2.journal of non-crystalline solids volume 159 issue 3 1993 [doi 10.1016%2f0022-3093%2893%2990225-m]...

ourna, of Non C sta,,ine So,iris 159 1993 213-221 North-Holland

Structural studies in lead germanate glasses: EXAFS and vibrational spectroscopy

S.J.L. R ibeiro a j . Dexpert -Ghys b B. Pir iou b and V.R. Maste laro c lnstituto de Quimica - UNESP, CP 355, 14800-900, Araraquara, SP, Brazil

h Laboratoire des Eldments de Transition dans les Solides, CNRS, 1, Pl. A. Briand, 92195, Meudon c~;dex. France c LURE, Unit,ersit~ Paris Sud, 91405, Orsay, France

Received 9 September 1992 Revised manuscript received 31 December 1992

The Raman, IR absorption and EXAFS spectra at the Ge K-edge and Pb Lm-edge of eight lead germanate glasses, with general formula xPbO(1-x )GeO 2 with x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60, have been measured. The occurrence of [GeO 6] units besides [GeO 4] could not be deduced unambiguously from the data. The vibrational and EXAFS data agree with a progressive depolymerization of the network. Starting from all Ge atoms linked to four bridging oxygens in GeO 2 (x = 0), the number of tetrahedral units with one or two non-bridging oxygens increases with x. At low content, Pb ~ + ions act as modifiers in the germanate structure, but to a lesser extent than an equivalent number of alkaline ions.

I. Introduction

Glasses and crystals in the PbO-GeO 2 system are well known. The phase relations and crystal- lization of glasses have been established [1]. A previous investigation by means of europium as a local probe in glasses of the related system PbF 2- GeO 2 has been published [2].

Alkali germanate glasses (xM20(1 - x)GeO 2, M += Li +, Na + or K +) have also been the subject of many investigations. Most conclude that there is a partial change of the germanium coordination number from four (Ge TM) in glassy GeO 2 to six (Ge vI) when x increases. This change would induce the so-called 'germanate anomaly' observable for several physical properties such as the refractive indices [3], densities [4], elastic con- stants and molar volumes [5]. The identification of Ge w as well as Ge TM has been corroborated by

Correspondence to: Dr S.J.L. Ribeiro, Instituto de Qulmica - UNESP, CP 355, 14800-900, Araraquara, SP, Brazil.Tel: + 55- 162 322 022. Telefax: + 55-162 227 932. E-mail: uearq@brfa- pesp.bitnet.

IR absorption [6], Raman scattering [7], X-ray diffraction [8] and EXAFS [9,10]. From these measurements it may be concluded that the proportion of Ge w is 20% of the total Ge atoms, at 0.15~

214 S.J.L. Ribeiro et al. / Lead germanate glasses

amount of change are less than in alkali germanates [14]. Recently, Ruller et al. [15] have referred to this change in coordination number in the explanation of some properties of sodium- lead germanate glasses.

Vibrational data (IR absorption and Raman scattering) have also been interpreted as exhibit- ing features characteristic of Ge vI [16,17], although Canale et al. [18] have shown that assignments of vibrational frequencies based on [GeO6] octahedra were very difficult to make since the broad features observed could obscure characteristic bands.

In this work we describe the investigations by vibrational spectroscopy (IR absorption and Ra- man scattering) and EXAFS at the Ge K-edge and Pb Liii-edge of several lead germanate glasses with 0.2 4 x 4 0.6. We discuss our results by con- sidering two questions. First, is there any evidence, from our experimental data, supporting the observation of Ge vI groups in this composition range? Second, to what extent do lead atoms play the role of modifiers in the germanate network? For this second question we compare our results with the above-cited works on alkali and lead germanates, with some Raman data of glasses containing heavy metal oxides [19], and with results for lead silicate glasses [20] and lead borate glasses [21]. In the following, the notation Ge TM and Ge w will refer to germanate entities with respectively the tetrahedral and octahedral coordination, whereas the different tetrahedral entities containing 4, 3, 2, 1 and 0 bridging oxygens (and obviously the complement to 4 non-bridging oxygens) will be labelled Q4 to Q0 as is usual in NMR notation.

