STRUCTURE OF Bi2O3−SiO2 GLASSES
Tokuro Nanba*, Hiroyasu Tabuchi and Yoshinari Miura
Department of Environmental Chemistry and Materials, Okayama University,
Okayama 700-8530, Japan
tokuro_n@cc.okayama-u.ac.jp

While Bi2O3 has been so far regarded as a conditional glass former, the structural role of Bi ions in thebismuthate glasses has been still obscure. In the present study, Bi2O3−SiO2 system was chosen toinvestigate the glass structure. According to 29Si MAS-NMR and X-ray radial distribution analyses,
similarities in local structure around bismuth and oxygen sites were confirmed between
50Bi2O3·50SiO2 glass and a Bi2SiO5 crystal. It was concluded that Bi−O−Bi networks were commonly
present in Bi2O2 layers and 1-dimensional SiO3 chains interlinked the layers. It was also concluded
that Si−O−Si networks were indispensable for the glass formation(Keywords: glass structure, NMR, XPS, diffraction method)present in the glass. It was expected that Bi ionsplayed a role as a network former, NWF in theBi2O3−Li2O glass. While coordination number of
Bi ions has been reported in the combination ofsome typical NWFs such as B2O3 and P2O5 [3−5],structural roles of Bi ions have not been clarified.
In the present study, Bi2O3−SiO2 system waschosen to examine the glass structure, where thestructural information was collected from thespectroscopic methods such as optical absorption,
29
Si MAS-NMR and XPS, and the radial
distribution analyses were also performed from
X-ray diffraction measurements. According to theinformation, the structural role of Bi ions in theglass formation was discussed.

1. Introduction
Pb-containing
glasseshaveexcellent
properties, such as high refractive indices andlow melting temperatures, and hence they havebeen widely used in various applications, such astable wares, optical lenses and solder glasses forelectronic components. Bi and Pb are placed next
to each other on the periodic table, and Bi3+ andPb2+ ions have the same 6s2 electronicconfigurations. Therefore, a number ofsimilarities have been confirmed in variousproperties. In toxicity, however, Bi is much safer
than Pb. For these reasons, Bi-based materials
have been expected as substitutes for thePb-containing materials, and the novel glasssystems have been explored in glass industries.
It is well known that Bi2O3 does not vitrify
without additives, but the addition of other oxides
enables its vitrification. It has been reported thatBi ions in oxide glasses have 5- or 6-foldcoordination state [1]. Bi ions in trivalent state
possess lone pair electrons in the outer most 6s2shell. It has been accepted that the 6s2 lone pairelectrons are localized on Bi ions and they do notpartake in Bi−O bonds. It has been also acceptedthat BiOn polyhedra were distorted due to thelone pair electrons, and the distortion in BiOn
units would make the structural analyses for theBi-containing glasses difficult.
According to radial distribution analyses,
Watanabe et al. [2] revealed that Bi2O3−Li2O
glass had a similar structure to a Bi2O4 crystal. In
Bi2O4 crystal, Bi ions are in a mixed valence stateof 3+ and 5+, and in the Bi2O3−Li2O glass,however, it was confirmed that Bi ions were all ina trivalent state and large amount of defects, suchas positive electron holes on oxide ions were2. Experimental
The glasses were prepared in the compositionof xBi2O3·(100−x)SiO2 (x = 35 − 65 mol%), and
0.1 mol% Fe2O3 was also added to the batches for
reducing the spin lattice relaxation time in 29Si
MAS-NMR measurements. At x = 35, 40 and 50,
the batches were melted at 1050 − 1200 ºC in a Pt
crucible, and at x = 60 and 65, they were melted
at 950 − 1010 ºC in a Au crucible. The melts
were press-quenched by Cu plates, andsometimes the crucible bottom was dipped inwater obtaining higher quenching rate.
The glasses obtained were evaluated byvarious methods. Thermo-gravimetric analysis(TGA) and differential thermal analysis (DTA)
were performed under the atmospheres of dry air
and nitrogen flow. Diffuse reflectance spectra of
the powdered specimens were measured with aspectrophotometer equipped with an integrating
sphere, and the reflectance spectra measured
were converted into absorption spectra by using a1

density and r is the distance from any reference
atom center. Reliability of the data was checkedby reverse Fourier transformation. XPS and 29Si
MAS-NMR measurements were done for PbO−
SiO2 glasses, making a comparison in localstructures.

