ISSN 2075�1133, Inorganic Materials: Applied Research, 2013, Vol. 4, No. 3, pp. 265–271. © Pleiades Publishing, Ltd., 2013.Original Russian Text © S.V. Stefanovsky, A.A. Shiryaev, Ya.V. Zubavichus, J.J. Purans, 2012, published in Fizika i Khimiya Obrabotki Materialov, 2012, No. 5, pp. 76–82.

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INTRODUCTION

Strontium is one of the main fission products ofuranium. During the processing of spent nuclear fuel,strontium isotopes, including the major 90Sr isotope,are transferred into liquid radioactive waste (RW),which can be solidified to produce permanent chemi�cally and radiation resistant forms suitable for subse�quent long term storage [1, 2]. One of the most effec�tive techniques for RW treatment is vitrification inelectric furnaces with Joule or induction heating [2].The possibility of partitioning of high level RW to sep�arate a relatively short�lived cesium–strontium frac�tion and a long�lived actinide or actinide–rare earthfraction is also under discussion. The first one can besubjected to a further partitioning to separate Cs and Srto be used as ionizing radiation sources in radioisotopedevices, and these sources can be in a glassy form [3].

At present, unpartitioned RW is subject to vitrifica�tion, and the Sr concentration in the glasses is nothigher than a few tenths of a percent [4]. At these lowconcentrations, it has no effect on the macropropertiesof the glass, except for chemical resistance, where theparameters of leaching of radioactive Sr ions are impor�tant for environmental control. Since the leachability ofSr depends directly on the form in which it is present inthe glass, it is relevant to obtain information on thestructural position of Sr and its interphase distributionin the case of formation of polyphase materials.

It is known that the doubly charged Sr2+ ion is atypical glass network modifier and can have variablelocal environment. In different glasses and crystallinecompounds, the coordination number (CN) of Sr withrespect to oxygen varies from 6 to 12 [5], and the radiusof the Sr2+ ion varies from 1.18 (CN = 6) to 1.44 Å(CN = 12) [6]. The behavior of Sr2+ is similar to thatof Ca2+ because of the close values of ionic radii andpolarizability. Monograph [7] addressing the behaviorof Sr in glasses reports only the data known up to 1978.

Studies on the structural position of Sr ions inglasses are limited. Results of IR spectroscopy ofstrontium containing glasses are reported in [7]; theysuggest that the Sr2+ ions exhibit a significant polariz�ability and thus a deforming action with respect to sil�icon–oxygen and boron–oxygen groups and contrib�ute to the depolymerization of the structural networkof the glass; this leads to the splitting of the bands inthe vibrational spectra. According to IR and EPRspectroscopy data, the Sr2+ ions in boron containingglasses contribute to the transition of boron from thethreefold to fourfold coordination [7]. In the EPRspectra of irradiated glasses of the SrO–SiO2 system,three types of radiation�induced paramagnetic centersare formed; they are responsible for a narrow symmet�rical line with g = 2.0016, an anisotropic line with g =2.008, and a broad anisotropic line with geff = 1.970. Asthe SrO concentration in the glasses increases, theintensity of the first of the lines, which is typical of

Local Environment of Strontium Ions in Borosilicateand Aluminophosphate Glasses

S. V. Stefanovskya, A. A. Shiryaevb, Ya. V. Zubavichusc, and J. J. Puransd

a Federal State Unitary Enterprise “ Radon,” Moscow, Russiab Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia

c National Research Center Kurchatov Institute, Moscow, Russiad Institute of Solid State Physics, Academy of Sciences of Latvia, Riga, Latvia

e�mail: profstef@mtu�net.ru; shiriaev@psyche.as.ru; yzubav@gmail.com; purans@cfi.lu.lvReceived July 14, 2012

Abstract—Parameters of the local environment of the Sr2+ ions in borosilicate glasses with a high level wastesurrogate were determined by analyzing the of X�ray absorption fine structure spectra. It is found that the Sr2+

ions mostly have an oxygen environment with a trigonal bipyramidal configuration; the average Sr–O dis�tance in the first coordination shell is about 2.53–2.54 Å. The second coordination shell of strontium in theglasses is weak; the nearest neighboring atom is located at a distance of 3.55–3.58 Å. In the structure of all thestudied glasses, the first coordination shell of strontium is split into two subshells because of a distortion ofthe coordination polyhedron, while the second and subsequent coordination shells become apparent upon anincrease in the SrO concentration.