2. Experimental details

Glasses were obtained by melting appropriate mixtures of the starting oxides (GeO 2 99.999% and PbO 99.99% Aldrich) in a platinum crucible at 1000C for one hour followed by quenching on a steel plate at about 200C (cold water quenching was necessary for the composition x = 0.60). The amorphous state was measured by X-ray diffraction and the compositions were deter-

mined by Pb and Ge inductive plasma emission spectroscopy analysis. Samples were obtained with the following compositions: x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60. For the last one, which had very poor optical quality, only IR spectra were obtained.

Crystalline PbGeO 3 was obtained by a solid- state reaction between PbO and GeO 2. Powder mixtures of the appropriate amounts of the oxides were pelletized and placed in platinum cru- cibles at 600C for 6 h. The pellets were ground to powders, and again pelletized and treated for 6 h at 600C. X-ray diffractograms agreed well with the one given in ref. [22].

The experimental set-up for Raman measurements was an argon ion laser (Spectra-Physics 165), a Coderg PHO double monochromator spectrometer, a Hammamatsu 1477 photomulti- plier and a Keithley 177 multimeter. The output of the multimeter was linked to a 16-bit BFM 187 computer for storage and processing. Spectra were obtained at room temperature with the clas- sical 90 geometry between the incident beam and scattered light. Samples were cut and pol- ished with diamond paste and were investigated under parallel (VV) and perpendicular (VH) polarizations. The spectral slit widths were routinely set at 6 cm -1 and the spectrometer was cali- brated with the lines of a low-pressure mercury lamp.

Infrared absorption spectra were obtained with powders, in KBr pellets with a FTIR Nicolet 730FX (resolution set at 4 cm-1).

Room-temperature transmission EXAFS measurements were obtained at the germanium K- edge and lead Lm-edge. We utilized different spectrometers (EXAFS I station - Si (331) chan- nel-cut monochromator and EXAFS III station - Si (331) double crystal monochromator) with ion chambers as detectors. The spectra were recorded using the DCI storage ring at Orsay (France) which was typically operating at an energy of 1.85 GeV and 250 mA current. The ground samples (~< 20 p~m in size) were homogeneously layered on kapton strips.

The EXAFS oscillation curves, x(k ) , obtained by a standard procedure [23], were Fourier trans- formed using k3x(k ) (Ge K-edge) and k2x(k )

S.J.L. Ribeiro et al. / Lead germanate glasses 215

(Pb Lni-edge) weighting. A Kaiser apodization function was applied over 5 to 11 .~-1 at the Ge K-edge and over 3.7 to 9 A-1 at the Pb L nl-edge, with r = 1.5 in both cases. Using a single scattering theory and the plane-wave approximation, the normalized oscillatory part of the absorption spectra is described as

[ f,("rr, k)lN~ x(k ) = - ~_~ kR 2 s in [2kR i + 4hi(k)]

i

exp( - 2o-i2k 2) exp( -2R i /A ) . (1)

In this formula N/ represents the number of atoms at a distance R i from the absorber, ~r i is a damping coefficient due to thermal and structural distributions of distances, A is the mean free path of the photoelectron and f i(k) and c~i(k) are the amplitude and phase-shift functions, which were obtained from standard compounds. The energy threshold was adjusted in all cases. The values for o- must be taken as relative differences between the samples and the reference compounds. These were trigonal GeO 2 (N= 4, R = 1.739 ,~) and orthorhombic PbO (N= 4, R 1 = 2.20 ,~ (2 oxygens) and R 2 = 2.49 A (2 oxygens)). In the analysis we have taken for PbO a mean Pb-O distance of 2.34 A and N= 4.

