of basic oxide such as CaO,P(Q3) is destroyed into P(Q2) resulting P±Oÿ±Ca2�

bond. The distributions of structural sites, XP�Qn�,derived from the peak area of individual compo-

nent are expressed by the theoretical ones derivedfrom the following equations:

XP�Q2� � �CaO�=�P2O5�;XP�Q3� � ��P2O5� ÿ �CaO��=�P2O5�:

Fig. 7 represents the distribution of each P(Qn)determined by the deconvolution of the NMRspectra. The normalized standard errors, s ��P jyobs

i ÿ ycalci j=

Pyobs

i �, of the deconvolution ofthe 31P NMR were 6.4% in average. The Ramansignal at 1300 and 1160 cmÿ1 assigned to P(Q3)and P(Q2), respectively [16]. The decrease of theformer band and increase of the latter band with xcoincide with the distribution of P(Q3) and P(Q2)derived from the 31P NMR spectra. Although thechemical shifts di�ers among two glass systems,the composition dependence of the P(Qn) distri-butions against CaO content agrees well with thevalues of that of MgO glasses. However, whenx � 0:5, the peak of P(Q1) appeared at )10 ppm inthe CaO±P2O5 glasses as shown in Fig. 1(a). Thispeak disappeared in the MgO±P2O5 glasses. It isconsidered that P2O5 evaporated from CaO±P2O5

melt. This result is also supposed from the Raman

Fig. 6. Correlation between glass composition and the popu-

lation of threefold boron atoms from 11B MAS-NMR spectra

in 0.139Na2O±0.673SiO2±(0:188ÿ x)Al2O3±xB2O3 (a) and

0.188Na2O±0.673SiO2±(0:139ÿ x)Al2O3±xB2O3 (b) systems.

Here, r is ��Na2O� ÿ �Al2O3��=��Na2O� ÿ �Al2O3� � �B2O3��.

Fig. 7. Experimentally determined P(Qn) distribution as a

function of MO content (x) in the xMO±(1ÿ x)P2O5 glasses.

Here, the circular and triangular symbols are the CaO and

MgO systems, respectively. Error bar: see text.

54 H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59

spectra. Thus, the weak peak around 1010 cmÿ1

corresponding to P(Q1) appeared as shown in Fig.1(b) [16]. As quoted in the Na2O±P2O5 glasses, theconversion of P(Q3) to P(Q2) is quantitatively es-tablished by the addition of CaO or MgO. If thereaction can be described by an acid±base concept,it is understood that P2O5 is a strong acid andreacted quantitatively with CaO and MgO. Thechemical shift of each phosphate site depends onthe kind of metal ions attached to each site. Be-cause of larger electronegativity of Mg than Ca,the positions of P(Q1) and P(Q2) in MgO±P2O5

glasses shift to higher magnetic ®eld side thanthose of CaO±P2O5 glasses.

4.2. Alkaline earth phosphosilicate glasses

Stebbins [27] observed the presence of 5-co-ordinated silicon atom at )150 ppm in a fast-quenched K2Si4O9 glass. The abundance of the5-coordinated silicon site for all Si(Qn) was 0.1%and coexisted Si(Q6) was less than 0.02%. This sitecould also be detected in phosphorous-freeNa2Si4O9 and Cs2Si4O9 glasses [28]. Nogami et al.[21] found also a signal around )170 ppm. How-ever, they concluded that the peak did not corre-spond to the SiO5 group but spinning side band,because the peak position varied with spinningspeed. We also measured NMR spectra with dif-ferent spinning speed. However, appreciable dif-ferences could not be found out between 4 and 6.5kHz. This additional signal did not always appear.Generally, it was observed only when Si(Q4) andSi(Q6) coexist. If this signal is due to a spinningside band, it should appear in the NMR spectrumof 0.05SiO2±0.30Na2O±0.65P2O5 glass [16], or thepresent 0.05SiO2±0.50CaO±0.45P2O5 glass in Fig.2(a), where all structural sites are Si(Q6) or Si(Q4),respectively. Thus, we tentatively assign this signalto the 5-coordinated silicon, hereafter denoted bySi(Q5). The distributions of the structural unitsderived from the peak area are shown in Fig. 8 as afunction of f �� ��P2O5� ÿ �MO� ÿ �B2O3�ÿ�Na2O��=�SiO2��. Here, MO [21], B2O3 [22] andNa2O [16] are preferentially bonded to PO4

