27Al
MAS NMR study of charge compensation mechanisms for [4]Al tetrahedra in an
aluminoborosilicate glass containing various alkali and alkaline-earth cations
A. Quintas, O. Majérus, D. Caurant, M. Lenoir
Laboratoire de Chimie de la Matière Condensée de Paris, UMR CNRS 7574, ENSCP, 11 rue
Pierre et Marie Curie, 75231 Paris cedex 05, France.
T. Charpentier
CEA Saclay, Laboratoire de Structure et Dynamique par Résonance Magnétique,
DSM/DRECAM/SCM – CEA CNRS URA 331, Gif-sur-Yvette, 91191, France.
Abstract
The aluminium environment in an aluminoborosilicate glass bearing 3.0 mol% alumina
with simultaneously alkali (14.4 mol%) and alkaline earth (6.3 mol%) oxides, is
investigated using 27Al MAS and 3Q-MAS NMR spectroscopy. A quantitative analysis of
the NMR data is performed in order to extract the distribution of 27Al NMR parameters
and to determine the influence of the nature of the alkali or alkaline earth cation.The
mean
27Al
isotropic chemical shift and mean quadrupole coupling constant PQ
respectively increase by + 5 ppm and + 2 MHz between the Cs- and Li-bearing glasses,
in agreement with literature values in pure alkali-aluminosilicate glasses. These
variations can be therefore attributed to the alkali cations acting for the charge
compensation of [4]Al tetrahedra. In contrast no variation is found when substituting the
alkaline earth cation type, clearly demonstrating that the
[4]Al
tetrahedra preferentially
involve alkali cations for their charge compensation in these peralkaline glasses.
1
Introduction
The presence of alumina in glass improves its properties such as chemical durability and
mechanical strength. Moreover, partly because it raises the viscosity, it can help reducing the
melt tendency to phase separation or devitrification, resulting in glasses with enhanced
thermal stability [1]. For all these reasons, 2-12 wt% alumina are generally added in nuclear
waste glasses [2]. In this field of application, glass formulation studies put in evidence the
great sensitivity of the glass crystallisation tendency to small Al2O3 content variations. Rare
earth-rich aluminoborosilicate glasses containing alkali and/or alkaline-earth cations give rise
to crystallisation of a rare earth silicate phase with apatite structure (Ca2RE8(SiO4)6O2 or
NaRE9(SiO4)6O2) [3,4]. For a given rare earth content and paralkaline compositions, this
crystallisation propensity is clearly enhanced when the amount of Al2O3 increases in the glass.
Similarly, calcic aluminosilicate glasses bearing TiO2, ZrO2 and Nd2O3 give rise to enhanced
zirconolite crystallisation (Ca1-x,Ndx)Zr(Ti2-x,Alx)O7 in the bulk when the Al2O3 content is
increased from 8 wt% (no crystallisation) to 15 wt% (very high zirconolite nucleation rate)
[5]. An explanation resides in the fact that Al2O3 is strongly acid and reacts with alkali (M2O)
and alkaline-earth (MO) oxides to form AlO4- species charge compensated by alkalis and
alkaline-earths in the glass. The excess of MO and M2O oxides, by reacting with SiO2,
depolymerizes the aluminosilicate network and forms non-bridging oxygens (NBO’s),
suitable for the stabilization of high field strength cations such as rare-earths RE3+, as well as
Ti4+ and Zr4+. If the Al2O3 content is high, then the amount of M2O and MO oxides is not
enough to dissolve high field strength cations, which tend to phase separate. When B2O3 is
present, alkalis still react first with Al2O3 and then with B2O3 to form charge compensated
(BO4-)M+ units (Al2O3 is more acidic than B2O3) [6,7]. In these systems, Al2O3 can react and
2
be charge compensated as tetrahedral species by alkalis, alkaline-earths and rare-earths [7],
whereas B2O3 is less reactive with alkaline-earths and rare-earths. Indeed, the amount of BO4species decrease in favour of BO3 species when alkali are replaced by alkaline-earth or when
the rare-earth content is increased in the glass [6].
