Structure of high-temperature NaF-AlF3-Al2O3 melts: A multinuclear NMR study

Article abstract of DOI:10.1021/jp013114l

Fluoroaluminate molten salts are used in the Hall-Heroult industrial process for the production of aluminum by electrolysis. To better understand the mechanism of the dissolution of alumina (Al₂O₃) in cryolitic melts, we have studied

Full text of DOI:10.1021/jp013114l

1862
J. Phys. Chem. B 2002, 106, 1862-1868
Structure of High-Temperature NaF-AlF3-Al2O3 Melts: A Multinuclear NMR Study
Vincent Lacassagne,† Catherine Bessada,*,† Pierre Florian,† Sylvie Bouvet,‡ Benoist Ollivier,‡
Jean-Pierre Coutures,† and Dominique Massiot†
Centre de Recherche sur les Mate´riaux a` Haute Tempe´rature, CRMHT-CNRS, 1D aVenue de la Recherche
Scientifique, 45071 Orle´ans Cedex 2, France, and CRV Pechiney, BP 27, 38340 Voreppe, France
ReceiVed: August 10, 2001; In Final Form: NoVember 29, 2001
Fluoroaluminate molten salts are used in the Hall-Heroult industrial process for the production of aluminum
by electrolysis. To better understand the mechanism of the dissolution of alumina (Al2O3) in cryolitic melts,
we have studied the structure of these liquids up to 1025 °C by in-situ NMR of 27Al, 23Na, 19F, and 17O, using
a laser-heated experimental setup. From these experiments we can propose a quantification of the different
-
aluminum-bearing species AlF4 , AlF52-, and AlF63- in the liquid and give a direct experimental evidence of
the existence of at least two different oxy-fluoride species, Al2OF62- and Al2O2F42-, when dissolving alumina
in the fluoride liquid. The obtained results are compared with other spectroscopic data obtained by in-situ
Raman spectroscopy.
1. Introduction
between the different available environment, (rapid as compared
to NMR time scales ranging from 102 to 108 Hz). Consequently,
the observed peak position is the average of the chemical shifts
of individual species, weighted by their respective populations.
In a pioneering high-temperature NMR study, Stebbins et al.5
reported 27Al chemical shifts for four compositions of the NaF-
AlF3-Al2O3 system that were systematically much higher in
the liquid than in the related solid phases. They emphasized
the dramatic effect of composition and temperature on the liquid
structure and proposed an evolution of aluminum environment
toward a higher fraction of 4-fold coordinated environments of
aluminum. Using the laser-heated NMR setup developed in
Orleans,29 we studied the 27Al NMR signature for a wider range
of high-temperature liquid compositions of the NaF-AlF3
system and compared the obtained results to existing Raman
data,7 showing that both spectroscopies quantitatively agreed
in describing the structure of the liquid phase in terms of AlF4,
AlF5, and AlF6 ionic complexes. We also reported well-
separated 27Al chemical shifts ranges for the different types of
aluminum coordinations in fluoroaluminates,8 including the AlF4
coordination, measured at 38 ppm on the high-temperature NMR
spectrum of the NaAlF4 melt where Raman spectroscopy gave
Cryolitic liquids (NaF-AlF3 binary system) with dissolved
aluminum oxide (Al2O3) are used for the production of metallic
aluminum by the Hall-Heroult electrolytic process.1 These
corrosive and experimentally difficult-to-handle liquids have
been the subject of numerous studies including experimental
(density, viscosity, electrical conductivity) approach and model-
ing.1 More recently Raman2-4 and NMR5-8 spectroscopies, with
in-situ studies at high temperature, have been used experimen-
tally to better describe the structure of the cryolitic liquids and
compare the experimental point of view to results obtained from
thermodynamical9-15 or computational modeling.16-22 Gilbert
et al.2 have extensively studied the structure of fluoroaluminate
molten salts and proposed a quantitative description of the melt
in terms of ionic complexes. The model described by these
authors combines results from Raman spectroscopy and ther-
modynamics data, and involves three fluoroaluminate species
AlF4-, AlF52-, and AlF63- together with free fluorine. However
the existence of major amounts of AlF52- remains a subject of
a number of discussions.