3. Results

3.1. Raman scattering

Figure 1 shows Raman scattering spectra for four samples with x = 0.20, 0.33, 0.50 and 0.56. These are reduced spectra, corrected for the thermal population effect, following the relation I R = Iw[n(~o) + 1] -t. I and I R are the observed and reduced Raman scattering intensity at wavenumber o) and n(w)= [exp(hw/kT) - 1] - l is the Bose population coefficient [24]. Spectra agree well with those presented by Canale et al. [18] despite the fact that these authors did not work with polarization data.

Comparing the general aspects of the spectra it may be seen that the scattered intensity in the mid-frequency range (200 to 600 cm- 1) relative to the one in the high-frequency range (600 to 1100

F-

i i m

(] i \, Ji (

,/' X = 0.20

/ J

l j h~ l i , , , l . . . . . . . . . . . . .

~0 I000

b A

',U /

i t

0 500 iO00

RAMAN SIIWT {cm "tj

Fig. 1. Reduced Raman scattering spectra for the glasses xPbO(1-x)GeO2: (a) parallel (VV) and (b) perpendicular

(VH) polarizations.

cm 1) decreases when the amount of lead oxide increases.

A strong polarized band is seen at approximately 450 cm -1 for the samples x = 0.20 and 0.25. It is accompanied by a shoulder near 920 cm 1 in the polarized (VV) spectra and a better resolved band at about 930 cm i in the depolarized (VH) spectra, both decreasing for higher PbO content but Still present up to the composition x = 0.40.

A rather narrow band is observed for all polarized spectra, at -- 820 cm i for x = 0.20 shifting to --790 cm t at x=0.56. A broader line appears in the VH spectra at frequencies ranging from -~800 cm-1 at x=0.20 to ---730 cm-1 at x = 0.56. Note that for this last sample an intense component at about 790 cm-1 is observed in the VH spectrum. This feature is attributed to the mixing of the VV and VH components. It appears for this composition which lies near the limits of the glassy state in this system. Some inhomogeneity at the microscopic scale could have altered the relative orientation of the incident and collected lights in a sample of poor optical quality.

For the composition x = 0.50 and for those with higher lead contents, we note in VV spectra, besides the shift of the line at about 800 cm ]

9O

3O

%-

0 W .3 Z

Z

r" I--

500 1OOO

RAMAN SHIFT (cr~ll

Fig. 2. Raman scattering spectrum for powdered PbGeO3, corrected for thermal population effect. Points denote para-

sitic lines of the argon plasma.

contents is an artifact linked to the mathematical reduction treatment.

No characteristic feature at 600 or 650 cm- could be isolated in the composition range 20 to 30% PbO as was the case in Li, Na, K germanate glasses [7]. Nevertheless, a shoulder at about 580 cm- 1 occurs in the VV spectra for 0.20 ~< x ~< 0.33.

The Raman spectrum for PbGeO 3 is presented in fig. 2. Polarization information is lost as the sample was a powder. Maxima were observed in the high-frequency part at 747, 782 and 815 cm- 1.

towards lower wavenumbers, the appearance of a very weak shoulder at approximately 730 cm-1.

A broad band appears in both orientations for all samples in the region 520-550 cm -1. At the same time a band at -- 370 cm-1 is observable in all spectra and is more intense for VH polarization. Its intensity relative to the one at about 540 cm-1 increases with lead content.

A narrow band is detected at 140 cm-~ for the compositions x = 0.50, 0.53 and 0.56. The small feature at 100 cm -~ observed even at low lead


1000 600

WAVENUMBER(cn~ 1)

Fig. 3. IR absorption spectra for the glasses xPbO(1-x ) - GeO 2.

3.2. IR absorption

Figure 3 presents IR absorption spectra (400- 1500 cm -~) for the glasses studied. They agree well with those presented in refs. [17] and [18], and contain a broad and strong band that shifts

,,~ X Ld

x=O

..J

r~ O

I--

4 6 8 I0 12 I 2 3 4 5 6

k(~ -I) R(~) Fig. 4. Ge K-edge EXAFS spectra and their Fourier transforms for the glasses xPbO(1-x )GeO2, GeO 2 hexagonal ('quartz-type'-denoted q) and glassy GeO 2. (a) EXAFS spec-

tra (x(k)); (b) Fourier transform modulus (F(R)).


from 720 cm -1 for x = 0.60 to 830 cm -~ for x = 0.20. Besides this shift, components at about 925 and 660 cm-t are resolved for the samples with x = 0.20, 0.25, 0.33 and 0.40. With increase in lead content, these shoulders are not resolved.

