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Journal of Non-Crystalline Solids 354 (2008) 2038–2044

Structures of lanthanum and yttrium aluminosilicate glassesdetermined by X-ray and neutron diffraction

I. Pozdnyakova a,*, N. Sadiki b, L. Hennet a, V. Cristiglio c, A. Bytchkov d, G.J. Cuello c,J.P. Coutures b, D.L. Price a

a CNRS-CRMHT, 1d avenue de la Recherche Scientifique, 45071 Orleans cedex 2, Franceb PROMES, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France

c ILL, 6 rue Jules Horowitz, BP 156, 38042 Grenoble cedex 9, Franced ESRF, 6 rue Jules Horowitz, BP 220, 38043 Grenoble cedex, France

Received 5 March 2007; received in revised form 17 November 2007Available online 8 January 2008

Abstract

We have measured the structures of lanthanum and yttrium aluminosilicate glasses by X-ray and neutron diffraction and determinedthe interatomic distances and nearest-neighbor coordination numbers. The results obtained with the two techniques are in good agree-ment with each other and with recent NMR studies. The Si–O and Al–O coordination numbers are found to be 4 and 4.5, respectively.All the glasses show pronounced intermediate-range order that exhibits a reduced length scale with increasing La or Y content.� 2007 Elsevier B.V. All rights reserved.

PACS: 61.10.�i; 61.12.�q; 61.43.Fs

Keywords: Neutron diffraction/scattering; Synchrotron radiation; X-ray diffraction; Aluminosilicates; Short-range order

1. Introduction

Aluminosilicate glasses containing rare-earth elementcations such as Y3+ and La3+ are interesting for a varietyof technological applications, as well as for elucidating gen-eral principles of glass formation and structure. Theseglasses have unusually high glass transition temperatures(�900 �C) [1–4], high hardness (�8 GPa) and elastic mod-ulus (�100 GPa) [5–8], and good chemical durability (nor-malized losses at high surface:volume ratio of Si, Al, andLn �10�5–10�7 g/m2 J) [9–11]. Rare-earth aluminosilicatebased glasses have been successfully used as a laser ionhosts, optical lenses, seals, and as in vivo radiation deliveryvehicles [12]. Also these glasses, with or without smallamounts of alkali modifiers, can be considered as model

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.11.012

* Corresponding author. Tel.: +33 238257693; fax: +33 238638103.E-mail address: pozdnya@cnrs-orleans.fr (I. Pozdnyakova).

systems for the study of a potential matrix for the storageof long-lived actinides.

The physical properties of a glass are closely related toits atomic structure. Despite their industrial importance,the information about the structure of Ln3+ contained alu-minosilicate glasses is not complete. Generally an alumino-silicate glass is thought of as a more or less orderednetwork of corner-shared Si–O and Al–O tetrahedra. Ifno network-modifying cations (e.g., K+, Na+, Ca2+, etc.)are added, then the tetrahedra are linked together by bridg-ing oxygens (BO) so that Al–O–Si chains are formed. Theaddition of cations leads to the formation of non-bridgingoxygens (NBO) linking the Si–O and Al–O tetrahedra withones based on cations [13]. The effect of the modifying cat-ion on the formation of NBO has been studied extensivelyin the case of alkali and alkali-earth modifiers ([14] and bib-liography therein). Furthermore, the coordination of the Alis questionable even for the simple SiO2–Al2O3 system,

I. Pozdnyakova et al. / Journal of Non-Crystalline Solids 354 (2008) 2038–2044 2039

where small amounts of AlO5 and AlO6 polyhedra havebeen found [15].

Since Si and Al have similar scattering cross-sections forboth X-rays and neutrons, it is difficult to separate the con-tributions of Si–O and Al–O polyhedra to the total atomicpair distribution function. However, using high-energy X-rays and sophisticated data collection and data treatmentstrategy, the Si–O and Al–O bonds were resolved in Ca2+

aluminosilicate glass [14]. It was clearly demonstrated thatNBO’s are located on Si–O but not on Al–O tetrahedra,and that the connectivity of the Si/Al–O network decreaseswith increasing the Ca2+concentration.