Kubelka-Munk function. XPS measurements
were carried out, where the glass samples werefractured in an ultrahigh vacuum (~ 1 × 10−7 Pa),and the fresh surfaces were immediately analyzed
to prevent surface degradation. A monochromatic
Al-Kα (hν = 1486.6 eV) was irradiated to thefresh surfaces, and the surface charge wascontrolled by means of covering the surface with
an electrically grounded Ni mesh screen as well
as flooding with low energy electrons (~ 5 eV)[6]. Crystal powders were also analyzed applying
the same charge neutralization method. C1s peakof an adventitious hydrocarbon was adopted as acalibration standard (284.6 eV). 29Si MAS-NMR
spectra were obtained at 59.6 MHz (7.05 T) with
5.0 µs pulses and 1.0 s recycle delays. The
chemical shift standards used were poly(dimethyl siloxane) (PDMS). X-ray scattering
measurement was carried out with powdered
specimens, being irradiated with a monochromatic Mo-Kα (λ = 0.7107 Å) radiation. Total
correlation function, T(r) = 4πrρ(r), anddifferential correlation function, D(r) = 4πr[ρ(r)− ρ0], were obtained by a Fourier transformationof the interference function, where ρ(r) is thedensity function, ρ 0 is the average number
Bi2O3 [3+]x=40

Bi2O4 [3+&5+]

x=50

Bi4O7 [3+&5+]

x=60

Bi2O5 [5+]x=65

3. Results
Optical absorption spectra were measured inorder to evaluate the valency of Bi ions. As
shown in Fig. 1, the absorption edge of thepresent glasses are located at around 3 eV, whichis almost the same with the bismuthate crystals,
Bi2O3 and Bi4(SiO4)3 containing Bi3+ ions. It istherefore concluded that Bi ions in the glasses arepresent in a trivalent state and the amount of Bi5+ions are negligible if present. Furthermore,neither weight gains under air flow nor weightlosses under N2 flow were confirmed in TG
measurements, leading to a conclusion that
positive electron holes on oxide ions and oxygen
defects which have been confirmed in
Bi2O3−Li2O glass were negligible in the present
Bi2O3−SiO2 glass.
Fig. 2 shows O1s XPS spectra. With
increasing Bi2O3 content, the O1s signal shifts tothe lower binding energy side, indicating theincrease in polarizability of oxide ions due to theaddition of Bi ions with higher polarizability. In
the present glasses, oxide ions are classified into
three groups, Si–O–Si, Si–O–Bi and Bi–O–Bi. In
the observed O1s XPS spectra, however, only
two components are extracted. The lower binding

Bi4(SiO4)3 [3+]5+Bi

Si-O-Si3+Bi

Si-O-Bi+Bi-O-Bi(α/s) / arb.units

Intensity / arb.units

6560
50
40
x=35
1.0

2.0
3.0
Photon Energy (eV)4.0534Fig. 1. Optical absorption spectra obtained from thediffuse reflectance measurements for xBi2O3·(100−x)SiO2 glasses and the bismuthate crystals withvarious valence states of Bi ions.

532
530
528
Binding energy / eV

526

Fig. 2. O1s XPS spectra and the separated components
for xBi2O3·(100−x)SiO2 glasses.