The main feature observed in the IR spectrum (not presented here) for crystalline PbGeO 3 is a strong peak at 753 cm- t, which is about the same frequency observed for the lead metagermanate glass (750 cm-~). There are also two other sharp components at 856 and 908 cm -~.

3.3. EXAFS spectra

Table 1 Fit results for xPbO(1 - x)GeO 2 glasses (Ge K-edge). Phase and amplitude from hexagonal GeO 2 (N= 4 and R= 1.739

Sample (x) N~ o RGe-o (A) A'G,:-O (A) (+_0.1) (+0.01) (0.005)

0 4.0 1.74 -- 0.010 0.20 4.2 1.75 0.040 0.25 4.2 1.75 0.028 0.33 4.3 1.75 0.041 0.40 4.5 1.76 0.030 0.50 4.5 1.77 0.041 0.53 4.8 1.78 0.039

The normalized EXAFs (g(k)) spectra and the Fourier transforms (modulus) for six lead germanate glasses at the Ge K-edge are displayed in figs. 4(a) and (b) respectively. Spectra for glassy GeO 2 and trigonal quartz-type GeO 2 are also shown. The spectra are similar for all the samples investigated.

x w

uR

! ~,2/%~ 0.20

' ' ~2 Fk 0.25

lCU d ~.o / p bb

; T ;",

~ ~/ \ 0.40

q ~ o ~ 0.50

%fo.~ ,~ 0.53

6 ? 8 9 I0

k(~-I)

Fig. 5. First-shel l f i l tered EXAFS spect ra (00o) and calculated curves (---) obta ined with parameters f rom table 1, for

glasses xPbO(1 - x )GeO 2.

One-shell fits were tried and fig. 5 presents the filtered g(k) EXAFS spectra and the calculated curves obtained with the parameters shown in table 1. With respect to the crystalline trigonal GeO 2 used as reference, we observed a tendency to an increase in the number of oxygen atoms in the first coordination shell, N, and an increase in the average Ge-O distances, R, with increase in lead content. These two features will be discussed in the following.

Furthermore, one may observe for the Fourier transforms (fig. 4(b)) the presence of a second peak which decreases in intensity with increasing lead content. We attribute this peak to second neighbour effects, a feature which is rather difficult to observe in glassy systems.

Spectra at the Pb Lm-edge are not presented here, but some general observations can be made. A very poor signal-to-noise ratio was observed mainly due to some intrinsic factors such as the high absorption cross-section for X-rays presented by lead atoms, the weak scattering power for oxygen atoms and the large disorder in atomic distances usually observed in lead compounds. Nevertheless, one-shell fits were tried giving N = 3 _+ 0.5 and R = 2.35 _+ 0.05 ,~ for all samples.

4. Discussion

In ref. [17] Kolesova stated that starting at x = 0.04 in xPbO(1-x )GeO 2 glasses, a component at 700 cm- l in IR might be related to Ge v'. This component should increase continuously


with increase in lead content but it would be hidden by a shift of the strongest band occurring at higher wavenumbers. Canale et al. [18] have given the interpretation that the shoulders observed for the high-frequency band for x = 0.20 could be due to the convolution of two different bands related to different glass phases. The IR absorption spectra presented in fig. 3 also exhibit a shoulder at around 685 cm-1 for the two lowest PbO contents investigated in this work (x = 0.20 and 0.25). We cannot determine whether it is still present under the main component in the spectra of samples with higher x.