These considerations are very useful in order to formulate a glass with given glassceramization properties. From a structural point of view, they are reflected in the distribution
of basic network units (SiO4 Qn units, where n is the number of bridging oxygens, AlO4, BO4
and BO3 units), and local charge compensation mechanisms. In this paper, we are interested
in the local charge compensation of AlO4- tetrahedra, when both alkali and alkaline-earth
cations are present in the glass. By considering that alkali oxides are more basic than alkalineearth oxides, it could be predicted that AlO4- species are preferentially charge-compensated
by alkalis. However, calcium oxide in particular demonstrates strong affinity for Al2O3, as
shown by the high number of calcic aluminate and aluminosilicate crystalline compounds
existing in the CaO-Al2O3 and CaO-Al2O3-SiO2 systems [1]. Recent
43
Ca MAS NMR data
show that Ca2+ ions acquire a charge-compensating character (associated with lower
quadrupolar charge coupling constant and isotropic chemical shift), when mixed with Na+
ions in an aluminosilicate network [8]. X-ray absorption data and molecular dynamics models
(MD) do not reveal significant changes of the Na+ and Ca2+ sites with Na/Ca substitution in
aluminosilicate compositions [9]. A trend for higher coordination of Ca as Na2O is replaced
by CaO is detected by MD and attributed to an enhanced charge-compensating character.
Moreover, as in silicate glasses [10,11], non-random Ca-Na mixing and pairs association are
inferred from these MD models. Thus the question remains open, mostly because of the few
experimental studies so far.
The structure of an aluminoborosilicate glass of molar composition: 61.9 SiO2 – 8.9 B2O3 –
14.4 Na2O – 3.0 Al2O3 – 6.3 CaO – 1.9 ZrO2 – 3.6 RE2O3 (RE = Nd or La), has been
3
thoroughly investigated because this glass is the basis for new nuclear waste glass
formulations designed to increase the waste concentration [3, 6, 7]. In particular, the type of
alkali and alkaline-earth cations has been systematically varied in order to investigate changes
in the network structure and rare-earth site distributions (rare earths serve as surrogates for
most fission products and actinides). In this paper, we explore the consequences of changing
the alkali type (MCa glass series), or the alkaline-earth type (NaM glass series) on the
27
Al
MAS NMR spectra and infer results about the charge compensation mechanisms in this glass
composition.
Experimental
Two series of glasses have been synthesized using the procedure described in [3]:
-
Glasses of the MCa series, M = Li, Na, K, Rb, Cs, differ in the nature of the alkali ion,
the alkaline-earth ion being Ca2+ as in the base composition,
-
Glasses of the NaM series, M = Mg, Ca, Ba, Sr, differ in the nature of the alkalineearth ion, the alkali ion being Na+ as in the base composition.
Alkali and alkaline-earth oxides were inserted as carbonates in the powder mixture, except Rb
and Cs which were inserted as nitrates. All glasses looked homogeneous and transparent. The
CsCa glass melt was clearly more visquous than other melts when poured from 1400°C onto
the cold copper plate. The chemical compositions checked by ICP-AES and electron
microprobe analysis are within 2 mol% close to the nominal ones. Glass transition
temperatures and molar volumes strongly vary within these glass series and in particular the
MCa glass series (alkali ions are the most abundant in the composition), denoting significant
changes in chemical bonding and structure, which were investigated by Raman and
multinuclear MAS NMR spectroscopy [to be published]. The rare earth coordination shell is
4
only slightly sensitive to the nature of the alkali or alkaline-earth cation serving for its charge
compensation, indicating that rare earth ions are able to fulfil their preferred bonding
requirements in this system [12], whatever the network changes associated to the different
alkali or alkaline-earth cation types and whatever the nature of these cations.
