Thermal transformation of quaternary compounds in NaF-CaF2-AlF3 system

Article abstract of DOI:10.1016/j.jssc.2009.05.043

Details of quaternary compounds formation in the system NaF-CaF₂-AlF₃ are specified. To achieve this aim, the samples of phases NaCaAlF₆ and Na₂Ca₃Al₂F₁₄ have been obtained by hi

Full text of DOI:10.1016/j.jssc.2009.05.043

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Journal of Solid State Chemistry 182 (2009) 2246–2251
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Journal of Solid State Chemistry
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Thermal transformation of quaternary compounds in NaF–CaF2–AlF3 system
Ã
Julia N. Zaitseva a, Igor S. Yakimov b, Sergei D. Kirik a,b,
a Institute of Chemistry and Chemical Technology SB RAS, K. Marx St., 42, Krasnoyarsk 660049, Russia
b Siberian Federal University, Svobodny Str., 79, Krasnoyarsk 660041, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Details of quaternary compounds formation in the system NaF–CaF2–AlF3 are specified. To achieve this
aim, the samples of phases NaCaAlF6 and Na2Ca3Al2F14 have been obtained by high-temperature solid-
phase synthesis. Their thermal behavior when heated up to 800 1C has been studied using the methods
of high-temperature X-ray diffraction (XRD) and thermal analysis (TA). The system under consideration
can be regarded as a quasibinary section CaF2–NaAlF4, where at T ¼ 745–750 1C invariant equilibrium is
implemented with the phases CaF2–NaCaAlF6–Na2Ca3Al2F14–(liquid melt)–(NaAlF4). The peculiarity of
the equilibrium is NaAlF4 metastability at normal pressure. Below the equilibrium temperature the
quaternary phase Na2Ca3Al2F14 is stable and NaCaAlF6 above this temperature. The phase NaCaAlF6
fixed by rapid quenching from high temperatures and when heated up to 640 1C decomposes, yielding
Na2Ca3Al2F14. Further heating in vacuum at temperature up to 740 1C results in decomposition of
Na2Ca3Al2F14 into CaF2 and Na3AlF6. The expected reverse transformation of Na2Ca3Al2F14 into NaCaAlF6
has not been observed under experimental conditions. Transformations in bulk samples reveal direct
and reverse transformation of quaternary phases.
Received 30 March 2009
Received in revised form
26 May 2009
Accepted 28 May 2009
Available online 9 June 2009
Keywords:
Phase equilibrium diagram of the system
NaF–AlF3–CaF2
Calcium–sodium–aluminum fluorides
High-temperature XRD
Aluminum production
Industrial electrolytes
Synopsis: Thermal transformation of the quaternary compounds in system (NaF–CaF2–AlF3) was
investigated using high-temperature X-ray diffraction (XRD) and thermal analysis (TA). In the system
the invariant equilibrium is implemented with the phases CaF2–NaCaAlF6–Na2Ca3Al2F14–(liquid
melt)–(NaAlF4) at T ¼ 745–750 1C.
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
aluminum fluoride, or in another term—excess of AlF3 concentra-
tion [1]. The quantitative phase determination by XRD in
electrolytes containing calcium presents methodical problem. In
the course of crystallization calcium fluoride forms three phases:
CaF2 (fluorite), NaCaAlF6 and Na2Ca3Al2F14 (calcium cryolites).
The measurement of NaCaAlF6 meets a problem because of its
poor crystallinity. It has been mentioned in literature that the
correlation between calcium phases changes depending both on
electrolyte composition and sampling procedure [3,6,7]. Unfortu-
nately, the equilibrium system of calcium-containing phases is
still unclear, which does not allow one to substantiate adequate
electrolyte sampling procedure providing samples of a required
quality.