Aluminosilicate glasses with a rare-earth element as athird component are especially interesting since they falloutside the traditional range of glass modifiers, in whichlow-field-strength cations either modify the network orcompensate the charge of network AlO3�

4 groups. To thebest of our knowledge, there have been no X-ray or neu-tron studies of Ln3+ aluminosilicates. The La2O3–Al2O3–SiO2 and Y2O3–Al2O3–SiO2 systems (defined later) havebeen studied mostly by spectroscopic techniques such asultrasonic [16], Raman [3], FTIR [3,4,17], and NMR [17–20] spectroscopies. In the earlier works, it was suggestedthat these glasses do not have significant structure beyondthe short-range, and that there is a wide distribution ofNBO [3]. In the latest NMR study it was confirmed thatthese glasses have a significant fraction of 5-co-ordinatedAl, and that the concentration of high-co-ordinated Al spe-cies depends on the nature and concentration of the rare-earth modifier [20].

The combination of neutron and X-ray diffraction canprovide structural information at the partial level in glasses[21]. In this paper we present measurements of the localstructure in La and Y aluminosilicate glasses with thetwo techniques, which made it possible to extract detailedstructural information including interatomic distancesand coordination numbers. The results obtained are ingood agreement with each other and with the NMR studiesperformed on the same compositions [20].

2. Experimental

2.1. Sample preparation

Four compositions were chosen, with 30 and 10 wt% ofLa and Y rare-earth oxides and with 50–60 wt% silica inorder to meet the requirements for possible commercializa-tion of these glasses for actinide waste storage. These sam-

Table 1Compositions and densities of the glasses studied [22]

Sample Composition (wt%) Composition (mol%)

Ln2O3 Al2O3 SiO2 La2O3 Al2O3

LAS1 30 20 50 8.22 17.50LAS3 10 30 60 2.32 22.23YAS1 30 20 50 11.43 16.89YAS3 10 30 60 3.31 22.00

ples will be denoted LASn and YASn with n = 1 and 3(Table 1). Glasses were synthesized from the oxides(La2O3 99.0%, Y2O3 99.4%, Al2O3 99.9%, SiO2), wellmixed and melted for 2 min on a water-cooled aluminumplate connected to a vertical laboratory solar furnace of2 kW power and heat flux of 900–1000 W/m2. The La2O3

was heat-treated in an alumina crucible for 2 h at1100 �C both before and after mixing in order to eliminateanionic impurities. Transparent quasi-spherical glass drop-lets between 2 and 6 mm in diameter were obtained. Theglass compositions were measured by SEM–EDX analysis[22].

2.2. X-ray diffraction

The X-ray experiments were performed at the ID15beam line [23] at the ESRF (Grenoble, France). The incom-ing beam energy was 88.91 keV, corresponding to a wave-length of 0.14 A, and the detection system was a MAR345online image plate scanner (2300 � 2300 pixels, pixel size0.15 lm). A 5-mm-diameter cylindrical beam stop wasplaced in the direct beam. The distance between sampleand image plate was 400 mm, giving a usable Q range of0.8–16 A�1. One-dimensional diffraction patterns wereobtained by integrating the diffraction rings of the two-dimensional patterns with MATLAB� and FIT2D soft-ware packages [24].

2.3. Neutron diffraction

The neutron experiments were performed on the D4Cspectrometer [25,26] at ILL (Grenoble, France). Themonochromator was a vertically focused Cu[200] singlecrystal giving an incoming wavelength of 0.7 A. The dif-fracted neutrons were measured with a group of nine detec-tors, each with an 8�-angular range and separated from theadjacent ones by 7�. In order to get a continuous measure-ment over the 1.3–140� angular range and limit the effect ofthe relative cell efficiencies, we recorded the diffractedintensities at five detector positions.