In alkali fluoroaluminate solid compounds, aluminum is
present only in octahedral coordination with fluorine. Their 27Al
chemical shifts range between -13 and +1.4 ppm and are
typically more shielded than the AlO6 octahedra in oxide
compounds.23-27 Only a few studies report lower coordination
numbers for Al in fluorides: Kohn et al.26 have described the
27Al MAS NMR spectra of glasses of jadeite mixed with
cryolite, in terms of AlV and AlVI coordination at 22 and -5
ppm, respectively; Herron et al.28 reported a 27Al chemical shift
-
evidence of only AlF4 species.(Table 1).
The aim of this contribution is to present new results obtained
by a multinuclear (19F, 27Al, and 23Na) NMR study of the high-
temperature liquids of the NaF-AlF3 binary systems and to
discuss the mechanism of dissolution of alumina (Al2O3) in
cryolite-based liquids from the observation of 17O. Due to its
high polarizability 17O has a large range of chemical shifts and
is thus very sensitive to the structure of its local environment
in crystalline, amorphous, or liquid materials.30-32 Its observ-
ability is limited only by its very low intrinsic sensitivity due
to low natural abundance (0.037%) that can be circumvented
by isotopic enrichment. The combination of information ob-
tained from the observation by NMR of all the different nuclei
present in these high-temperature melts enables us to discuss
previously proposed models of the liquid structure and to give
the first experimental evidence of the presence of at least two
oxyfluoride species upon dissolution of alumina.
-
at 49 ppm for the tetrahedral anion AlF4 in a [1,8-bis-
(dimethylamino)naphthaleneH+] [AlF4-] saturated solution.
In these liquids, for all the observed nuclei, the high-
temperature NMR spectrum consists, in a single, narrow line,
characterized by its position (isotropic chemical shift) and its
line width. This single sharp line reflects rapid exchange
* Corresponding author. Fax: +33(0) 238 638 103. E-mail: bessada@cnrs-
orleans.fr.
† CRMHT-CNRS.
‡ CRV Pechiney.
10.1021/jp013114l CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/02/2002

Structure of High-Temperature NaF-AlF3-Al2O3 Melts
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1863
TABLE 1: 27Al Chemical Shifts Values for Al-O and Al-F
TABLE 2: Typical Acquisition Conditions Used for
High-Temperature NMR Experiments
Coordination8
frequency
number
pulse
recycle
aluminum
coordination
δ
Al (ppm)
nucleus (MHz) (9.4 T) of scans length (µs) delay (s)
reference
19F
376.3
104.2
105.8
54.2
8
64
64
64
π/2
π/8
π/8
π/2
1
CFCl3 1 M
Al(NO3)3 1 M
NaCl 1 M
H2O
5-
AlO4
AlO5
AlO6
90/55
30/40
20/-20
27Al
23Na
17O
0.5
0.5
0.5
7-
9-
-
AlF4
AlF5
AlF6
38
20
1.4/-13
2-
27Al, 23Na, and 19F solid-state MAS NMR spectra have also
been performed at room temperature on stable compounds of
the system NaF-AlF3, to correlate the experimental chemical
shifts and the structural features given by crystallographic data.
The solid-state NMR data were collected with the same
spectrometer (9.4 T) using a 4 mm double bearing MAS probe
from Bruker for a 15 kHz spinning rate. The acquisition
parameters were chosen to optimize the resolution: short pulse
lengths (0.5 µs), recycle times of 1 to 5 s, and a high number
of scans (1024). We have reported in Table 3, the cor-
respondence between experimental NMR chemical shifts and
coordination for a given nucleus.
For the 17O, the chemical shifts reported by Walter et al.30
for OAl410+ sites in the different types of alumina (δ,θ,η,R) lie
between 72 and 75 ppm. It should be noticed that due to its
high quadrupolar coupling (CQ ≈ 1.5-2 MHz), and its low
amount in the baths, the 17O signal will be visible in our
experimental conditions only when involved in the liquid phase.
3-
2. Experimental Section
Samples have been prepared in a glovebox under an argon
atmosphere by mixing suitable proportions of natural cryolite
(from Ivigtut), AlF3 (Inland, 99.9% purified by two sublimations
at 1000 °C), NaF (Aldrich, 99.9%), and R-Al2O3 (APA 05).
The 17O-enriched alumina has been synthesized in the laboratory
from 46% 17O-enriched H2O and aluminum isopropoxide.
Samples were preheated in a furnace at 1025 °C under argon
before NMR experiment. Weight losses were checked to be
always less than 2%.