Phase diagrams of cryolite-containing systems [1] have mainly
been studied focusing at the ‘‘liquidus’’ part for determining
the temperatures of the melt crystallization. However, for the
purposes of electrolyte composition control by XRD subsolidus
part appears to have great importance. The available informa-
tion about this temperature range of the system is fragmentary
and contradictory [1,8,9]. The ternary system Na3AlF6–AlF3–CaF2
which is a part of more general system NaF–AlF3–CaF2 was
investigated during the 20th century. As early as in 1912, Fedotiev
and Ilinskii [10], using the data of crystal-optical analysis, made
the first description of the system, having found crystalline
The significance of the quasiternary system Na3AlF6–AlF3–CaF2
diagram could hardly be overestimated for the aluminum
production [1,2]. It was repeatedly investigated in 20th century.
However, there are still blank spots which induce some problems
with the electrolysis control. Because of the high volatility
of electrolyte components at the electrolysis temperature opera-
tion there is a necessity to correct the electrolyte composition
from time to time [3–5]. The correction is carried out after an
electrolyte analysis. The electrolyte composition is defined by
X-ray diffraction (XRD) using crystallized samples. Their composi-
tion is presented by a number of crystal phases: Na3AlF6 (cryolite),
Na5Al3F14 (chiolite), CaF2 (fluorite) and others. The procedure
of quantitative XRD provides the concentration of each present
phase converted into the concentration of chemical components.
Particularly, the content of CaF2 is determined, and the key
technological parameter—cryolite ratio (CR)—is calculated:
CR ¼ NaF (mol)/AlF3 (mol)—molar ratio of sodium fluoride to
Ã
Corresponding author at: Institute of Chemistry and Chemical Technology SB
RAS, K. Marx St., 42, Krasnoyarsk 660049, Russia.
E-mail address: kirik@icct.ru (S.D. Kirik).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.05.043

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2247
phases: NaF, Na3AlF6, Na5Al3F14, AlF3, CaF2. Pfundt and Zimmer-
2. Experimental
mann [11] established one more compound existing in this
system—NaCaAlF6—which melts incongruently. A diagram frag-
ment limited to the compounds Na3AlF6–AlF3–CaF2 was reported
by Craig and Brawn [8] in 1980. The work revealed the existence of
two compounds with the following composition: NaCaAl2F9 and
NaCaAlF6. As a whole, the following phases can crystallize in the
system: CaF2, Na3AlF6, Na5Al3F14, Ca2AlF7, CaAlF5, AlF3, NaCaAlF6,
NaCaAl2F9. Later, in 1985, Baggio [12] made a correction and found
out that the real composition of the phase NaCaAl2F9 corresponds
to the compound Na2Ca3Al2F14. Soon after it was confirmed by
crystal structure investigations [13–15]. Anufrieva et al. [9] in
1985, reported a diagram of the system NaF–AlF3–CaF2, according
to which seven chemical compounds are formed including three
congruently melting compounds: Na3AlF6, NaAlF4, CaAlF5, and
four incongruently melting ones: Na5Al3F14, NaCaAlF6, NaCaAl2F9,
Ca2AlF7.
2.1. Synthesis of NaCaAlF6 and Na2Ca3Al2F14
Crystal phases NaCaAlF6 and Na2Ca3Al2F14 were synthesized
from stoichiometric mixtures of individual fluorides AlF3, NaF and
CaF2 by high-temperature solid-phase synthesis. Temperature
regimes were chosen with respect to recommendations [12]. The
carefully blended mixture of fine powders of initial components of
10 g mass was loaded into platinum crucible with a head and was
placed in a heated furnace at T ¼ 740 1C for an hour. Then, in case
of NaCaAlF6 the crucible was taken out of the furnace and air-
cooled. The mass loss when synthesizing was about 1.2 mass%. In
case of Na2Ca3Al2F14 after keeping in furnace for an hour at 740 1C,
the temperature was reduced to 565 1C and kept for 20 h. The
mass loss was about 1.4%. The substances obtained were analyzed
by XRD. XRD patterns are given in Fig. 1a, b.