3. Results

3.1. Structure factors

A recent description of X-ray and neutron diffractionfrom liquids and glasses has been given by Fischer et al.[27]. In this work weighted average structure factors S(Q)

Al/Si Al/Ln Density, (g/cm3) from Ref. [22]

SiO2

74.29 0.202 1.86 3.70275.45 0.314 12.3 2.85171.67 0.236 1.49 3.04274.69 0.279 4.86 2.769

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in the Faber–Ziman formalism were extracted from theexperimental data with the procedure of Wagner [28].For X-ray diffraction S(Q) is related to the mean differen-tial scattering cross-section per atom through the relation

drdX¼Xn

i¼1

ci fij j2 þXn

i¼1

cifi

����������2

ðSðQÞ � 1Þ; ð1Þ

where ci and fi are respectively the atomic concentrationand the atomic scattering factor of species i present in thesample [29,30]. For neutron diffraction the fi are replacedby the Q-independent coherent scattering lengths bi [31].

In order to obtain the mean differential scattering cross-section per atom, the scattered intensity was corrected fordetector efficiency, background scattering, multiple scatter-ing, absorption, and inelastic scattering. For X-rays, theauto-normalization technique was used: at high-Q values

Fig. 1. X-ray and neutron structure factors for LAS1 and LAS3 glasses.The three top curves are shifted up by 0.5, 1.5 and 2 for clarity.

Table 2X-ray (X) and neutron (N) weighting factors of the partial structure factors

LAS1 LAS3

X N X N

Si–O 0.223 0.234 0.290 0Al–O 0.098 0.093 0.154 0La–O/Y–O 0.172 0.088 0.056 0O–O 0.194 0.499 0.253 0Si–Si 0.064 0.027 0.083 0Si–Al 0.056 0.022 0.032 0Si–La/Si–Y 0.099 0.021 0.088 0Al–Al 0.012 0.004 0.024 0Al–La/Al–Y 0.044 0.008 0.017 0La–La/Y-Y 0.038 0.004 0.003 � 0

The X-ray factors have been calculated at Q = 0 for an energy of 99.9 keV.

(S(Q) � 1) ? 0. For neutrons, intensity normalizationwas achieved by comparison with a standard vanadiumsample.

Fig. 1 shows the X-ray and neutron S(Q)’s for LAS1and LAS3 glasses. The X-ray S(Q)’s are different for thetwo compositions, but the neutron S(Q)’s do not show sig-nificant structural changes with lanthanum content. Thiscan be explained by the fact that the neutron weighting fac-tors for La-X atomic pairs (X = Si, Al, La, O) are lowerthan the X-ray ones (Table 2), which fall from 35.3% forLAS1 to 16.4% for LAS3. Fig. 2 shows the correspondingS(Q)’s for YAS1 and YAS3 glasses, in which a similarbehavior is observed.

3.2. Pair correlation functions

The pair correlation function g(r) is calculated fromS(Q) with the Fourier transform

gðrÞ ¼ 1þ 1

2p2q0

Z Qmax

0

QðSðQÞ � 1Þ sin Qrr

MðQÞdQ; ð2Þ

where q0 is the number of atoms per unit volume. M(Q) is amodification function used to force the integrand to gosmoothly to zero at Qmax; we used the Lorch modificationfunction M(Q) = sin (Qp/Qmax)/(Qp/Qmax). Like S(Q), g(r)is a weighted sum of partial functions:

gðrÞ ¼X

i;j

W ijgijðrÞ ¼X

i;j

cicjfif �jjP

cifij2gijðrÞ: ð3Þ

Based on the densities reported in Table 1 [22], we used thevalues for q0 of 0.087, 0.078, 0.076 and 0.077 atom/A3 forLAS1, LAS3, YAS1 and YAS3, respectively.

The pair distribution functions g(r) are shown in Figs. 3and 4, respectively. Again, in contrast to the X-ray ones,the neutron g(r)’s are not very sensitive to the lanthanumor yttrium content. The first peak found at 1.67 A in allthe g(r)’s arises from the combination of Si–O and Al–Onearest-neighbor distances: the resolution in our pair distri-bution functions (Dr � 2p/Qmax) is not sufficient to resolvethem. In addition, as seen in Table 2, the X-ray and neu-tron weighting factors for Si–O and Al–O bonds are quite

YAS1 YAS3

X N X N

.250 0.220 0.218 0.300 0.251

.119 0.092 0.082 0.160 0.119

.024 0.183 0.122 0.040 0.022

.534 0.200 0.488 0.262 0.535

.029 0.060 0.024 0.086 0.030

.028 0.050 0.018 0.091 0.028

.006 0.100 0.027 0.023 0.005

.007 0.011 0.003 0.024 0.007

.003 0.042 0.010 0.012 0.0030.042 0.008 0.002 � 0

Fig. 2. X-ray and neutron structure factors for YAS1 and YAS3 glasses.The three top curves are shifted up by 0.5, 1.5 and 2 for clarity.