Raman spectra for alkali germanate glasses (xM20(1 -x )GeO 2, M = Na or K) have well-resolved bands in the region between 600 and 650 cm -1 for compositions around x = 0.15. These bands are atributed to Ge w polyhedra and are supposed to be present also for lithium germanate glasses but as a part of a broader band [7]. In the compound 2Li209GeO2, where the germanate skeleton contains chains of Ge TM te- trahedra connected by Ge vl octahedra in a network involving only bridging oxygens, the main bands are at 566 and 595 cm-1 [7].

In Raman spectra (fig. 1), no characteristic band is observed in the 600 to 700 cm-~ region, which could be unequivocally assigned to Ge w groups. The shoulder near 600 cm- ~ for x = 0.20, 0.25 and 0.33 may be related to these groups, but this assignment is speculative. A component at about 735 cm -~ appears in the high-frequency Raman band for compositions x >/0.50. This amount of lead oxide is higher than that at which the discontinuity in physical properties related to Ge w atoms has been observed (between x = 0.20 and 0.40 from ref. [14]), so that we find no reason to attribute it to Ge w polyhedra and we rather interpret it in relation to depolymerization of the GeW-based skeleton, as discussed below.

The analysis of EXAFS spectra for xNa20- (1 -x )GeO 2 glasses developed in ref. [10] is in good agreement with the observation of the 'germanate anomaly' in a well-defined range of compositions. Starting from R = 1.73 A in a Ge TM network, the average Ge-O distance increases with the increasinog amount of Ge w, up to a maximum of 1.76 A for x = 0.20 before decreas-

ing again. Also, pressure-induced coordination changes in crystalline and vitreous GeO 2 have been confirmed by the observation of increases in Ge-O distances obtained from one-shell fits to- gether with XANES measurements [12]. Two- shell fits for the Ge K-edge EXAFS spectra in xL i20(1 -x )GeO 2 glasses are presented in ref. [9]. The conclusion of the authors is that the number of Ge vI increases markedly between x = 0.05 and x = 0.15, then it is constant to x = 0.25 (the maximum modifier content investigated in that work).

The results of our EXAFS analysis (fig. 5 and table 1) show an increase in Ge-O distances and Ge coordination numbers with increase in PbO content in the whole composition range. More- over, the results for glassy GeO2, in excellent agreement with the literature [10], show that the data treatment was satisfactory.

We may rationalize these results in two ways. The first interpretation is the appearence of Ge w groups, in a continuously increasing proportion up to the limit of the glass domain. If the shoulder at 600 cm 1 in our Raman spectra is assigned to Ge w groups, they occur in a limited range (0.20 ~x ~ 0.33) and not above x = 0.33. This interpretation seems not to be satisfactory.

The second way must involve a decrease of the disorder with increasing lead content. The De- bye-Waller disorder parameter, or, and the coordination number, N, are strongly correlated in the usual least-squares fitting procedures [23]. An increase in N or a decrease in or may cause the same effects in the EXAFS amplitudes. Actually, N is a scale parameter for the whole EXAFS spectrum while ~r is a damping factor, with its influence being observed for large values of k [23]. The spectra obtained could be extended only up to 11 A-~ in k-space so that it became difficult to resolve the contributions from these two parameters. EXAFS measurements with higher signal-to-noise ratio on a larger scale in k-space are necessary to discuss this point accu- rately.

The variation of the average distance, R, with x deduced from the EXAFS fits (table 1) can be reliably related to the increase in Ge-O distance as the depolymerization, or the average number

S.J.L. Ribeiro et al. / Lead germanate glasses" 219

of non-bridging oxygens (NBOs) linked to the germanium, increases. In crystalline PbsGe30~, for instance, the structure of which is made of Q~ and Q0, two Ge-O distances are encountered: 1.75 and 1.79 A [26], while it is 1.739 A. in crystalline GeO 2 (all Q4). Another evidence for depolymerization is the disappearence of the second-shell peak in the Fourier transforms with increasing lead content. This peak is not analyzed in this work but it must be related to Ge-Ge interactions in a polymerized network [10].