On the basis of the differential thermal analysis (TA) data
for the system Na3AlF6–NaCaAlF6 Craig and Brawn [8] found
that NaCaAlF6 existed in three polymorphous modifications
with the transition temperatures 610 and 722 1C. The phase
NaCaAlF6 is retained at 600 1C for 20 h; however, with increasing
the time of annealing up to 100 h, NaCaAlF6 is converted into
NaCaAl2F9 (as mentioned above, the correct composition Na2-
Ca3Al2F14). Wide ranges of solid solutions were not found in the
system [16].
On the whole, the data available in literature describe the
crystallization of phases in the system. However, the phase
composition of industrial electrolytes is more diversified than
that expected according to the phase equilibrium diagram. It has
to be noted that the revealed crystallization areas of the
compounds NaCaAlF6, Na2Ca3Al2F14, Ca2AlF7 and CaAlF5, as well
as eutectic and peritectic points given in the above mentioned
works [8,9], do not coincide in many cases. The most substantial is
the fact that the data available are not sufficient to explain phase
formation mechanism observed when sampling in industry and to
formulate sound recommendations for sampling.
In the present paper details of the quaternary phase crystal-
lization in the system NaF–CaF2–AlF3 are specified. The individual
phases NaCaAlF6, Na2Ca3Al2F14 were synthesized and high-
temperature XRD and thermal studies of their transformations
were carried out at heating. When cooled the behavior of these
phases in the electrolyte composition depending on the cooling
rate and electrolyte composition (CR) has been studied. The effect
of temperature annealing in the subsolidus area on decomposition
and formation of calcium-containing phases in electrolytes has
been investigated as well.
XRD data are in agreement with XRD reference data ICDD-
JCPDS-PDF-2 [17,18]. The phases Na5Al3F14, CaF2 for Na2Ca3Al2F14
have been recorded as admixtures.
2.2. Synthesis of calcium-containing electrolytes
To study the behavior of the calcium-containing phases in
electrolyte medium the electrolytes E1, E2, E3 with CR 1.9, 2.4, 2.7,
respectively, and the same CaF2 concentration equal to 8 mass%
were synthesized from the correspondent mixtures of the
fluorides AlF3, NaF, and CaF2. The mixtures of the initial fluorides
with the mass of 10 g were melted in a closed platinum crucible
in a furnace at the temperature range from 960 to 1000 1C
(depending on the CR value), for 20 min. Later, three different
ways of cooling were used: (1) the sample was cast into a massive
metallic mold at 25 1C, (2) the sample was taken out of the furnace
and air-cooled; (3) the sample was cooled in the furnace at a rate
of 8 1C/min up to one of the fixed temperatures of 560, 610 and
710 1C; then, the electrolyte was kept at these temperatures for
30 min, and air-cooled.
2.3. TA
TA of the samples of the NaCaAlF6 and Na2Ca3Al2F14 phases
was carried out using a MOM Q-1000 Derivatogragh in platinum
crucibles with a head and a weight of 500 mg, in the temperature
range from 25 to 860 1C in air with a heating rate of 10 1C/min.
Thermograms are given in Fig. 2.
8000
7000
6000
5000
4000
3000
2000
1000
8000
6000
4000
2000
0
0
10 20 30 40 50 60 70 80
10 20 30 40 50 60 70 80
2 Theta, deg.
2 Theta, deg.
Fig. 1. (a) X-ray diffraction patterns of NaCaAlF6 [18]; (b) X-ray diffraction patterns of Na2Ca3Al2F14 [17].