27
Al MAS and MQMAS NMR spectra of all glasses were collected on a Bruker Avance II
500WB spectrometer (magnetic field 11.75 T), operating at a Larmor frequency of 130.06
MHz. A commercial Bruker CPMAS BL4 WVT probe with 4mm o.d. ZrO2 rotors and a
spinning speed of 12.5 kHz was used. Aluminum-27 chemical shifts are reported in ppm
relative to an external sample of 1.0 M aqueous Al(NO3)3 at 0 ppm. To obtain quantitative
MAS spectra, a single hard pulse of 1 μs in length was used, with radiofrequency (rf) fields of
about 50 kHz, and recycle delays of 1s. Triple quantum 27Al MQMAS (3QMAS) spectra were
acquired on NaCa, LiCa, MgNa and Ca glasses, with the z-filter pulse sequence [13]: p1 - t1
(MQ evolution) - p2 – tau - p3 – t2 (acquisition). For all experiments, the z-filter delay tau
was set to one rotor period. The optimized lengths of the triple quantum excitation and
reconversion pulses were p1=6 μs and p2=2 μs, with an rf field of 100 kHz, the soft 90° pulse
was set to 5μs with an rf field of 25 kHz. 42 t1 increments of 10μs with 360 FID’s per t1 were
collected with a recycle delay of 0.5s. All data were processed and fitted with a homemade
program [14].
Results
27
Al MAS NMR spectra of all glasses are displayed in Figure 1 (right). The 27Al MAS NMR
lines are typically asymmetric with a steeper slope on the low field side. The gradually
decreasing slope toward high fields is due to the quadrupole-induced shift (QIS), which
depends quadratically on the quadrupole coupling constant (CQ) [15]. Indeed, due to their
5
structural disorder, each aluminum site in glasses is characterized by a distribution of NMR
parameters CQ (quadrupole coupling constant), Q (asymmetry parameters) and iso (isotropic
chemical shift).
27
Al nuclei characterized by high CQ cover a large range of QIS and thus
smear out the high field side of the NMR line, whereas 27Al nuclei with smaller CQ have close
QIS which overlap on the low field side. In addition, the isotropic chemical shift iso is
distributed and contributes to the broadening of the MAS NMR spectra.
27
Al 3Q-MAS NMR spectra shown in Figure 1 (left) give an estimation of the wideness of
these distributions. The quadrupolar interaction parameters (CQ, Q) as well as the iso are
distributed in these glasses and contribute to the width of the
27
Al MAS line by about the
same extent. In all glasses, only one distribution of Al sites is put in evidence in the 3Q-MAS
NMR spectra (Figure 2). This distribution centered at a chemical shift of about 57 ppm in
MAS spectra can be attributed to fourfold coordinated aluminum AlIV, according to the
literature [16].
The 3Q-MAS spectra were fitted using a Normal (Gaussian) distribution of isotropic chemical
shift diso and a Gaussian Isotropic Model (GIM) for the distribution of quadrupolar
interaction parameters CQ and Q [ 14]. The non-quantitativeness of 3Q-MAS spectra, due to
the CQ-dependent efficiency of the coherence transfers induced by the 3Q-MAS sequence,
was taken into account in fitting the spectra, as described in [17]. From the reconstructed
distributions of iso and quadrupolar interaction parameters CQ and Q, mean values were
calculated. Because of the disorder, Q is badly constrained in glasses and the quadrupole
coupling parameter PQ (=CQ*sqrt(1+Q2/3)) is then preferred. Here Q was 0.61 for all
glasses. Satisfactorily, mean values for iso and PQ, extracted according to this 3Q-MAS NMR
fitting procedure, were very close to the mean values extracted from the MAS spectra using
MAS NMR lineshapes and the same kind of parameter distribution. This demonstrates both
the accuracy of the 3Q-MAS fitting procedure and the fact that MAS data alone are sufficient
6
to extract mean NMR parameters, in the case of these glasses that only contain one
distribution of sites for Al [6]. Accordingly, all
27
Al MAS NMR spectra were systematically
fitted to get mean iso and PQ values. Fits are shown in Figure 1 and mean parameters from
the fit are reported in Figure 2 and Table 1.