In the binary NaF-AlF3 system we have investigated a
composition range extending from 20 to 50% mol of AlF3 and
the NaF end member. AlF3, the second end member of this
binary diagram, sublimates before melting.
Mixtures of cryolite and 17O-enriched Al2O3 (0.6 to 8.2 mol
% of Al2O3) cover the whole range of alumina solubility in
cryolite (up to 6 mol %) and the beginning of the saturation
domain.
3. Results and Discussion
The high-temperature NMR experiments have been acquired
using the previously described high-temperature laser-heated
system29,33-35 developed at CRMHT Orle´ans (France). The
sample is contained in a high-purity boron nitride (BN AX05
Carborundum) crucible, tightly closed by a screwed BN cap,
and put inside the RF coil, at the magnetic center of the cryo-
magnet. The NMR axial saddle coil is thermally isolated by a
ceramic shield. The crucible is directly heated by a continuous
CO2 laser (λ ) 10.6 µm) passing axially through the probe-
head. With this design, temperature cannot be measured by a
thermocouple during NMR experiments. The temperature
calibration is carried out in two steps: calibration of temperature
versus laser power using a thermocouple located inside the BN
crucible controlled by in-situ observation of phase transitions
by NMR. The R f â transition and melting in cryolite are
clearly marked by a net 27Al signal modification. Respecting
the same heating and cooling procedure, the temperatures
reported below are accurate within (5 °C. As liquid fluoride
melts are very corrosive and subject to non congruent vaporiza-
tion, the experimental time spent in the liquid phase has been
minimized (≈5 min) to avoid evolution of the composition
during the experiment. The chemical compositions of the
samples were obtained by wet chemical analysis and cross
checked by high-resolution NMR and Rietveld analysis of their
X-ray diffraction patterns before and after high-temperature
NMR experiments. Weight loss measurements confirmed the
absence of evaporation out of the crucible.
3.1. NaF-AlF3. 27Al, 23Na, 19F NMR spectra of the different
melts consist in a single Lorentzian line, characteristic of rapid
exchange between the different chemical species in the liquid.
The typical line widths of the lines are constant, of the order of
100 to 200 Hz, and mainly due to principal field inhomogeneity
as could be checked by relaxation time measurements.
We have reported in Figure 1 the evolution of 27Al, 23Na,
and 19F chemical shifts over the composition range, between
20 and 50 mol % of AlF3, with the 23Na and 19F chemical shifts
measured in pure liquid NaF. Going from NaF to AlF3 we
observe a continuous decrease, respectively increase, of δ23Na
and δ27Al. In both cases we notice a change of slope between
the compositions of chiolite (Na5Al3F14) and cryolite (Na3AlF6).
In contrast with 27Al and 23Na the position of the 19F line rapidly
grows from liquid NaF (δ ) -228 ppm), stabilizes between
chiolite and cryolite at ≈ -192 ppm, and lowers for higher
contents of AlF3.
As already described in a preceding paper,7 from the evolution
of the 27Al chemical shift it is clear that the ionic structure of
these melts consists of more than one type of aluminum-
containing ionic species, the proportion of which depends on
the composition. From the knowledge of the whole chemical
shift range for 6-fold aluminum coordinated with F, and the 38
-
ppm value measured for AlF4 complexes, it turns down all
3-
the structural description involving only AlF6 ionic species,
even as distorted AlF63- complexes.12,13,15,18 For a dissociation
-
All the NMR experiments have been carried out using a
Bruker DSX400 NMR spectrometer operating at 9.4 T. The
NMR spectra have been acquired using single pulse excitation,
and typical acquisition conditions are reported in Table 2. 27Al,
23Na, 19F, and 17O chemical shifts are referenced to 1 M aqueous
solutions Al(NO3)3, NaCl, CFCl3, and H2O, respectively, at room
temperature, and are accurate to (0.5 ppm. The reported 19F
spectra are corrected from a broad probe-head signal due to the
presence of Teflon in the probe assembly.
scheme involving only AlF4 and AlF63-, the distribution of
these two ionic species calculated directly from the experimental
NMR chemical shifts for the different compositions would not
coincide with any models previously described.14 Between
chiolite and cryolite cryolitic ratio, the anionic molar fractions
-
3-
calculated for AlF4 and AlF6 species would be nearly
identical and follow the same decreasing trend, while the
calculated equilibrium constant varies of 1 order of magnitude
(0.02 to 0.21 at 1025 °C) over the whole range of AlF3 content.