The Ge TM framework in alkaline germanate glasses has been investigated by Raman scattering [7,27]. The description in terms of NBOs used for silicate networks may be employed as well for germanates. In vitreous GeO 2 each Ge atom is linked to four bridging oxygens (BOs). Alkaline M + atoms break some of the Ge-O-Ge links and create Q3 where Ge atoms are surrounded by three BOs and one NBO. The composition corresponding to all Q3 atoms (in the absence of disproportionation) is the digermanate MzGe20 5. When all Ge atoms are Q2 (two BOs and two NBOs), the composition is MzGeO 3 (metagermanate). Of course, in a glass some dispersion in coordination numbers, bond distances and an- gles, may exist with respect to this simple description. Moreover disproportionation equilibria such as 2Ge(n) ~ Ge(n - 1) + Ge(n + 1) must be con- sidered. We may try to extract information on the germanate network from the Raman scattering investigations.

The spectra may be separated in three ranges. The low-frequency range (0 to 200 cm-~) is due to lattice modes and Pb-O vibrations. The mid-

frequency (200 to 600 cm-~) and high-frequency (600 to 1100 cm - l ) ranges are due to Ge-O vibrations. The decreasing scattering intensity in the mid-frequency range relative to the high- frequency range, with increasing modifier content, is a general observation for germanates as well as for silicate glasses [7,27]. It seems obvious that the increasing scattered intensity between 600 and 1100 cm- ~ is linked to a structural change from a totally poIymerized network towards a depolymerized one. Lines et al. [28] established a quantitative correlation between the ratios of in- tegrated intensities in both ranges and the amount of heavy metal oxide (for instance PbO) atribut- ing all scattered intensity in the mid-frequency

-1 range to BO modes. The band at --370 cm seems not to have the same decreasing pattern with increase in PbO concentration. In fact, Canale et al. [18] have assigned this band to a Pb -O stretching vibration along with deforma- tion modes of the germanium-oxygen network.

We have decomposed the high-frequency part of the spectra, assuming Gaussian band shapes for the components. VH spectra, which must contain only depolarized bands, were first decomposed and then VV spectra were similarly decomposed taking into account both polarized and depolarized bands; for the latter of which the frequency and line widths were determined in the VH analysis. Figure 6 shows the results for compositions x = 0.25 and 0.53, and table 2 summa- rizes the decompositions.

The depolarized A band may be related to a completely polymerized Q4 network as its disap- pearance for higher contents is accompanied by

Table 2 Parameters for the Gaussian functions utilized in the decompositions of the high-frequency ( > 600 cm t ) part of Raman spectra of xPbO(1 - x)GeO 2 glasses

x A(dp) B(dp) C(P) D(dp) E(P)

w y % w y % o~ y % w y % w 3' %

0.20 920 90 16 830 67 30 824 50 26 770 70 25 0.25 910 90 11 832 63 20 815 50 34 765 70 35 0.33 910 90 7 833 65 18 814 50 37 765 70 37 0.50 835 60 9 802 50 42 765 70 38 720 60 11 0.53 835 60 7 800 48 40 765 70 38 720 62 15

dp, depolarized; P, polarized; w, wavenumber (cm-I); y, bandwidth (cm- 1); %(i), area( i ) /E i areas


(o) ~ - - exp.

Z I--

(b) rr

660 -60 800 900 I~0 I100 WAVENUMBER (cm I)

Fig. 6. Gaussian decomposition for the high-frequency part of the VV Raman spectra. (a) x = 0.25; (b) x = 0.53. See text for

assignments of the components.

that of the strongly polarized band at 450 cm-1. This last band is the feature dominating the Ra- man spectrum of vitreous GeO 2 (Q4) [25].