Fig. 3. X-ray and neutron pair correlation functions for LAS1 and LAS3glasses. The three top curves are shifted up by 0.5, 1.5 and 2 for clarity.

Fig. 4. X-ray and neutron pair correlation functions for YAS1 and YAS3glasses. The three top curves are shifted up by 0.5, 1.5 and 2 for clarity.

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similar and it is not possible to distinguish between Si–Oand Al–O coordinations from the difference between X-ray and neutrons g(r)’s. To obtain this information, itwas necessary to use data from additional sources (see Sec-tion 4).

Since neutrons are more sensitive to O–O correlationsthan X-rays (Table 2), the second peak at �2.7 A in all fourneutron g(r)’s can be ascribed to O–O correlations. In the

X-ray g(r)’s, the second peaks occur at 2.58, 2.65 and2.3 A for LAS1, LAS3, and YAS1, respectively, while theshoulder is observed at 2.4 A for YAS3. The variation inthe second peak positions for the X-ray g(r)’s can beascribed to the different weighting factors for La–O/Y–Opairs for the different compositions.

The third peak in the X-ray g(r)’s is found at 3.32, 3.18,3.15 and 3.12 A for LAS1, LAS3, YAS1 and YAS3, respec-tively. This peak is due to a combination of cation–cationcorrelations and its shift to lower r is due to the changein the corresponding weighing factors with decreasing ofLa or Y content (Table 2). With neutrons the influenceof cation–cation correlations is very small and only ashoulder is observed. For the following peaks, the interpre-tation is more difficult; the fourth peak in all g(r)’s couldarise from next-nearest-neighbor Si–O and Al–Ocorrelations.

4. Discussion

In order to obtain pair-specific information, we fittedGaussian peaks to T(r) = 4pq0rg(r) for all the composi-tions studied. The resultants are presented in Figs. 5 and6 for the lanthanum and yttrium glasses, respectively. Sincethe resolution in g(r) does not allow us to see individualbonds, we used a synergistic approach: for starting param-eters we used the Si–O and Al–O bond lengths from data inwhich those bonds were resolved [14], and then fitted the X-ray and neutron g(r)’s with gaussian peaks. From these fitswe obtained the interatomic distances for the four firstpairs: Si–O, Al–O, O–O, and Ln–O, and coordinationnumbers for the Si–O and Al–O pairs: the coordination

Fig. 5. X-ray and neutron total correlation functions T(r) for LAS1 andLAS3 glasses, showing the Gaussian fits.

Fig. 6. X-ray and neutron total correlation functions T(r) for YAS1 andYAS3 glasses, showing the Gaussian fits.

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number Cni ðjÞ of atoms j about an atom i associated with a

Gaussian Tn can be calculated from

ZrT nðrÞdr ¼ W ij

cjCn

i ðjÞ: ð4Þ

Table 3Interatomic distances and coordination numbers derived from the Gaussian fi

r (A)Si–O Al–O

Sum of ionic radii �1.62 AlIV 1.76AlV 1.84AlIV 1.9

Errors ±0.03 ±0.03

LAS1 X 1.62 1.82N 1.62 1.84

LAS3 X 1.62 1.82N 1.62 1.82

YAS1 X 1.60 1.80N 1.60 1.80

YAS3 X 1.62 1.82N 1.62 1.82

The values obtained for the four compositions are given inTable 3 along with the estimated error limits. The mainsource of uncertainty is the limited Q range used for theFourier transform in Eq. (2) and the resulting appearanceof artificial features in g(r), although these are reduced bythe Lorch modification function. Uncertainties in the coor-

ts of the X-ray (X) and neutron (N) data

CNLa–O/Y–O O–O C Si–O Al–O

2.39/2.26 2.72

±0.05 ±0.1 ±0.5 ±0.5

2.34 2.70 4.1 4.52.36 2.70 4.2 4.6

2.36 2.66 4.0 4.42.36 2.66 4.1 4.5

2.26 2.62 3.9 4.52.22 2.64 4.0 4.5

2.24 2.62 4.0 4.62.20 2.64 4.0 4.4

I. Pozdnyakova et al. / Journal of Non-Crystalline Solids 354 (2008) 2038–2044 2043

dination numbers can also arise from uncertainties in thenumber density q0 and from the imperfect normalizationof S(Q). The sums of the corresponding atomic radii areshown for comparison.