Keywords: aluminophosphate glass, borosilicate glass, EXAFS, XANES, strontium

DOI: 10.1134/S207511331303012X

JOINING OF MATERIALS

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phase�separated glasses, decreases relative to the sec�ond, which is determined by the hole centers in the sil�icon–oxygen nodes, while the intensity of the thirdline, which is associated with the electrons trapped bythe Sr2+ ions, monotonically increases [8]. Nuclearmagnetic resonance (NMR) and IR spectroscopystudies of the behavior of Sr in phosphate glasses [9]showed that the introduction of Sr in the amount of afew units of weight percent leads to an increase in thedisorder of the glass and an acceleration of its degrada�tion. However, all these data do not make it possible todraw a conclusion about the structural state of the Sr2+

ions.Anomalous wide angle X�ray scattering (AWAXS)

was used to determine the interatomic Sr–O distancesin the first coordination shell and the CN of Sr withrespect to oxygen in glasses of the SrO–Al2O3–SiO2and Na2O–SrO–SiO2 systems; these parameters werefound to be 2.68 Å and 6 and 2.73 Å and 5, respectively[10]. X�ray absorption fine structure (XAFS) spectros�copy revealed that, in anhydrous and hydrated alumi�nosilicate glasses containing 0.27 to 0.67 wt % SrO,the Sr–O distance in the first coordination shell is2.51–2.56 Å and CN = 5.3–6.7 [11].

Our first attempt to use XAFS to determine thestate of the Sr2+ ions in the structure of borosilicateand aluminophosphate glasses intended for long�termstorage of RW was made in the early 1990s [12]; how�ever, the results were not widely released, and the firstreport was published only recently [13]. The Sr2+ ionsin glasses of either type have a CN with respect to oxy�gen of about 8 and the environment with a hexagonalbipyramidal configuration with six equatorial oxygenions at a distance of 2.52 and 2.58 Å and two axial ionsat a distance of 2.74 and 2.88 Å in borosilicate and alu�minophosphate glasses, respectively. Aluminoborosil�icate glasses containing of 2.6 to nearly 30 wt % SrOwere studied by XAFS in [14]; it was found that theCN of Sr with respect to oxygen is 4.4–6.5 at a Sr–Odistance in the first coordination shell of 2.52–2.55 Å.

Thus, the structural state of Sr in glasses heavilydepends on their composition and, possibly, the prep�aration technique. In this study, XAFS is used toexamine the local environment of Sr in glass materialswith a complex composition which are intended toimmobilize RW with a high content of Na and Al.

EXPERIMENTAL

The high level waste (HLW) surrogate was preparedvia mixing solutions of reagent�grade sodium, potas�sium, magnesium, calcium, aluminum, chromium,manganese, iron, and nickel nitrates and sodium sul�fate, drying the resulting solution in a desiccator, andcalcination of the solid residue at 600°C for 5 h. Theresulting calcine was ground in a porcelain mortar.The calcine powder was intermixed with a frit contain�ing 8 Li2O, 8 B2O3, 12 Na2O, and 72 SiO2 (wt %) atvarious weight ratios. The calculated compositions of

the materials are listed in Table 1. The mixtures wereheated in corundum crucibles at temperatures of 1150to 1300°C (depending on their composition) withholding at the final temperature for 1 h. A portion ofthe melt was poured onto a metal sheet (quenching,“q” samples); the rest was cooled in crucibles in theswitched off furnace (annealing, “a” samples).

Samples for XAFS spectroscopic studies were pre�pared and spectra were recorded as described in [15].The Sr K�edge spectra of the samples of the preparedmaterials and strontium carbonate (strontianiteSrCO3), which was used as a standard, were recordedat the Structural Materials Science (SMS) station onthe 1.3b channel of a synchrotron radiation source ofthe National Research Center Kurchatov Institute[16] in two ranges: the X�ray absorption near�edgestructure (XANES) and the extended X�ray absorp�tion fine structure (EXAFS) using the techniquedescribed in [16]. The IFEFFIT [17], FEFF8 [18],and Hama [19] software products were used for pro�cessing the spectra.

RESULTS AND DISCUSSION

Brief Description of the Materials

The glasses containing simulated RW were previ�ously studied in details by X�ray diffraction, electronmicroscopy, and IR spectroscopy [20, 21]. It wasfound that the samples 1q, 1a, and 3q are X�ray amor�phous, and sample 3a contains an inclusion of thespinel phase (Fig. 1), which is represented by two gen�erations of dendrite crystals. Spinel�1 has submicron(nanometric) dimensions; this makes determining itscomposition difficult; spinel�2 forms individual crystalswith a size of up to 25 µm. According to X�ray spectralanalysis, its composition corresponds to the formula(Mg0.04Mn0.03Fe0.02Ni0.91)

2+(Al0.19Cr0.91Fe0.90)3+O4. Stro�

ntium is entirely included in the structure of the glass.The structural network of the glasses is mostly formedby metasilicate chains with embedded AlO4, FeO4,and, apparently, BO4 tetrahedra. An appreciable frac�tion of three coordinated boron is present. Strontiumcations, along with cations of alkali (Li, Na, K, andCs) and other alkaline earth elements (Mg, Ca) arenetwork modifiers.