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CaF2
mass%
0.0 1.0
NaCaAlF6
Na2Ca3Al2F14
0.2
0.8
Ca2AlF7
0.4
0.6
Na2Ca3Al2F14
CaAlF5
0.6
0.4
NaCaAlF6
600
650
700
750
800
0.8
0.2
T, °C
Fig. 2. Thermograms for phases NaCaAlF6 and Na2Ca3Al2F14 at heating. The peak at
655 1C corresponds to transformation of NaCaAlF6 into Na2Ca3Al2F14, the peak at
T ¼ 695 1C corresponds to eutectics Na5Al3F14–NaAlF4, the peak at 725–735 1C
corresponds to peritectic Na5Al3F14–Na3AlF6, the peak at 745–750 1C corresponds
to decomposition of Na2Ca3Al2F14 into CaF2 and Na3AlF6.
1.0
0.0
NaF
0.0
1.0
AlF3
0.2
0.4
Na3AlF6
0.6
0.8
Na5Al3F14 NaAlF4
Fig. 3. Composition triangle of the system NaF–CaF2–AlF3. The positions of the
main phases occurring in the electrolyte are marked. The field of the ‘‘industrial
electrolyte’’ composition is marked by the color spot.
2.4. XRD analysis
XRD patterns were taken using X’Pert Pro (PANalitical)
diffractometer, equipped with PIXel detector with a graphite
1 - Na3AlF6
monochromator using CuK
801 with a step of 0.0261 (2
a
radiation, in the range from 101 to
), the total scanning time being
2 - Na5Al3F14
3 - NaCaAlF6
Y
3
12 min. The sample was prepared by a method of direct loading in
a sample holder with a diameter of 27 mm.
4 - Na2Ca3Al2F14
5 - CaF2
3
3
High-temperature XRD investigations were carried out using
X’Pert Pro (PANalitical) diffractometer with a high temperature
chamber NTK1200N ‘‘Anton Paar’’ in vacuum. The sample with
mass of 200 mg was prepared in a flat alumina sample holder with
a diameter of 10 mm. The temperature was increased at a rate
of 50 1C/min to certain value and then, for 5 min XRD patterns
3
3
3
I
3
3,4
3
4
4
4
4
4
4
3
4
4
3
3
4
II
4
were taken in the range from 51 to 801 (2Y). Then, the process was
repeated at the next temperature stage.
5
1
1
III
16 18 20 22 24 26 28 30 32 34 36 38 40
2 Theta, deg.
3. Results and discussion
Fig. 4. X-ray diffraction patterns of the sample of NaCaAlF6 taken in X-ray high-
temperature chamber I—at 25 1C, II—at 500 1C, III—at 500 1C, 1 h. At 500 1C
structural changes of the phase NaCaAlF6 are observed; the lines of the phase
Na2Ca3Al2F14 appear (middle pattern). After exposure during 1 h to 500 1C, the
Fig.
3 presents a composition triangle of the system
NaF–CaF2–AlF3 with the area of the ‘‘working electrolytes’’ being
marked, where the investigations were carried out; also marked
are the positions of the main calcium-containing phases occurring
in the cooled electrolyte samples. The presence of four and
sometimes five phases in the electrolyte samples contradicts
Gibbs’s phase rule and indicates non-equilibrium conditions
of their obtaining. The system peculiarity is the location of the
ternary phases on the section of CaF2–NaAlF4. This allows one to
confine the possible electrolyte composition field by the triangle
CaF2–Na3AlF6–NaAlF4. The double fluoride phases Ca2AlF7 and
CaAlF5 have never occurred in the electrolyte samples and it is a
well-grounded argument for marking this triangle field.
High-temperature XRD of the phases NaCaAlF6 and Na2-
Ca3Al2F14 were carried out by multistage heating in vacuum.
The NaCaAlF6 sample was heated with intermediate data obtained
at 100, 300, 400, 500, 550, 570, 600 1C. Some of the XRD patterns
are given in Fig. 4. More detailed information is presented in [19].
Below 400 1C the phase NaCaAlF6 undergoes structural
transformation retaining its composition. At 500 1C (Fig. 4) the
process of NaCaAlF6 decomposition with the formation of
phase NaCaAlF6 disappears; the content of Na2Ca3Al2F14, CaF2 and Na3AlF6
increases.