Considering their lineshape (Figure 1) and mean parameters (Table 1), 27Al MAS spectra do
not behave the same way in the NaM and CaM glass series. In the NaM glasses, that all
contain Na (14.4 mol%) and a varying type of alkaline-earth cation (6.3 mol%), the MAS
NMR lineshape and parameters remain constant with iso = 61.9 ± 0.7 ppm and Pq = 5.3 ± 0.2
MHz. Beside, in the MCa glasses, in which the type of the alkali cation changes, the MAS
NMR lineshape and parameters vary significantly. The parameters iso and CQ decrease from
62.4 to 59.1 ± 0.7 ppm and from 6.4 to 4.4 ± 0.2 MHz respectively from the LiCa to the CsCa
glass. More precisely, a sharp decrease occurs between the LiCa and KCa glasses, while it is
less pronounced between the KCa and CsCa glasses (Figure 2).
Discussion
In all investigated glasses, aluminum is fourfold coordinated as demonstrated by the 3Q-MAS
NMR spectra. In these conditions of resolution (B0 = 11.75 T), the detection level is estimated
at about 5% of the total aluminum content. This result is not surprising since the glass
composition studied here is peralkaline, ie ([M2O] + [CaO])/[Al2O3] = 7 > 1. Considering the
fact that B3+ in BO4 species, Zr4+ and RE3+ cations also involve alkalis and alkaline-earths for
their charge compensation within the glass, we still evaluate an excess of alkalis and alkalineearths with respect to Al3+ cations. In the NaCa glass for instance, we know that i) N4 =
[BO4]/[B] = 0.4 from 11B MAS NMR [6], ii) RE3+ cations exist in a 7- to 8-fold coordination
sphere with an excess charge of about – 0.4 [12], iii) Zr4+ cations are probably 6-coordinated
7
with an excess charge of -2, according to previous studies on very similar glasses [18], iv) the
total positive charge is 2([Na2O] + [CaO]) = 41.4 moles. Then, the positive charge
“consumed” for electrical balance of B3+, Zr4+ and RE3+ cations amounts to 2*8.9*0.4 (B3+) +
2*3.6*0.4 (RE3+) + 1.9*2 (Zr4+) = 13.8 moles. The available positive charge for Al3+ cations
is then 41.4 – 13.8 = 27.6 moles, while [Al] = 2*3 = 6 moles. These glasses are thus strongly
peralkaline with a pseudo R = [Na]/[Al] = 4.6, consequently aluminum is only fourfold
coordinated as observed for long in the literature. An exhaustive
27
Al high resolution 3Q-
MAS NMR study of the CaO-Al2O3-SiO2 glass system, operated at very high field (17.6 T),
could detect a proportion of fivefold coordinated
compositions with R = 3 [16].
[5]
Al species as high as 8% in peralkaline
However, for SiO2 molar content of 62 % as in our
composition, less than 5 % of [5]Al is detected which is consistent with our results.
The nature of the alkaline-earth cation does not affect the 27Al MAS NMR lineshape contrary
to the nature of the alkali cation. It is inferred from this observation that a great majority of
alkalis ensure the charge compensation of the AlO4 tetrahedra in the glass. Of course in this
glass composition, only 30 % of the positive charge available for the AlO4 charge
compensation is carried by Ca2+ ions (in other words, the ratio R = [CaO]/([Na2O]+[CaO]) =
30). However, any change in NMR parameters affecting 30 % of the
27
Al nuclei should be
detectable on the resultant MAS NMR line. Another glass series, in which the R ratio is
varied from 0 (Na+ ions only) to 50 without any phase separation (which happens for R >=
50), provides complementary information about this issue [6]. The
27
Al MAS NMR
parameters only slightly vary from R = 0 (iso = 62.1 ppm and PQ = 4.8 MHz) to R = 50 (iso =
61.2 ppm and PQ = 5.2 MHz). In the R = 100 glass (Ca2+ ions only), which is phase-separated,
these parameters are iso = 65 ppm and PQ = 7.2 MHz. By comparison, the MAS NMR
parameters for tetrahedral 27Al in Ca-aluminosilicate glasses with same SiO2 content are iso =
63.1 ppm and PQ = 8.1 MHz [16]. If the Ca2+ and Na+ ions were in the vicinity of AlO4
8
tetrahedra in the same proportions than in the nominal composition, then we would expect
NMR parameters to be intermediary between these extreme values (between 4.5 and 6.8
MHz). This is not the case since the NMR parameters stay close to that of the R = 0 glass up
to R = 50 at least (4.9 to 5.2 MHz). Thus, we conclude that the charge compensation of AlO4
tetrahedra is ensured by a proportion of alkali ions significantly superior to 70 % in all glasses
of the two series, ie whatever the nature of the alkali or alkaline-earth cations. As stated in the
introduction, the greater basicity of the alkali oxyde compared to alkaline earth oxide could
give account of this result, providing that both oxides are in excess relatively to alumina.