X�ray Absorption Spectra of the Strontium K Edge

The XANES spectra of the K absorption edge of Srin the glasses (samples 1q, 1a, 3q, and 3a) and stron�tium carbonate SrCO3 are shown in Fig. 2a. The max�imum of the main peak in the spectra of samples 1qand 1a is localized at 16111.2 ± 0.1 eV; in the spectraof samples 3q and 3a, it is shifted to 16110.1 and16112.6 eV, respectively; in strontianite, it is localizedat 16115.1 eV, which is attributed to variations in thecoordination environment of the Sr2+ ions.

Fourier transforms (FTs) of the EXAFS spectra ofthe Sr K absorption edge in the glasses and strontianite

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LOCAL ENVIRONMENT OF STRONTIUM IONS 267

with no phase shift correction are shown in Fig. 2b.The FT of the spectrum of strontianite strongly differsfrom those described in the literature [14, 22]. Thepeak due to the first coordination shell of Sr in stron�tianite is asymmetric because of the superposition ofseveral peaks resulting from five different Sr–O dis�tances (2.552 to 2.725 Å). In [14, 22], the actual aver�age Sr–O distance in the first coordination shell wasdetermined from EXAFS data as 2.59 and 2.61 Å,respectively. Approximately the same value of theaverage Sr–O distance was obtained in our case.

In the FTs of the spectra of all the glasses, the firstcoordination shell is split into two subshells (Fig. 2b),and the local environment of the Sr2+ ions has a sym�metry close to axial. The shorter Sr–O1 distance is2.32–2.38 Å; the longer one is 2.80–2.85 Å (Table 2).The average CN of Sr with respect to oxygen in thefirst coordination shell is about 5 for all the samplesexcept 1a. The intensity ratio of the components of thefirst coordination shell of Sr is close to 3 : 2, and we canassume that the Sr2+ ions have an oxygen environmentwith a trigonal bipyramidal configuration with a D3hsymmetry, in which three equatorial oxygen ions arelocated at a shorter distance from the central Sr2+ ionand two axial ions are located at a longer distance.Accordingly, the average Sr–O distance in the firstcoordination shell is about 2.53–2.54 Å. In sample 1a,the CN of Sr with respect to oxygen is slightly higherand closer to 6, and the oxygen environment has aconfiguration of an axially elongated octahedron.

The second coordination shell of Sr in glasses 1q,1a, 3q, and 3a is weakly pronounced (Fig. 2b). Thenearest neighboring atom is located at a distance of3.55–3.58 Å (Table 2). An analysis of the data usingthe wavelet transformation (Fig. 3) and the results ofsimulations using various models of the local environ�ment of strontium suggest that the second coordina�tion shell includes not only oxygen but also, probably,boron (the radius of the B3+ ions in the trigonal andtetrahedral coordination is 0.01 and 0.11 Å, respec�tively [6]). A peak at 4.40–4.50 Å can also occur; itapparently corresponds to the Sr–O–Si distance.

For comparison, Fig. 4 shows the FTs of theEXAFS spectra of the K absorption edge of Sr in boro�silicate and aluminophosphate glasses in the series(30 – x) Na2O, x SrO, 20 B2O3, 50 SiO2 and (27.6 – x)Na2O, x SrO, 20.6 Al2O3, 51.8 P2O5 (wt %). The com�position of the original phosphate glass corresponds tothe base composition of the glass manufactured at PAMayak [23]. In the structure of all the glasses, the firstcoordination shell of Sr is split into two subshells withSr–O distances of 2.38–2.42 Å (four oxygen ions) and2.74–2.87 Å (two oxygen ions) with an average Sr–Odistance of 2.53–2.60 Å (Table 2) and has a configura�tion of an axially elongated octahedron, and the localoxygen environment of strontium is the same as insample 1a (Fig. 2, Table 2). As the SrO concentrationin the glasses increases to about 10 mol %, the param�eters of the first coordination shell of the Sr2+ ions varyonly slightly.