Na2Ca3Al2F14 begins in accordance with the equation
3NaCaAlF6-Na2Ca3Al2F14+NaAlF4
Among the reaction products the phase NaAlF4 was not
observed; that is likely to be explained by its high instability. In
vacuum at 500 1C the complete transformation of NaCaAlF6 into
Na2Ca3Al2F14 was observed after 1 h as well as the beginning
of Na2Ca3Al2F14 decomposition with the formation of Na3AlF6
and CaF2.
Heating Na2Ca3Al2F14 in high-temperature XRD chamber has
shown that the phase remains approximately up to 600 1C and
then, decomposition begins. Only two decomposition products,
CaF2 and Na3AlF6, are shown in the X-ray patterns (Fig. 5), which
cannot explain the mass balance without assuming volatile phase

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1 - Na3AlF6
4
away from the reaction zone, decompose or react with other
medium components, according to the equations
2 - Na5Al3F14
4 - Na2Ca3Al2F14
5 - CaF2
4
4
3NaAlF4-Na3AlF6+2AlF3
4
4
4
4
4
5
5
4
2NaAlF4+Na3AlF6-Na5Al3F14
4
4
4
The conducted high-temperature experiments have demon-
strated that calcium-containing phases have different tempera-
ture intervals of stability. The phase NaCaAlF6 is metastable at low
temperatures. Its relaxation in the conditions of low pressures
4
5
4
2
4
4
4
2
I
5
5
begins at 500 1C and reveals the transformation into Na2Ca3Al2F14
.
No reverse transformation is observed in the experiment condi-
tions.
5
It is obvious that the results obtained only partially reflect
the processes occurring when cooling the electrolyte samples.
For this reason an additional number of experiments were
carried out connected with electrolyte sample cooling from the
melt condition at different rates, including exposure to different
temperatures during the process of cooling.
1
1
1
1
1
II
20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60
2 Theta, deg.
XRD analysis of the products has shown that no changes of the
phase composition occur when changing crystallization rate.
The following phases are observed: Na3AlF6, Na5Al3F14, NaCaAlF6,
Na2Ca3Al2F14 and CaF2. Faster cooling gives bigger amount
of NaCaAlF6. The cooling rate influences the microstructure of
particular phases. Specifically, with the increase of the cooling
rate the cryolite lines are broadened by 10%. The microstructure
of other phases changes very little. Calcium was present in three
phases: NaCaAlF6, Na2Ca3Al2F14 and CaF2, with the first phase
dominating in the sample with the cryolite ratio 2.7 and the
second one in the sample with the cryolite ratio 2.3 and 1.9.
Furnace cooling, including annealing at one of the follow-
ing temperatures: 740, 610, 560 1C, gives different results. For
example, annealing at 740 1C gives rise mainly to NaCaAlF6
crystallization, irrespectively of the cryolite ratio. Annealing at
610 and 560 1C leads to the presence of mainly Na2Ca3Al2F14 in the
sample.
Based on the results of the experiments carried out the
following conclusions can be made. The calcium cryolites formed
in the system (NaF–CaF2–AlF3) occupy various temperature ranges
in the framework of the equilibrium diagram. Na2Ca3Al2F14 is a
low temperature phase; NaCaAlF6 is a high temperature one. In
the cooled electrolyte samples the presence of the two indicated
phases simultaneously is a consequence of the non-equilibrium
cooling process at which NaCaAlF6 is not completely transformed
into Na2Ca3Al2F14 and at low temperatures remains in the
metastable condition. This circumstance explains the crystal
structure degradation and ‘‘diffusion’’ peaks on the diffrac-
tion pattern of this phase. It was experimentally proved that
dominating crystallization of Na2Ca3Al2F14 in industrial samples
of electrolytes with low CR is caused by the shift in the
composition of the system liquid component at cooling into the
concentration field confined with the temperature below 734 1C
(incongruent melting of Na5Al3F14) and above 695 1C (solidus
of the subsystem Na5Al3F14–AlF3) [1], in which crystallization of
Na2Ca3Al2F14 occurs.