groups. Therefore, we assume that the PO4 groups,which are not bonded to the MO, B2O3 and Na2O,

participate in the formation of the Si(Q6). Gener-ally, the normalized standard errors of the de-convolution of the NMR spectra were within11.7%. The distribution of Si(Q6) in xNa2O±0.05SiO2±(0:95ÿ x)P2O5 [16] and 0.30Na2O±0.05SiO2±0.65[(1ÿ x)P2O5±xB2O3] glasses [22] arealso shown in Fig. 8. The data of SrO±SiO2(5±36mol%)±P2O5 glasses by Nogami et al. [21] are alsoincluded. The population of the Si(Q6) increasedrapidly with an increase in f. However, the popu-lations of Si(Q6) ®rst increase with an increase inthe ratio but seems to have maximum in the pre-sent glasses. The reason why it has a maximum inCaO±P2O5±SiO2 glasses cannot be clari®ed. It maydepend on the content of SiO2. The distribution ofSi(Q5) has a similar manner with the above ratio.However, further quantitative discussion con-cerning the existence of the Si(Q5) in natural glassunder ambient pressure requires knowledge otherthan NMR spectra presented here.

The addition of 5 mol% SiO2 to CaO±P2O5

binary glasses causes a change in 31P NMR spec-tra. If the local structure of Si(Q6) is similar to that

Fig. 8. Distributions of the Si(Q6) units as a function of f

(� ��P2O5� ÿ �MO� ÿ �B2O3� ÿ �Na2O��=�SiO2�) in the xMO±

0.05SiO2±(0:95ÿ x)P2O5 system. s, }, �: this work; M: Ref.

[16]; �: Ref. [22]; d: Ref. [21]. Error bar: see text.

H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59 55

of crystalline SiP2O7, the Si±O±P bond should beformed. It means P(Q4), where only bridging ox-ygens are formed. The chemical shift of SiP2O7

synthesized here and BPO4 are )29.7 and )32.9ppm. If P(Q4) is formed in glasses it locates be-tween around )20 to )25 ppm of P(Q2) andaround )36 to )40 ppm of P(Q3). In the previouspaper we assigned the P(Q4) in Na2O±P2O5±B2O3±SiO2 glasses [22]. In the following the deconvolu-tion will be made by including P(Q4). In the de-convolution, the following will be taken intoconsideration:1. P(Q4) structural unit coexists in equimolecular

amounts with the Si(Q6) species.2. By addition of CaO, residual P(Q3) is destroyed

to form P(Q2) resulting in a P±Oÿ±Ca2� bond.Fig. 9 represents the distribution of each P(Qn).Generally, the normalized standard errors of thedeconvolution of the NMR spectra were within6.4%. The contribution of P(Q4) to the overallspectra has maximum when x is about 0.25. Thesolid lines are derived by assuming that SiO2 par-ticipates quantitatively in the formation of P(Q4)and the other P(Qn) can be formed in the samemanner seen in the binary CaO±P2O5 glasses.Thus, the distributions of P(Qn) are expressed bythe following equations:

XP�Q2� � �CaO�=�P2O5�;XP�Q3� � ��P2O5� ÿ �CaO� ÿ �SiO2��=�P2O5�;XP�Q4� � �SiO2�=�P2O5�:

This result suggests also that P2O5 prefers to reactwith CaO.

The Raman peaks at 1350 and 1200 cmÿ1 arerelated to the presence of Si(Q6) [16]. Actually, theappearance of these peaks is correlated with thepresence of a peak at )215 ppm in the 29Si NMRspectra. Larsen et al. [29] examined Raman spec-troscopy of SiO2±MgO±Al2O3±P2O5 glass andfound four peaks from 800 to 1500 cmÿ1. Twobands at 1165 and 1205 cmÿ1 were assigned to thesymmetrical and asymmetrical stretching vibrationof P(Q2). Thus, the signal at 1160 cmÿ1 (peak d)can be assigned to P(Q2). It is con®rmed from theresults of studies of binary CaO±P2O5 glasses.However, if the band at 1210 cmÿ1 (peak c) is as-signed to the vibration of P(Q2), the disappearanceof this signal in glasses of high x value contradictsthe change of 31P NMR spectra. Thus, we assignthis signal to the vibration related to Si(Q6). Thepeak intensity at 1300 cmÿ1 (peak b) decreaseswith an increase in x. Larsen et al. [29] assignedthis band (1305 cmÿ1) to the P¸O stretching ofP(Q3). By considering the Raman spectra of binaryCaO±P2O5 glasses and 31P NMR, the peak b canbe assigned to P(Q3). When x exceeds 0.45 newbroad signals around 1100 (peak e) and 1060 cmÿ1

(peak f) appeared. They are assigned to the vi-bration of P(Q1) [29,30]. The appearance of P(Q1)is con®rmed also from the 31P NMR spectra.