1864 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Lacassagne et al.
TABLE 3: Coordination and Experimental Chemical Shifts for Sodium and Aluminum Fluoride Solid Stable Phases
δ27Al (ppm)
(b)
δ23Na (ppm)
(b)
δ19F (ppm)
(a) (d)
-221
compound
AlF3
Na3AlF6
Na5Al3F14
Na5Al3F14
NaAlF4 (molten) AlF4
site
(a)
(c) compound
site
(a)
(c)
compound
site
3-
5-
5+
AlF6
AlF6
AlF6
AlF6
-15
-1
-13.2
1.4
NaF
Na3AlF6
NaF6
NaF6
NaF8
7
1
7.2
2.4
NaF
Na3AlF6
FNa6
0
4
FNa3Al5+ -189
3-
3-
3-
-
5-
7-
5-
-1.5 -1
-2.8 -3
38
-1 Na3AlF6
-12 -9.3
-8 Na5Al3F14 FNa3Al5+ -187 -189.5
-3 Na5Al3F14 NaF6
-7
-21
-6 Na5Al3F14 FNa4Al6+ -190 -191.4
11-
5+
Na5Al3F14 NaF12
-21 Na5Al3F14 FAl2
-162 -165
a This study. b Ref 24. c Ref 5. d Ref 36.
Figure 2. Linear correspondence between 27Al and 23Na chemical shifts
measured over the whole range of compositions.
Figure 1. 19F, 23Na, and 27Al chemical shifts in cryolitic melts at 1025
°C.
We can describe the observed chemical shifts in terms of
structural environment of the observed nucleus, by using the
correspondence between experimental chemical shifts and
coordination, described in Table 3 and established at room
temperature for solid stable compounds. From 18 to 50% AlF3,
the averaged coordination number of aluminum with fluorine
decreases from 5.5 to 4 and that of sodium increases from 8.5
to 11. The correlation between chemicals shifts of 23Na and
27Al is equivalent to a correlation between averaged coordination
numbers, which indicates that these melts can be described from
the Al-F as from the Na-F point of view.
It is interesting to notice that the observed singularity in both
the aluminum and sodium observed chemical shift closely
corresponds to behavior already observed for macroscopic
properties such as conductivity, density, and viscosity.1 We have
correlated the conductivity data measured in liquids of the binary
NaF-AlF3 system, with the average coordination evolution of
Na and Al atoms deduced from our high-temperature NMR data
(Figure 3). The conductivity decreases rapidly with increasing
the amount of AlF3 in the melt, and coincides rather well with
the coordination evolutions, respectively, increasing for Na and
Figure 3. Conductivity data1 reported vs Al and Na average coordina-
tion.
decreasing for Al. A modification in this evolution is evidenced
around 30 mol % AlF3 by a break of the slope. According to
the Raman description of such melts, this particular composition
corresponds to that with the minor amount of free fluorine, the
major fraction of AlF52-, and the emergence of AlF4-. This
observation confirms the high relation between the macroscopic
properties and the liquid microstructure in terms of anionic
species and their relative proportions.
To better describe the 19F chemical shifts evolution, we need
some additional information. In the dissociation scheme of the
cryolitic melts, fluorine is involved in four possible different
species: AlF4-, AlF52-, AlF63-, and F-. From the anionic
proportions reported by Gilbert et al.,2 we can calculate the
atomic fraction of fluorine inside each of these species.
Moreover, the 19F chemical shift of individual fluoroaluminate
anion can be directly deduced from our experimental data. The
value measured in pure molten NaF at -228 ppm is assigned

Structure of High-Temperature NaF-AlF3-Al2O3 Melts
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1865
Figure 4. 19F chemical shifts: experimental and calculated from the anionic fractions reported by Gilbert et al.2
to the free fluorine ion. For the melt of NaAlF4 composition
(50 mol % AlF3) where F- is involved only in AlF4-, the 19F
chemical shift is measured at -200 ppm (δFIV). The chemical
shift evolution has been fitted over the whole range of
composition (Figure 4), and we find a really satisfying modeling
with δF ) -188 ppm and δF ) -176 ppm. These results
The exact anionic structure of the oxyfluoride species is still
a matter of controversy. A number a studies yet converge clearly
2-
toward two major entities containing oxygen, namely Al2OF6
and Al2O2F42-, where the former appears to be the dominating
one at a low amount of alumina4,14,20 while the other is the
unique species for higher Al2O3 content.