Fig. 5. X-ray diffraction patterns of the sample Na2Ca3Al2F14 taken in X-ray high-
temperature chamber I—at 25 1C, II—at 600 1C. The phase Na2Ca3Al2F14 disappears,
the phases CaF2 and Na3AlF6 are observed.
contribution. Assuming that the decomposition occurs within the
framework of the quasibinary system CaF2–NaAlF4, the observed
process can be described by the equation system
Na2Ca3Al2F14-2NaAlF4+3CaF2
3NaAlF4-Na3AlF6+2AlF3
The presence of Na3AlF6 in the products indicates the fact that
the NaAlF4 stability at low pressure is limited and its decomposi-
tion takes place directly in the sample. No solid-phase decom-
position reaction of Na2Ca3Al2F14 into NaCaAlF6 and CaF2 is
observed.
The final products belong to a quasibinary system of eutectic
type, CaF2–Na3AlF6, with the melting point of the eutectic being
945 1C [16]. When cooled the phase transformation
-Na3AlF6 occurs in the system.
The existence of the phase NaAlF4 has been repeatedly
b-Na3AlF6-
a
mentioned in literature [1,9,20–24] but reliable information
is absent. The phase NaAlF4, according to [20], is formed by
peritectic reaction at 710 1C and decomposes already at 680 1C:
NaAlF4(s)-1/5 Na5Al3F14(s)+2/5 AlF3(s)
Holm et al. [21] published X-ray diffraction data for NaAlF4,
which were repeated in some papers [22,23]. The structure of
compound is not known. At the present study the phase NaAlF4
was not observed among the solid products of decomposition in
the sample; however, the traces of the substance were detected
in the products condensed in cooled parts of the X-ray high-
temperature chamber.
This is likely to cause the necessity of creating special
conditions for maintaining appropriate vapor pressure above the
substances in order to fix highly volatile and unstable compo-
nents. Due to the technical limitations, as contrasted to the
requirements given in this work, the transformations were carried
out either in vacuum or in the air, with the sample mass being
comparatively small. These conditions hindered NaAlF4 observa-
tions in the sample. The formed phase NaAlF4 could evaporate
Transformation of NaCaAlF6 into Na2Ca3Al2F14 at cooling
occurs during the reaction of decomposition, probably yielding
NaAlF4. When heated the reverse reaction does not take place
since unstable NaAlF4 is absent in the sample. Instead, the
decomposition reaction of the phase Na2Ca3Al2F14 on CaF2 and
NaAlF4 is observed with NaAlF4 finally decomposing into cryolite.
Thermograms of the phase heating of NaCaAlF6 and Na2-
Ca3Al2F14 (Fig. 2) can be fully interpreted from the point of view
of the processes described. For NaCaAlF6 at 655 1C the process of

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880
860
840
820
800
780
760
740
720
700
680
1 - Na3AlF6
2 - Na5Al3F14
3 - NaCaAlF6
4 - Na2Ca3Al2F14
1
1
2
4
1
1
1
1
4
3,4
1
1
3
1
2
2
4
I
1
2
1
1
1
4
4
4
4
1
2
0
20
Na2Ca3Al2F14
mass%
40
60
80
100
NaAlF4
2
II
CaF2
NaCaAlF6
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
2Theta, deg.
Fig. 6. The prospective equilibrium diagram of the quasibinary system CaF2–
NaAlF4.