2

Table 1. Isotropic chemical shifts (δ / ppm) andrelative intensities (I / %) of Qn units in xBi2O3·(100−x)SiO2 glasses. The uncertainties are 0.5 ppm forδ and 2% for I.

energy component is attributed to the oxide ionsin Si–O–Bi and Bi–O–Bi groups. It is expectedthat Bi–O bonds have much higher π-bonding
character than Si–O bonds, because the vacantBi6d orbitals spread out more widely than Si3d.
It is therefore supposed that electrons on oxideions are delocalized through the vacant Bi6dorbitals, and the oxide ions adjacent to Bi ionsare equalized in electron density, being observed
as one component in the O1s signal. It istherefore concluded that the higher binding
energy component is attributable to the oxideions in Si–O–Si units and the lower energy
component is to those in Si–O–Bi and Bi–O–Biunits.
Fig. 3 shows 29Si MAS-NMR spectra. The
relative amounts of Qn species (SiO4 units containingn bridging oxygens) were obtained from the peakseparation. As far as known to the authors, 29Si NMR
studies on the Bi2O3−SiO2 system have never been
published. Therefore, among the PbO−SiO 2systems [7−9], the 29Si chemical shifts in aPbF2−PbO−SiO2 system [7] were chosen as theinitial entries for the peak separations, which wereshown as the vertical lines in Fig. 3. The results ofthe deconvolutions are summarized in Table 1. The
relative amounts of bridging oxygens in Si−O−Si
units are estimated from the deconvolutions results,
and they are shown in Fig. 4 together with the data
evaluated from XPS. The amounts of Si−O−Six

I35 105.1 26
40 104.5 1350 104.0 560
65Fraction / %40

I95.9 3795.5 3693.2 28
91.6 18
91.4 16Q2−δI
87.7 3187.1 4185.6 4385.1 40
84.8 33

Q1Q0−δ

I
78.8 678.8 1079.3 2479.7 38
79.4 45−δI

74.6 474.7 6XPSNMR302010030

405060
Bi2O3 content, x / mol%70Fig. 4. Relative amount of oxide ions in Si−O−Si
bridges determined from O1s XPS and 29Si NMR inxBi2O3·(100–x)SiO2 glasses. The uncertainties are 2%in XPS and 5% in NMR.

bridges are in good agreement between XPS andNMR, indicating that the deconvolutions and thesubsequent assignments in XPS and NMR areappropriate for the present Bi2O3−SiO2 glasses.
Fig. 5 shows the differential correlation
functions, D(r)s obtained from X-ray diffraction
measurements. The peaks seen at 2.3 and 3.8 Å
are assigned to the neighboring Bi−O and Bi−Bipairs. With increasing Bi2O3 content, a broad
shoulder at around 2.7 Å increases in intensity,
suggesting the wider distribution of Bi−O bondlength and the distortion of BiOn polyhedra. The
peaks of Si−O bonds are supposed to be observed
at 1.6 Å but are not clearly seen in Fig. 5, whichis due to the large difference in scattering
coefficient between Bi and Si atoms. Then, thecoordination numbers only around Bi ions areevaluated from the RDFs (Table 2). At x = 50, thecoordination numbers, NBi(O) and NBi(Bi) areapproximately consistent between the glass and65
Intensity / arb.units

Q3−δ

50

Q0Q1Q2 Q3 Q46050
40x=35-60

Q4−δ-80-100-120
Chemical shift, δ / ppmFig. 3. 29Si MAS-NMR spectra at 7.05 T for xBi2O3·(100−x)SiO2 glasses. The vertical lines indicate the29
Si chemical shifts of Qn units in PbF2−PbO−SiO2glasses [7].