Table 1. Calculated content of components in the glass samples with different composition

Oxides HLW oxides Frit

1 21BS 2BS 3BS 4BS 1 AP 2 AP 3 AP 4 AP

wt % mol % wt % mol %

Na2O 25.9 12.0 16.2 16.4 17.6 18.6 25.7 21.0 6.1 10.9 37.2 30.0 22.2 13.9

Al2O3 36.5 — 11.0 6.8 14.6 9.4 — — — — 20.6 21.3 22.1 22.9

Fe2O3 7.4 — 2.2 0.9 3.0 1.2 — — — — — — — —

Cr2O3 1.6 — 0.5 0.2 0.6 0.3 — — — — — — — —

NiO 3.3 — 1.0 0.8 1.3 1.2 — — — — — — — —

MnO 0.5 — 0.2 0.1 0.2 0.2 — — — — — — — —

MgO 0.9 — 0.3 0.4 0.3 0.6 — — — — — — — —

CaO 2.6 — 0.8 0.9 1.0 1.2 — — — — — — — —

SrO 3.0 — 0.9 0.5 1.2 0.8 3.1 6.3 9.6 13.1 4.9 10.2 15.8 21.9

K2O 7.6 — 2.3 1.5 3.0 2.1 — — — — — — — —

Cs2O 5.0 — 1.5 0.3 2.0 0.5 — — — — — — — —

SO3 5.6 — 1.7 1.3 2.3 1.8 — — — — — — — —

Li2O — 8.0 5.6 11.8 4.8 10.5 — — — — — — — —

B2O3 — 8.0 5.6 5.1 4.8 4.5 18.3 18.7 9.1 19.5 — — — —

SiO2 — 72.0 50.4 52.8 43.2 47.1 53.0 54.0 55.2 56.5 — — — —

P2O5 — — — — — — — — — — 37.2 38.5 39.9 41.3

HLW oxides

100.0 — 30.0 — 40.0 — Model glasses

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In borosilicate glass 1BS with a low (3.1 mol %)content of SrO, the second coordination shell of theSr2+ ions is clearly pronounced, while it is absent inglass 2BS (6.3 mol % SrO) and gradually becomesapparent with increasing SrO concentration (glasses3BS and 4BS). This anomaly is not observed in thealuminophosphate glasses: the relative intensity of thepeaks due to the second coordination shell of the Sr2+

ions increases with increasing SrO concentration inthe entire range of compositions. The second shell,along with the first one, can be represented as a super�position of two peaks, which can be attributed to theSr–Me (Me = B, Si, Sr) and Sr–O2 distances.

In the aluminophosphate glasses, the first coordi�nation shell of Sr is more symmetrical than in the

borosilicate glasses. The Sr–O distance is 2.58 ± 0.02 Åand slightly depends on the SrO concentration. TheCN of strontium with respect to oxygen is about 6,which corresponds to an octahedral environment. Thesecond coordination shell is quite clearly pronouncedonly in glasses with high SrO concentrations (3AP and4AP); in addition, the distance of 3.59–3.64 Å appar�ently corresponds to Sr–O–P; that of 3.95–4.00 Å, toSr–O–Al.

In the FTs of the EXAFS spectrum of the K absorp�tion edge of strontium in glass 4AP, peaks correspond�ing to distances of ~5.01 and ~5.68 Å are observed;they can be attributed to Sr–O2 and Sr–M, includingSr–Sr, of more distant coordination shells; this indi�

(b)

(c)

(а) 100 μm

50 μm

50 μm

1

2

3

Fig. 1. Backscattered electron SEM image of sample 3q: (a) general view, (b, c) details of the image: (1) spinel, (2) glass 1, and(3) glass 2.

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LOCAL ENVIRONMENT OF STRONTIUM IONS 269

cates that the structure of this glass is significantlyordered.

A comparison of the XAFS data for the glasses withdifferent compositions suggests that the local environ�ment of the Sr2+ ions is not strongly dependent on thechemical composition of the glasses; the concentra�tion of SrO and Sr2+ ions, and the glass preparationtechnique. For all the borosilicate glasses studied byXAFS in our experiments and in [14], the average Sr–Odistance in the first coordination shell of the Sr2+ ionsis 2.50–2.60 Å, which is slightly lower than or approx�

imately the same as that in crystalline compounds, forexample, SrCO3 (2.59–2.61 Å) [14, 22]. Higher valuesof Sr–O distances in the first coordination shell wereobtained in [10]; however, this can be attributed eitherto significant differences in the composition of theglasses or to specific features of the AWAXS technique.

The fact that the total CN of Sr with respect to oxy�gen in the first coordination shell in all the glassesstudied weakly depends on their composition suggeststhat the Sr2+ ions form their own oxygen environment.