4.3. 0.30MO±0.05SiO2±0.65P2O5 glasses

A weak peak at )165 ppm appears also clearlywhen Si(Q4) and Si(Q6) coexist. The population ofSi(Q6) is also shown in Fig. 8. The population ofSi(Q6) increased in the order of MgO < CaO <SrO;BaO� Na2O. In the previous paper [16], ithas become apparent that the population in-creased with a decrease in basicity of the glasses,i.e., the [Si(Q6)]/[Si(Qn)] ratio had a role of acid±base indicator. These results suggest that the bas-icity of the basic oxide increases in the order of

Fig. 9. Experimentally determined P(Qn) distribution as a

function of CaO content (x) in the xCaO±0.05SiO2±

(0:95ÿ x)P2O5 system. Error bar: see text.

56 H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59

MgO < CaO < SrO;BaO� Na2O. Nogami et al.[21] reported the percentage of Si(Q6) of 1SiO2±5SrO±14P2O5, whose composition is similar to thepresent glass, i.e., 0.05SiO2±0.30SrO±0.65P2O5,was 75%. The present data agree well with thatvalue. The stronger the basicity of the basic oxide,the larger the tendency to react with strong acidsuch as P2O5. Thus, SiO2 reacts only with theP2O5, so that formation of Si(Q6) is preferable inBaO and Na2O containing glasses.

The 31P NMR spectra of ternary 0.30MO±0.05SiO2±0.65P2O5 are asymmetric and have ashoulder in high magnetic ®eld, whose intensitybecomes larger in the order of MgO < CaO <SrO < BaO. Because SiO2 reacts with P2O5 toform Si(Q6) and P(Q4) in BaO±SiO2±P2O5 glass,the peak width becomes broader. The order of thepeak position is understood by the di�erence of theelectronegativities among alkaline earth metals.

4.4. MO�M 02O�±SiO2±Al2O3±B2O3 glasses

The change of the NMR spectra can be un-derstood by a competition among the three net-work-forming oxides (SiO2, B2O3 and Al2O3) withbasic oxides. In binary glass melts containingNa2O, the thermodynamic acid strength was de-termined by EMF measurements to be P2O5 >B2O3 > SiO2 [31]. The optical basicity increased inthe order of P2O5 < B2O3 < SiO2 < Al2O3 [32].Thus, in sodium borosilicate glasses, it is com-monly understood that Na2O reacts mainly withB2O3. The homogeneous glass separates into twophases by heat treatment, one of which is an alkali-rich borate phase and the other is SiO2. By con-sidering the electronegativity of aluminum oraqueous system, Al2O3 is thought to be anamphoteric oxide. Maekawa et al. [26] investigated

the structural study of sodium aluminosilicateglasses by 29Si NMR. The reaction among Si(Qn),2Si�Qn� � Si�Qnÿ1� � Si�Qn�1�, seemed to proceedto the left direction with increasing aluminumoxide contents because Al2O3 dissolves in silicateas NaAlO2. As shown in the ®gure, it is observedthat the pro®les of 27Al NMR do not change withreplacing Al2O3 by B2O3. On the other hand, thepro®le of 29Si NMR of 0.673SiO2±0.139Al2O3±0.188Na2O glass is symmetric whose position lo-cated around )95 ppm (see Fig. 5(a), x � 0). Bysubstitution of Al2O3 for B2O3, peak width in-creases at ®rst and the spectrum consists of at leasttwo peaks when B2O3 content becomes 0.139. Thepeak position moves toward high magnetic ®eldsand a peak appeared at )90 ppm, which is due tothe formation of Si(Q3). This means that excisionof silicate network proceeds with an increase inB2O3 content. Table 1 represents the distributionof Si(Q3) determined by the deconvolution of the29Si NMR spectra, X obs