3units. Fig. 6 shows the relative amount of oxide ionsin Si−O−Si, Si−O−Bi and Bi−O−Bi units estimatedin this way, and the results for PbO−SiO2 glass arealso shown in Fig. 6. As shown by the dotted line, all
the bridging Si−O−Si oxygens would turn into
non-bridging Si−O−(Bi,Pb) oxygens at O/Si ratio =4.0, if Bi and Pb ions acted as a network modifier,
NWM. In practice, however, 30% of oxide ions arepresent as bridging oxygen at O/Si = 4.0(40Bi2O3·60SiO2). The deviation from thetheoretical line indicates the formation of oxygens
not bonding to Si atoms, that is, the formation ofBi−O−Bi networks. In the high Bi2O3 content glasses(x ≥ 60), over 50% of oxygens are present in theBi−O−Bi networks. It is also noted that the Si−O−Sibridges remain even at the upper limit of the glass
forming region (O/Si = 7.6), suggesting that theSi−O−Si bridges are indispensable to the glass
formation.

crystal, suggesting the similarity in coordination
structure around Bi ions. In RDFs, however, apeak at around 5 Å is commonly observed for thecrystals (dotted lines), while no peak is seen for
the glasses (solid lines). In the crystals, the 5 Åpeak is attributed to the second neighboring
Bi−Bi pairs across SiO4 or BiOn units,Bi−(Si/Bi)−Bi. It is therefore expected thatrandom distribution of the second neighboring
BiOn units results in the broadening andweakening of the 5 Å peak for the glasses.
Table 2. Number of the nearest neighbors around Biions, NBi for xBi2O3·(100–x)SiO2 glasses, which wasestimated from a peak fitting (uncertainty = ±0.2). The
NBi values in the bismuth silicate crystals, Bi4(SiO4)3,Bi2SiO5 and Bi12SiO20 are also shown in theparentheses.
x35NBi(O) 6.0
NBi(Bi) 7.440
6.1 (6.0)
7.6 (5.0)

60
6.7
9.4

86
— ( 7.0)— (10.0)

Bi2O3−SiO2SiOSi
SiOBiIBiOBi

Bi-Bi40

PbO−SiO2SiOSi
SiOPb
PbOPb

Bi-O
100

86

30SiOSi theory80602010500x=40Fraction / %D(r)=4πr[ρ(r)-ρ0] / 103 el2·Å-250
6.3 (6.0)
8.0 (8.0)6040200-10024Radius, r / Å682Fig. 5. Differential correlation functions, D(r)s forxBi2O3·(100–x)SiO2 glasses (solid lines, experiment)
and the bismuth silicate crystals, Bi4(SiO4)3 (x=40),
Bi2SiO5 (x=50) and Bi12SiO20 (x=86) (dotted lines,simulation).

34567(O/Si) atomic ratio /−8Fig. 6. Fraction of various oxygen sites in the glass
systems of Bi2O3−SiO2 and PbO−SiO2 determinedfrom 29Si NMR analyses. The dotted line indicates thetheoretical amount of typical bridging oxygen inSi−O−Si units, assuming Bi and Pb ions as NWMs.

4. Discussion

Among the bismuth silicate crystals, Q0 units are
exclusively present in Bi4(SiO4)3 [10] and Bi12SiO20[11], and in Bi2SiO5 [12], however, only Q2 units arepresent, forming continuous (SiO2+2/2)2− chains andinterlinking (Bi2O2)2+ double layers (Fig. 7). In theBi2SiO5 crystal, there are three crystallographic
oxygen sites; O(1) in Si−O−Si (20%), O(2) inFrom the Qn distributions determined from 29Si
NMR analyses (Table 1), the amounts of oxide ionsin the structural groups, Si−O−Si and Si−O−Bi areestimated, where it is assumed that oxide ions in theSi−O−Bi units were regarded as NBOs in 29Si NMR.
The residuals are attributable to the oxide ions notbonding to Si atoms, that is, oxide ions in Bi−O−Bi4networks. As also shown in Fig. 6, the glass forming
region overlaps between PbO−SiO2 and Bi2O3−SiO2glasses at (O/Si) = 3.5 − 4.5. In this region, theBi−O−Bi population is much higher than Pb−O−Pb,indicating that Bi ions are much more likely to act asa network former. According to Imaoka et al. [13],Pb ions prefer PbO3 sites to form PbO3 chains. The
difference in coordination structure between Bi andPb is a possible reason for the ability for network
formation. It should be noted again that the upper
limit of (O/Si) ratio is different between PbO−SiO2
and Bi2O3−SiO2 systems but the lower limit ofSi−O−Si population is almost the same at 15%. It
suggests that Si−O−Si networks are indispensablefor the glass formation even when BiOn or PbOn
units join in the glass networks.