Dirken et al. [15] have extracted reliable
27
Al quadrupole parameters from off-resonance
nutation NMR experiments, in alkali aluminosilicate glass samples with alkalis = Li, Na, K,
Rb or Cs. The
27
Al quadrupolar coupling constants PQ they reported for all samples are
systematically shifted by about -1 MHz with respect to our values. Origin of this shift is not
clear yet but could be due to the different NMR techniques employed and/or to structural
effects related to the different aluminosilicate compositions. However, the variations in PQ
from one alkali type to the other are very close in the two studies. In particular,
27
Al PQ
associated to the three bigger alkalis Cs, Rb and K fall in the same range (within 0.3 MHz)
while they jump to higher values for the smaller alkalis Na (+0.8 MHz) and Li (+2.0 Mhz).
By using a simple electrostatic model to calculate the electric field gradient (EFG) at the Al
nucleus, it is possible to correctly predict the rate of increase of the PQ value with the
increasing alkali field strength (z/r2 where z and r are the charge and ionic radius of the alkali
ion). Indeed, Na+ ions and the strongly polarizing Li+ ions are closer to the Al nuclei and
deviate the surrounding negative charge of oxygens out of the spherical symmetry. The
authors supposed that this effect was the most important in determining the PQ of 27Al nuclei.
As this effect is quantitatively the same in our glasses, which however have globally different
chemical composition and network topology, we think our study confirms their supposition.
9
At last, the 27Al isotropic chemical shift raises from 59.1 ppm (CsCa glass) to 62.4 ppm (LiCa
glass). Again, this shift to the high field side is comparable to the + 5 ppm shift observed in
pure alkali aluminosilicate glasses by Dirken et al. [15]. Therefore it may be attributed
directly to the nature of the alkali neighbour. Using the observed inversed linearity between
the 27Al iso and the Al-O-Si angle in crystalline aluminosilicates [19], the negative iso shift in
glass may be linked to an opening of the Al-O-Si angle with larger alkalis. The network
accomodation of the cationic volume, and/or the weaker interaction between the bridging
oxygen and the big alkali cation, could explain this opening.
Conclusion
The dependence of the Al environment on the nature of the alkali or alkaline-earth ion has
been extensively investigated in a peralkaline aluminoborosilicate glass composition, by mean
of
27
Al MQ-MAS and MAS NMR. In this complex multicomponent glass, the mean
27
Al
isotropic chemical shift and quadrupole coupling constant are sensitive to the nature of the
alkali cation, in the same way as in pure alkali aluminosilicate glasses. Both iso and CQ
increase by + 4 ppm and + 2.1 MHz respectively, from Cs-bearing glass to Li-bearing glass,
with a jump between K- and Na-bearing glasses. This evolution approximately scales with the
evolution of the field strength of the alkali. When changing the nature of the alkaline earth
present in the glass or when raising the amount of alkaline earth at the expense of alkali, the
mean 27Al NMR parameters do not change markedly. It is inferred from this result that alkali
ions are preferently involved in the charge compensation of the
[4]
Al tetrahedra compared to
alkaline earth ions. The greater basicity of alkali oxide with respect to alkaline earth oxide
may explain this discrimination, in the particular case of this glass in which both oxides are in
excess with respect to alumina.
10
References
[1]
Mysen, B.O. and Richet, P., Silicate Glasses and Melts, Developments in
Geochemistry Vol. 10, Elsevier, Amsterdam, 544 pp.