From Table 3 it can be seen that the calculated inter-atomic distances are in good agreement with each otherand fall close to the sum of the corresponding ionic radii.Within the estimated errors, the Si–O interatomic distancesand coordination numbers are composition-independent,with values of about 1.6 A and 4, respectively. Thus, a con-tinuous tetrahedral network with well-defined Si–O dis-tances is formed for all the compositions studied. Ourresults are not inconsistent with the work of Petkov et al.[14] who identified NBO’s at higher modifier concentra-tions (>30 mol%) than we have here.

On the other hand, the Al–O coordination numbers arefound to be higher than four. Together with the fact thatthe Al–O interatomic distances are larger than the sum ofionic radii of AlIV and O, this result supports the recentfindings from NMR measurements on the same samples[20] of a significant presence of AlV and AlVI species inLa and Y aluminosilicates. However, we do not see any dif-ference between La and Y containing glasses, whereas theNMR results show a larger amount of AlV in the YAScompositions; which might be expected from the higherfield strength of yttrium compared with lanthanum.

There are no significant changes in for the different Lnconcentrations, and the O–O distances are the same forall the compounds studied, within the estimated errors.

The first sharp diffraction peak (FSDP) in the structurefactor S(Q) is an indicator of intermediate-range order(IRO) [32,33]. Taking their positions from the X-rayS(Q)’s, we get characteristic values Q1r1 (Q1 is the wavevec-tor corresponding to the first peak in S(Q), r1 is the firstpeak in g(r) of 3.2, 3.0, 3.3, and 2.8 (±0.1) for LAS1,LAS3, YAS1, and YAS3, correspondingly. These valuesfall close to those typical for systems with tetrahedral coor-dination such as elemental semiconducting glasses andequi-atomic liquid alloys of AM type, where A is an alkalimetal and M a group-IV metal [34]. The IRO in aluminumsilicate glasses is related to the distribution of the 3- and 4-membered rings formed by the BO atoms [15]. There is adependence of both FSDP position and intensity on Lncontent: higher Q-values and higher peak intensities areobserved at higher Ln concentration. The move to higherQ, implying a reduced length scale for the IRO may suggestabout the decreasing of the connectivity of Si/Al–O tetra-hedral network with addition of the modifying ion, thesame effect that has been observed in [14] for Ca–alumino-silicates, while the increase in intensity may simply reflectthe increase in La–X weighting factors referred to above.

The glasses studied here are ternary compounds withaverage pair correlation functions composed of ten partialfunctions, one for each atomic pair. It is thus quite difficultto make a quantitative interpretation even for the firstcoordination sphere. Element-specific experiments such asanomalous X-ray scattering [35,36] or EXAFS at the La,

Y, Al and Si absorption edges would provide additionalinformation on the local structure of these glasses.

5. Conclusion

We have reported an investigation of the local order inlanthanum and yttrium aluminosilicate glasses with X-rayand neutron diffraction. The Si–O, Al–O and O–O firstinteratomic distances are found to be independent of Laor Y content. The Si–O and Al–O coordination numbersare found to be 4 ± 0.5 and 4.5 ± 0.5, respectively, alsoindependent of composition. The high coordination num-ber found for Al supports the presence of highly co-ordi-nated species previously found by NMR spectroscopy.

All the glasses show pronounced IRO characterized by aFSDP that, in the X-ray structure factors, moves to higherQ and increases in intensity with higher Ln content.

Acknowledgments

The authors are grateful to the staff of the ID15 beamline at ESRF and the D4C spectrometer at ILL for theirtechnical assistance. This work was financially supportedby the CNRS and the CEA through the GDR ‘Nomade’and by the regional council of the ‘Region Centre’.

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