1617016150

1613016110

1609016070

16050 543210

μ(k) χ(k)k2

R�ΔR, Å

1q

1a

3q

3a

SrCO3

1q

1a

3q

3a

SrCO3

Sr–O1 (I)Sr–O1 (II)

Sr–

M1

Sr–

M2

Sr–

O

Sr–

CS

r–O

–C

Sr–

Sr

Sr–

O

(а) (b)

E, V

Fig. 2. XANES spectra and FTs of the EXAFS spectra of the K absorption edge of Sr in the studied glasses and strontianite SrCO3ignoring the phase shift.

Table 2. Parameters of the computer simulation of the EXAFS spectra of the K absorption edge of Sr in the glasses (interatomic distances are given with the phase shift correction; the Debye–Waller factor was recorded at a level of 0.003 in allcases)

ParametersGlasses with an RW simulator Model glasses

1q 1a 3q 3a 1BS 2BS 3BS 4BS 1 AP 2 AP 3 AP 4 AP

R–O1av, Å 2.53 2.50 2.51 2.56 2.55 2.53 2.56 2.60 2.58 2.58 2.58 2.58

N1av 5.0 ± 0.2 6.2 ± 0.2 5.0 ± 0.2 5.0 ± 0.2 6.0 ± 0.2 6.0 ± 0.2 5.9 ± 0.2 5.8 ± 0.2 6.0 ± 0.2 6.0 ± 0.2 6.0 ± 0.2 5.9 ± 0.2

R–O1(I), Å 2.37 2.32 2.35 2.38 2.38 2.39 2.41 2.42

R–O1(II), Å 2.81 2.80 2.82 2.85 2.80 2.74 2.79 2.87

Sr–M1, Å 3.58 3.55 3.56 3.58 3.93 3.90 3.85 3.99 3.59 3.59 3.63 3.64

N(II) 2.0 2.0 1.9 1.9 2.0 2.0 2.2 2.2

Sr–M2, Å 4.41 4.40 4.48 4.50 4.00 4.00 4.00 3.95

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As regards the second coordination shell, it can besuggested that major contribution to the second coor�dination shell of strontium is rather from different ele�ments, the concentration of which in the glass is by anorder of magnitude higher, i.e., B, O, Si, and P thanfrom Sr. The contribution of Sr can become appre�ciable only at fairly high concentrations of SrO andmostly in the aluminophosphate glasses, in which theintensity of the second coordination shell of stron�tium gradually increases with increasing SrO con�

centration. In glass 4AP with the highest content ofSrO (~22 mol %), more distant coordination shellsare also evident.

The main difference of our results from the pub�lished data [9, 11, 14] is the splitting of the peak corre�sponding to the first coordination shell of the Sr2+ ionsin borosilicate glasses, which results from a distortionof the strontium–oxygen polyhedra. The causes of thisare apparently attributed to the structural features ofthe anionic motif of the structural network of theglasses.

CONCLUSIONS

In borosilicate glasses with a complex compositionsimulating vitrified RW and containing 0.9–1.2 wt %(0.5–0.8 mol %) SrO, the CN of the Sr2+ ions withrespect to oxygen in the first coordination shell is closeto 5–6 at an average Sr–O distance of 2.53 ± 0.03 Å.The splitting of the peak indicates the axial distortionof the strontium–oxygen polyhedron. The secondcoordination shell of strontium is weakly pronounced.In the model glasses of the Na2O–SrO–B2O3–SiO2system, the CN of strontium in the first shell is ~6 atapproximately the same Sr–O distances. In the glassesof the Na2O–SrO–Al2O3–P2O5 system, the averageSr–O distance in the first shell is ~2.58 Å, and the CNis also close to 6. As the SrO concentration increases,

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

–10–15 –5 0 5 10 15 20 25

R, Å

k, Å–1

Fig. 3. Amplitude of the wavelet transform of the EXAFSspectrum of the K absorption edge of Sr in sample 3q.

543210

χ(k)k2

R�ΔR, Å 543210

χ(k)k2

R�ΔR, Å

1BS

2BS

3BS

4BS

1AP

2AP

3AP4AP

Sr–

O1

(I)

Sr–

O1

(II)

Sr–

M1

Sr–

M2

Sr–

M2

Sr–

M1

Sr–

O1

(II)S

r–O

1 (I

)(а) (b)

Fig. 4. FTs of the EXAFS spectra of the K absorption edge of Sr in model (a) sodium borosilicate and (b) aluminophosphateglasses with allowance for the phase shift.

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LOCAL ENVIRONMENT OF STRONTIUM IONS 271

the second and subsequent coordination shells ofstrontium become apparent.

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Translated by M. Timoshinina