The B and D components may be assigned to the Q3 and Q2 species respectively. The frequencies for alkali and alkaline-earth germanates given in refs. [7] and [27] are: Q3 (digermanate): 810 (VV), 860 and 780 cm -1 (VH); Q2 (metagermanate): 800 (vv) , 800 and 750 cm-1 (VH). With the general observation that a higher degree of depolymerization leads to lower vibration frequencies in this range, our assignment is consistent, consistent as well with the fact that D increases at the expense of B for increasing x. Surely the situation is more complex. Following observations on alkali germanate glasses, one ex- pects in fact two resolved partly depolarized bands for each species in this region, and in lead germanate compounds (PbGeO 4, PbsGe3010 two or even three modes characterize one species [29]. In lead germanate glasses we observe a consider- able overlap of the components.

We note that the intensities of the C and D components increase in the same way versus x, so C may be ascribed to Q2 as well. This assign-

ment is in good agreement with the spectrum of lead metagermanate (fig. 2) where one observes a band at 815 cm-t and a pair at 782 and 747 cm-1 that correspond to C and D respectively.

The lowest-frequency component, E, could be due to Q0. A corresponding Raman mode occurs in PbsGe30]] near 710 cm - t [29]. After these data, the C and D components must also include the contribution of the species Q1. These assignments for the five components we have isolated agree with the characteristic frequencies ex- tracted in ref. [29], except for orthogermanate (Q0) reported at 760 cm-~ on average, instead of the 720 cm-1 observed in our glasses.

One may then conclude that lead plays the role of a modifier of the germanate network, but to a lesser extent than the alkaline M + ions. From the data in ref. [7] one observes the presence of Q4 structure for x ~ 0.2 M20, whereas in lead germanates evidence for a fully polymerized network, given by the A mode, is observed for x ~< 0.4. This observation seems to be a general trend of lead oxide which to some extent has a glass network-forming character since it favours covalent directed Pb-O rather than ionic M + ... O - bonds. This point has been discussed, for instance, for lead silicate glasses. From the Raman and IR reflectivity data for these systems obtained by one of us [20], the existence of PbO 4 pyramids in the glass from two peaks at 95 and 138 cm-] which are also seen in PbO were proposed. Similar conclusions were proposed for lead borate glasses [21]. The band at 140 cm -1 is visible in our spectra for compositions x >/0.50 PbO, although it appears with a very weak intensity compared with the characteristic bands of the germanate. It appears more strongly with respect to the silicate bands [20]. This difference may be easily understood if one considers observations reported in ref. [27] that the intensities of the Raman spectra for germanates are generally much higher than those for silicates. The same low- frequency features are observed in lead borate glasses [21]. As discussed in this last reference, the highly polarized line at 140 cm-t is due to the symmetric vibration of the PbO 4 tetragonal pyramid. Because of the lone electronic pair of Pb 2+, fourfold or threefold coordinations are en-


countered in many oxygenated compounds, crys- tallized or glasses.

Low coordination of lead in our samples is verified by the EXAFS analysis at Pb Lm-edge. The mean values for Pb -O distances and coordination numbers (R = 2.35 + 0.05 .& and N = 3 _+ 0.5) may be of value in the construction of a structural model. In lead silicate glasses, for ex- ample, different models can be constructed con- sidering the coordination number for lead as 3, 4 or 6 [30].

5. Conclusions

The vibrational and EXAFS studies carried out for lead germanate glasses xPbO(1 -x )GeO2 are satisfactorily interpreted by a structural change starting from a polymerized germanate network, with all Ge atoms linked to four bridging oxygens (Q4), to a depolymerized structure made basically of tetrahedral [GeO 4] units with one to four non-bridging oxygens (0 3 to Q0).

Pb at low content plays the role of modifier in the germanate glass structure, but to a lesser extent than an equivalent amount of alkaline ions (M ). Low coordination numbers suggest pyrami- dal structures for the range of compositions studied.

S.J.L.R. acknowledges CAPES and FUN- DUNESP (Brazilian agencies) for financial sup- port, Dr A. Michalowicz for the EXAFS data treatment software, Dr O.R. Nascimento for the spectral decomposition software, and Dr J.P. Itie for the glassy GeO 2 sample. The authors also acknowledge the referees for helpful suggestions.

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