Fig. 7. X-ray diffraction patterns of the industrial electrolyte CR 2.33, CaF2 ¼ 7.9
mass%; I—initial electrolyte the basic phases are Na3AlF6 and Na5Al3F14, calcium is
distributed on two phases—NaCaAlF6, Na2Ca3Al2F14; II—the sample calcinated at
640 1C 10 min, losses of weight 0.1 mass%; The basic phases—Na3AlF6 and
transformation into the phase Na2Ca3Al2F14 is observed. At 745 1C
this phase is decomposed into Na3AlF6 and CaF2. On the sample
heating curve of the phase Na2Ca3Al2F14 there is a number
of small peculiarities under 655 1C caused by the admixtures. The
temperature of 695 1C corresponds to the eutectic transformation,
Na5Al3F14–AlF3, and the temperature of 735 1C to the peritectic
Na5Al3F14, calcium is in one phase—Na2Ca3Al2F14
.
heated up to 640 1C decomposes, yielding Na2Ca3Al2F14. The
expected reverse transformation of Na2Ca3Al2F14 into NaCaAlF6
has not been observed under experimental conditions by high-
temperature XRD method. Heating and quenching bulk samples
reveal direct and reverse transformation of quaternary phases.
The transformations studied allow us to recommend a heat
regime strategy for cooling electrolyte samples which are selected
for the composition monitoring. Cooling with the short-time delay
(annealing) in the temperature range of 560–640 1C, in which
the full transformation of the phase NaCaAlF6 into Na2Ca3Al2F14
occurs, permits obtaining industrial electrolyte samples which
would be more suitable for the measurement by the XRD method
and application of full profile refinement as data processing
methods [25,26].
one, Na3AlF6–Na5Al3F14
. The decomposition of Na2Ca3Al2F14
occurs at 745 1C.
The transformations considered can be described in the
context of the quasibinary system CaF2–NaAlF4 with invariant
five phase equilibrium of CaF2–NaCaAlF6–Na2Ca3Al2F14–(liquid
melt)–(NaAlF4) at 745 1C. The subsolidus part is given in Fig. 6.
The peculiarity of the system is the presence of an unstable
and volatile compound, namely of NaAlF4 in the right part of
the system. Decomposition of the phase NaAlF4, for example,
into Na5Al3F14 and AlF3 or Na3AlF6 and AlF3, makes the system
split into two subsystems with different equilibrium pressures.
It solves the problem of the contradiction to Gibbs’s phase rule,
which appears since the number of the present phases (five)
exceeds the permissible number of phases (four) which are in the
equilibrium in the three component system at constant pressure.
This gives the basis for the interpretation of the transformations
observed. In particular, to approach the equilibrium from below,
no formation of NaCaAlF6 occurs, if any external pressure is not
applied to the system. In the course of the high-temperature X-ray
investigations, which allow the fixation of the solid-phase system
components; this condition was not created for technical reasons.
Nevertheless, experiments on heating and cooling the bulk
sample exhibit the possible presence of both calcium-containing
phases simultaneously. Behavior of calcium-containing phases
in the samples of industrial electrolyte is illustrated in Fig. 7.
The X-ray pattern of the initial electrolyte (Fig. 7, top) shows
two phases NaCaAlF6 and Na2Ca3Al2F14 containing calcium.
After calcinations at 640 1C for 10 min the same sample has lost
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0.1 mass% and calcium has been presented in Na2Ca3Al2F14
.
Thus, quaternary compounds in the system NaF–CaF2–AlF3 are
presented in phases NaCaAlF6 and Na2Ca3Al2F14. They are a part of
quasibinary section CaF2–NaAlF4. In the system at T ¼ 745–750 1C
the invariant equilibrium is implemented with the phases CaF2–
NaCaAlF6–Na2Ca3Al2F14–(liquid melt)–(NaAlF4). The quaternary
phase Na2Ca3Al2F14 is stable below the equilibrium temperature,
and NaCaAlF6 above this temperature. The phase NaCaAlF6 can be
fixed in metastable state by rapid quenching; however, when
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