V
VI
show the strong dependence of 19F chemical shifts with the
amounts of free fluorine ions in the melts: the increase in
chemical shift between 0 and 25 mol % AlF3 can be correlated
with the decrease of the anionic fraction of F- ion.
Thus, the interpretation of both 27Al and 19F chemical shifts
variations in terms of fluoroaluminate species confirm the
coherence of such a description in cryolitic melts.
The 17O chemical shift decreases from 25 to 8.5 ppm, in the
range of 0.6 to 3.8 mol % of alumina, and then remains constant
above 3.8%. The high sensibility of the 17O chemical shift with
the added alumina content shows that the average local
environment of oxygen atoms is strongly modified up to 3.8%.
From these direct observations we can already propose from
the evolution of the 17O chemical shift that, in the melt up to
3.8 mol % Al2O3, more than one species exist, while above,
the chemical shift being constant, we can suppose only one
species exists. In the same manner, the variations of the 27Al δ
iso, up to the solubility limit, show that the alumina dissolution
is always effective.
3.2. NaF-AlF3-Al2O3. The NMR spectra obtained for the
four nuclei of the system, 27Al, 23Na, 19F, and 17O, are reported
on Figure 5 for the different Al2O3 additions (0.6 to 8.2 mol
%).
Except for aluminum spectra where broadening of the lines
is observed with increase of the dissolved amount of alumina,
for the other nuclei, the evolution of the line width is not
significant.
Oxygen atoms in these complexes are most probably involved
in bridging bonds of the type Al-O-Al and
A small, but significant 17O signal is observed from 0.6 mol
% Al2O3. The intensity of the line increases with the amount
of alumina dissolved and indicates the increase of the number
of 17O nuclei in the melt up to the maximum value at the melt
saturation.
The 27Al spectra show also a strong evolution with alumina
addition, and shift toward higher chemical shifts. No significant
evolution of 19F and 23Na signatures are observed and the
chemical shifts appear rather constant over the whole range of
compositions. In Figure 6, the chemical shift data are reported
vs the mol % of Al2O3 for the four nuclei.
as it has been evidenced by Gilbert et al. from Raman
spectroscopic measurements.4 The chemical shifts evolution we
have observed for the 17O and 27Al spectra really account for
the existence of two different oxyfluoride species depending
on the Al2O3 content. We have then based our description on
2-
the two species described above: at 0.6 mol % only the Al2OF6
The evolutions observed for 17O and 27Al are nearly sym-
metrical except for the highest values of alumina content after
which the chemical shifts are constant. These simultaneous
evolutions indicate in the melt the existence of Al-O-F species,
in addition to fluoroaluminate species already described for the
NaF-AlF3 binary system. The 27Al chemical shifts increase
from 20 to 47 ppm over the range 0 to 6.6 mol % alumina, and
stay constant at 47 ppm for adding higher than 6.6%.This strong
variation indicates that the average local environment of
aluminum is highly modified by the addition of alumina. The
chemical shift values become rapidly higher than 38 ppm, which
is the maximum value measured in the binary system NaF-
AlF3 for AlF4-. It means that new chemical species with higher
chemical shift are to be considered in the melt. The chemical
shift remains constant above 6.6% and indicates the saturation
of the melt, with alumina (beyond 6%).
2-
is present, while the Al2O2F4 appears to be the unique
oxyfluoride for highest Al2O3 contents. We can extract then the
corresponding chemical shifts values for each individual species
as δo Al2OF6 ) 25 ppm, and δo Al2O2F4 ) 8.5 ppm.
If we make the assumption that only these two species are
present, the 17O chemical shift, δ(O), can be expressed as
2-
2-
δ(O) ) XOAl OF 2-.δO
2- + XO
2-.δO
2-
Al2O2F4
(1)
Al2OF6
Al2O2F4
2
6
with
X OAl OF 2-+ XO
2- ) 1
(2)
Al2O2F4
2
6
XAOl OF and XAOl O F are the atomic fractions of oxygen in
2-
2-
2 2 4
2
6
2-
Al2OF6 and Al2O2F42-, respectively.

1866 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Lacassagne et al.
Figure 5. 17O, 27Al, 19F, 23Na spectra of 17O-enriched alumina dissolution in cryolite at 1000 °C.
Figure 6. 17O, 27Al, 19F, 23Na chemical shifts evolution vs amount of alumina in cryolite (1000 °C).