[2]
Donald, I. W., Metcalfe, B. L., and Taylor, R. N. J., J. of Mat. Sci., 1997, 32, 5851.
[3]
Bardez, I., Caurant, D., Dussossoy, J.-L., Loiseau, P., Gervais, C., Ribot, F., Neuville,
D. R., Baffier, N. and Fillet, C., Nucl. Sci. and Eng., 2006, 153, 272.
[4]
Qian, M., Li, L., Li, H. and Strachan, D. M., J. of Non-Cryst. Solids, 2004, 333, 1.
[5]
Caurant, D., Loiseau, P., Bardez, I. and Gervais, C., J. Mater. Sci., 2007, in press.
[6]
Quintas, A., Charpentier, T., Majérus, O. and Caurant, D., Appl. Mag. Res., 2007,
submitted.
[7]
Bardez, I., Caurant, D., Loiseau, P., Baffier, N., Dussossoy, J.-L., Gervais, C., Ribot,
F. and Neuville, D. R., Phys. Chem. Glasses, 2005, 46 (4), 320.
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Angeli, F., Gaillard, M., Jollivet, P. and Charpentier, T., Chem. Phys. Lett., 2007,
440, 324.
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Cormier, L. and Neuville, D. R., Chem. Geol., 2004, 213, 103.
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Lee, S. K. and Stebbins, J. F., J. Phys. Chem. B, 2003, 107, 3141.
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Cormack, A. N. and Du, J., J. of Non-Cryst. Solids, 2001, 293-295, 283.
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Quintas, A., Majérus, O., Caurant, D., Lenoir, M., Klementiev, K. and Webb, A., J. of
Non-Cryst. Solids, 2007, accepted.
[13]
Amoureux, J.-P., Fernandez, C. and Steuernagel, S., J. Mag. Res., 1996, 123, 116.
[14]
Charpentier T.:PhD Thesis, University of Paris XI, France (1998). Angeli, F.,
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[15]
Dirken, P. J., Nachtegaal, G. H. and Kentgens, A. P. M., Sol. State. Nucl. Mag. Res.,
1995, 5, 189.
11
[16]
Neuville, D. R., Cormier, L. and Massiot, D., Chem. Geol., 2006, 229 (1-3), 173.
[17]
Angeli, F., Charpentier, T., Faucon, P. and Petit, J.-C., J. Phys. Chem. B, 1999, 103,
10356.
[18]
Galoisy, L., Pélegrin, E., Arrio, M.-A., Ildefonse, P. and Calas, G., J. Am. Ceram.
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Angeli, F., Delaye, J.-M., Charpentier, T., Petit, J.-C., Ghaleb, D. and Faucon, P.,
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12
Tables.
Mean iso
Mean PQ
iso
(± 0.1 ppm)
(± 0.1 MHz)
(± 2 ppm)
LiCa
62.4
6.44
5.0
NaCa (R = 30)
61.9
5.24
4.6
KCa
60.5
4.47
4.5
RbCa
60.0
4.55
4.2
CsCa
59.1
4.45
4.2
MgNa
61.6
5.17
4.7
SrNa
62.1
5.28
4.6
BaNa
61.9
5.32
4.4
NaCa (R = 0)
62.1
4.77
4.5
NaCa (R = 50)
61.2
5.19
4.9
NaCa (R = 100)*
65.0
7.21
-
Table 1.
27
Al NMR parameters from the simulations of the MAS NMR lines. R is the ratio
[CaO]/([CaO]+[Na2O]), from Ref. [6]. * The R = 100 glass was phase-separated and therefore
is put apart in the series.
13
Figure captions
Figure 1.
Left: Experimental (solid lines) and simulated (dashed lines) Al Triple-quantum MQMAS
spectra of caLi (top) and MgNa (bottom) glass. Right: Experimental (solid lines) and
simulated (dashed lines) Al MAS spectra of the glass series.
Figure 2.
Mean values of the 27Al quadrupolar coupling parameter Pq agains the isotropic chemical
shift diso extracted from analysis of MAS spectra (circles) and MQMAS spectra (diamonds).
14
Figure 1.
15
Figure 2.
16