Si�Q3�. The proportion ofSi(Q3) increases with an increase in B2O3 content(x). However, the spectra could not be deconvo-luted when x < 0:093. This result suggests thatNaAlO2 species coordinates the secondary coor-dination-sphere of Si(Q4) species and peak posi-tion of Si(Q4) moves to lower magnetic ®eld side.The population of Si(Q3) is also shown in Table 1.These values were calculated from Al2O3 contentand the proportion of B(Q4) determined by thedeconvolution of the 11B NMR spectra. The pop-ulation of Si(Q3) can be calculated by assumingresidual Na2O reacts with SiO2, X calc

Si�Q3�. The valuesagree well with the data determined from the 29SiNMR spectra. Fig. 10 represents a relation be-tween populations of 4-coordinated boron atomsand r (���Na2O� ÿ �Al2O3��=��Na2O� ÿ �Al2O3���B2O3���; here the data of Figs. 5(c) and 6 are used.

Table 1

Distribution of Si(Q3) determined by the deconvolution of the 11B and 29Si NMR spectra of 0:188Na2O±0:673SiO2±

�0:139ÿ x�Al2O3±xB2O3 glasses

x Na� containing species X calcSi�Q3� X obs

Si�Q3�NaAlO2 B(Q4)

0.093 0.046 0.062 0:238� 0:018a 0:230� 0:008a

0.116 0.023 0.081 0:250� 0:024 0:230� 0:009

0.139 0 0.097 0:270� 0:038 0:273� 0:012

a See text.

H. Yamashita et al. / Journal of Non-Crystalline Solids 270 (2000) 48±59 57

Generally, the normalized standard errors of thedeconvolution of the NMR spectra were within9.9%. The fraction of 4-coordinated boron in-creases with an increase in r. With further increasein r, it decreases. A solid line is the theoreticalvalue which is expected in xNa2O±(1ÿ x)B2O3

binary glasses; the populations of 4-coordinatedboron atoms, N4 � x=�1ÿ x�. Experimental valuescoincide well with the theoretical line. One seesthat Na2O reacts preferentially with Al2O3 andonly the residual Na2O reacts with B2O3 to form 4-coordinated boron atoms. Di�ering from B2O3,Al2O3 cannot be present in threefold coordinationwithout sodium ions. This agrees with Al2O3 beingconsidered as an acidic oxide in Na2O±Al2O3±B2O3 melts by EMF measurements [31].

5. Conclusions

The MAS-NMR spectra of 11B, 27Al, 29Si and31P of MO±P2O5, MO±SiO2±P2O5 and MO(M0

2O)±SiO2±Al2O3±B2O3 (M�Mg, Ca, Sr and Ba,M0 � Na) glasses were examined. In binary MO±P2O5 glasses the distributions of phosphate sites,P(Qn), can be expressed by a theoretical predictionthat P2O5 reacts quantitatively with MO. In MO±SiO2±P2O5 glasses, Si(Q6) and Si(Q4) were detect-ed. The fraction of Si(Q6) increases in the order of

MgO < CaO < SrO;BaO < Na2O. In xCaO±0.05SiO2±(0:95ÿ x)P2O5 glasses, the population of6-coordinated silicon increases with an increase inf �� ��P2O5� ÿ �MO� ÿ �B2O3� ÿ �Na2O��=�SiO2���and has a maximum at f � 9. The signal due tothe 5-coordinated silicon atoms is also observed inglasses when Si(Q4) and Si(Q6) coexist. In Na2O±SiO2±Al2O3±B2O3 glasses, where three network-forming oxides such as SiO2, Al2O3 and B2O3 co-exist, Al2O3 reacts preferably with Na2O. Thepopulations of 4-coordinated boron atoms, N4, areexpressed well with r=�1ÿ r�, where r ���Na2O� ÿ �Al2O3��=��Na2O� ÿ �Al2O3� � �B2O3��.It agrees with the theoretical expectation usuallyseen in the binary sodium borate glasses that thepercent of 4-coordinated boron atoms, N4, in-creases and has a maximum with an increase in x.The Raman signals at 1210 and 1350 cmÿ1 arecorrelated with the NMR signal of Si(Q6) at )215ppm which is also seen in CaO±P2O5±SiO2 glasses.

Acknowledgements

The authors wish to thank the Advanced In-strumentation Center for Chemical Analysis andthe Center for Cooperative Research and Devel-opment, Ehime University, for the NMR andX-ray di�raction measurements, respectively. Thiswork was supported in part by a grant-in-aid forscienti®c research (no. 09450327) from the Minis-try of Education, Science, Sports and Culture,Japan.

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