Si−O−Bi (40%) and O(3) in Bi−O−Bi (40%). As
shown in Fig. 6, the populations of these oxygensites in the glasses determined by NMR are almost
the same with those in the crystal at x = 50 (O/Si =5.0). As shown in Table 2, the similarity incoordination number around Bi ions is also
confirmed at x = 50. From these results, it is finallyconcluded that 50Bi2O3·50SiO2 glass has a similar
structure to Bi2SiO5 crystal, and it is therefore
expected in the 50Bi2O3·50SiO2 glass that BiOn
units gather in (Bi2O2)2+ double layers sharing theiredges and Q2 units build continuous (SiO3)2− chains
between the Bi layers.

5. Conclusions

Bi2O3−SiO2 system was chosen to investigate
the structural role of Bi ions. It was confirmedthat Bi ions were present exclusively in a
trivalent state. The structural similarities wereconfirmed between 50Bi2O3·50SiO2 glass andBi2SiO5 crystal; the similarities in local structurearound bismuth and oxide ions were recognizedfrom X-ray radial distribution and 29Si NMRanalyses, respectively. In 50Bi2O3·50SiO2 glass,
Bi ions occupied BiO6 sites, and the BiO6 units
were surrounded by eight BiO6 units sharing theiredges. The oxygen sites were classified into threegroups, Si−O−Si, Si−O−Bi and Bi−O−Bi, whose
respective populations were estimated as 20%,
40% and 40% in 50Bi2O3·50SiO2 glass. It wasconcluded that the Bi−O−Bi bridges formedlayers, the Si−O−Si bridges formed chains of Q2
units, and the bismuthate layers were joined withthe silicate chains. The structural characteristics
were compared with PbO−SiO2 system. It was
found that the Bi−O−Bi networks existed over
the whole glass forming region, while thePb−O−Pb networks were recognized only in thePbO-rich region at (O/Si) > 3. The population ofBi−O−Bi bridges was higher than that ofPb−O−Pb bridges, indicating that Bi ions prefer toact as a network former than Pb ions. In Bi2O3−SiO2system, however, Bi−O−Bi networks alone could
not form glass, and Si−O−Si networks wereindispensable for the glass formation.

Fig. 7. Crystal structure of Bi2SiO5.
S.G. = Cmc21(36), a=15.17, b=5.47, c=5.31 Å [12].

The Qn distributions in PbO−SiO2 glass obtainedin this study show a similar trend to the previous 29Si
NMR studies [7−9]. As compared with Bi2O3−SiO2system (Fig. 6), PbO−SiO2 system has narrower
glass forming region in the scale of (O/Si) ratio (<4.5). At (O/Si) ≤ 3, the Si−O−Si population in PbO−
SiO2 glass is approximately in accordance with thetheoretical prediction and Pb−O−Pb bridges are notpresent, suggesting that Pb ions exclusively act as anetwork modifier in this region and act as a network
former only in the higher (O/Si) region. To the bestof the authors′ knowledge, the quantitative
estimation of Pb−O−Pb bridges has never beenreported. As for Bi2O3−SiO2 glass, Bi−O−Bi groups
are confirmed over the whole glass forming region,
indicating that Bi ions prefer to join in the glass

Acknowledgments
The authors would like to thank Dr. T. Watanabe ofToyoda Gosei Co., Ltd. for the technical assistance.

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6