From these relations and the experimental chemical shifts, it
is possible to calculate the fractions of each oxyfluoride species
over the whole range of alumina additions (Figure 7).
The interpretation of the 27Al chemical shifts evolution is
rather more complex because of the presence of fluoroaluminates
species (AlFx) in addition to the oxyfluorides. We assume in a
first approach that the relative proportions of the AlFx species
are not modified by alumina additions in molten cryolite. We

Structure of High-Temperature NaF-AlF3-Al2O3 Melts
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1867
Figure 7. Evolution of oxygen fractions included in the oxyfluoro-aluminate species and directly calculated from the 17O chemical shifts at 1000
°C.
Figure 8. Anionic fractions of the different species in the molten cryolite alumina mixtures.
can then express the 27Al chemical shift as
the saturation, allows us to describe the evolution of the atomic
fraction of each nuclei and to deduce the anionic fraction of
the different species present in the melt (Figure 8). The fraction
of free fluorine slightly decreases over the whole range of
composition. The global fraction of fluorine inside the fluoro-
aluminate species decreases with increasing the alumina content,
but keeps highly majority.
Al
Al2O2F4
δ(Al) ) X
2-.δAl
2- + XAl 2-.δAl
+
2-
Al2O2F4
Al2OF6
Al2OF6
Al
X
.δAl
(3)
(4)
AlFx Na3AlF6
with
Al
Al2O2F4
2- + XAl 2- + XAAllFx ) 1
4. Conclusion
X
Al2OF6
High-temperature NMR study of melts in NaF-AlF3 and
NaF-AlF3-Al2O3 systems, by means of selective observation
of the different nuclei involved in these systems: 27Al, 23Na,
19F, and 17O, provides a powerful tool for the structural
description of such complex liquids.
and thus propose for the 27Al of the two different oxyfluoride
species:
Al
δ
2-) 50 ( 0.5 ppm and δAl
2-) 58.5 ( 0.5 ppm
Al2O2F4
Al2OF6
In the binary NaF-AlF3, the chemical shifts evolutions
For the NMR point of view, the local configuration around
coincide rather well with the existence of AlF4-, AlF52-, and
2-
3-
the aluminum atom in each of the two anions Al2OF6 and
AlF6 complexes, with an averaged coordination of 5 for
2-
Al2O2F4 can be described as a tetrahedral environment,
aluminum between cryolite and chiolite compositions, i.e., for
0.25 < X°(AlF3) < 0.38.
respectively, AlOF3 and AlO2F2. Compared to the two “extreme”
AlO4 (δ ) 80 ppm) and AlF4 (δ ) 38 ppm), the two 27Al
chemical shifts values extracted experimentally are clearly
intermediate between, and a difference of ≈10 ppm signs the
substitution of one fluorine by one oxygen atom in the first
coordination sphere of aluminum.
17O NMR gives a selective view of the alumina dissolution
in molten cryolite, because of its direct signature of the
oxyfluoride complexes. From the variations of the 17O chemical
shift it is necessary to consider at least two different oxyflu-
orinated species: Al2OF62-, at low alumina content, and
Al2O2F42-, that becomes the major species for higher amounts
of alumina.
The evolution of 17O and 27Al chemical shifts over the whole
domain of alumina addition, from a very low amount to above

1868 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Lacassagne et al.
(15) Danek, V.; Ostvold T. Acta Chem. Scand. 1995, 49, 411-416.
(16) Belashchenko, D. K.; Unikel, I. A. Inorg. Chem. 1997, 333 (11),
1168-1172.
Starting from simple assumptions, we have calculated the
atomic fractions of oxygen present inside the different species.
The combination of the different information given by the whole
set of nuclei allows us to determine the anionic fractions of
each species present in the melt and their evolution with alumina
additions.
(17) Liska, M.; Perichta, P.; Turi Nagy, L. J. Non Cryst. Solids 1995,
192 & 193, 309-311.
(18) Akdeniz, Z.; Tosi, M. P. Philos. Mag. B 1991, 2, 167-179.
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Acknowledgment. Pechiney is gratefully acknowledged for
a doctoral fellowship to one of the authors (V.L.) and for
financial support. Financial support from EEC ARI contract no.
HPRICT-1999-00042, CNRS and Re´gion Centre is also ac-
knowledged. B. Gilbert and E. Robert are thanked for helpful
discussions.
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