Structural aspects of the formation of solid solutions in the NaF-KF-AlF3 system

Journal of Solid State Chemistry 252 (2017) 1–7

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Structural aspects of the formation of solid solutions in the NaF-KF-AlF3 system

MARK

Alexander S. Samoiloa, Yulia N. Zaitsevab, Peter S. Dubinina, Oksana E. Piksinaa, Sergei ⁎ G. Ruzhnikova, Igor S. Yakimova, Sergei D. Kirika, a b

Siberian Federal University, Krasnoyarsk, Russia Institute of Chemistry and Chemical Technology SB RAS, Krasnoyarsk, Russia

A R T I C L E I N F O

A BS T RAC T

Keywords: NaF-KF-AlF3 system Solid solution X-ray powder diffraction Crystal structure refinement

The formation of solid solutions in the ternary system NaF-KF-AlF3 has been studied by X-ray diffraction and thermal analysis. Chiolite has been shown to form solid solutions with the composition (Na(5−x)Kx)Al3F14, in the limited range of 0 < x < 0.4. The lattice parameters change in the intervals for (a) 7.010 (3) −7.050 (3) Å, for (c) 10.365 (10) −10.400 (10) Å. According to the investigation of the crystal structure, potassium cations substitute sodium only in the 2-fold position in the amount of ~40%. The solid solutions are stable in the range from room to melting point temperature. A wide range of solid solutions based on β-cryolite (Na3AlF6) and elpasolite (K2NaAlF6) above 540 °C has been studied in detail. It is only the 8-fold cationic position in the β-cryolite structure which appears to have contributed into the substitution in the full range of solid solutions. The solid solution decays into a mixture of α-Na3AlF6 and K2NaAlF6 upon calcination below 540 °C., followed by further cooling without changing the α-Na3AlF6 composition. Elpasolitе initially containing an excess of sodium ions, has yielded cryolite and stoichiometric K2NaAlF6 below 340 °C. The phase K2NaAl3F12 present in two polymorphic forms, has not formed a wide range of solid solutions; however, a slight excess of potassium ions has improved the stability of the high-temperature form.

1. Introduction It is well known that cryolite-alumina melt is used for the electrolytic aluminum production. This explains the basic and practical interest to the system NaF-AlF3-KF [1]. At present the main features of multicomponent fluoride melts are well established. Some of them are presented in Fig. 1 and shortly summarized below. Three crystalline phases, namely, cryolite Na3AlF6, chiolite Na5Al3F14 and NaAlF4 are known in the binary system NaF-AlF3. [2] Cryolite melts congruently at 1011.6 °C having temperature polymorphs transition at 542 °C [1]. Chiolite melts incongruently at 737–739 °C. The NaAlF4 phase is metastable under normal conditions. It can be obtained by quenching the corresponding melt from 700 °C. The phase decomposes into Na5Al3F14 and AlF3 at heating between 400–700 °С [3–5]. Several phases were established in the system KF-AlF3 [6]. K3AlF6 melts congruently at 995 °C. It has some polymorphs with the phase transition at 132, 153, 306 оС [7]. KAlF4 melts congruently at 574 °C. The phase can directly be obtained from KF and AlF3 [8]. The compounds K2AlF5 and KAl4F13 were initially obtained by the hydrothermal synthesis in hydrofluoric acid solutions at relatively low

⁎

Corresponding author. E-mail address: [email protected] (S.D. Kirik).

http://dx.doi.org/10.1016/j.jssc.2017.04.037 Received 24 February 2017; Received in revised form 27 April 2017; Accepted 29 April 2017 Available online 04 May 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.

temperatures [9,10]. The binary system NaF-KF corresponds to a simple eutectic type with the eutectic point at T=721 °C and c(NaF)=40% (mol) [1]. The available data on the subsolidus part of the ternary system NaF-AlF3-KF are insufficient. The hydrothermal synthesis and structure of K2NaAl3F12 were presented in [11]. Later, the phase as well as its low-temperature modification were obtained from the high-temperature melt [12]. Both polymorphic forms have a wavy layered structure composed of [AlF6/2] octahedrons. The cations occupy the space between the layers. The phase transition is caused by the cation rearrangement. The Na3AlF6-K3AlF6 binary section is divided into two subsystems by elpasolite, K2NaAlF6, (Tmp 954 °C). There is an extensive range of solid solutions in the section [13]. The phases Na3AlF6 and K2NaAlF6 form the system at room temperature. Above the cryolite polymorphic transformation at 542 °C the system has a wide range of solid solutions. The high-temperature cubic form of Na3AlF6 is isostructural to elpasolite. The polymorphic transformation temperature of the solid solution decreases down to 340 °C in the vicinity of K2NaAlF6. Danielik et al. [14] investigated the system NaF-KF-AlF3 by thermal

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800 °C) and then, maintained for 40 min. The crystallization was performed by pouring the melt into a metal mold or by cooling it in the crucible at room temperature in the air. Additional thermal treatment of the samples was carried out in the closed platinum crucible in the furnace at 540 °C for 20 min in the air. According to the analysis of the resulting samples, the synthesis conditions did not lead to the appearance of oxygen-containing products. Fig. 1 depicts the triangle of the NaF-KF-AlF3 system with the indicated composition of the synthesized samples. There were four series. Series 1, 2 and 4 are characterized by the constant content of AlF3. Series 2 and 3 were limited to 20% (mol.) KF. In Series 3 the ratio NaF/AlF3 remained constant. Series 4 went through Na3AlF6 and K2NaAlF6. The structural features of both phases were investigated. 2.2. Research methods 2.2.1. X-ray diffraction X-ray powder patterns were obtained using the X′Pert PRO diffractometer (PANalytical) with CuKα radiation. PIXcel (PANalytical) equipped with a graphite monochromator was used as a detector. The sample was milled in an agate mortar and prepared by the direct loading. The scanning conditions: ranged from 3 to 100°(2θ), the step size being 0.013°, Δt – 50 s/step.

Fig. 1. The thernary system NaF-KF-AlF3 with the composition of the synthesized samples joined into series (1), (2), (3) and (4) with the indicated investigated areas of solid solutions.

analysis. They obtained the coordinates of the ternary eutectic points Е1: 36.3% (mol.) NaF, 62.7% (mol.) KF, 1.0% (mol.) AlF3; 711.2 °С and Е2: 51.9% (mol.) NaF, 27.4% (mol.) KF 20.7% (mol.) AlF3; 734.5 °С. On the whole, the available literature data concerning the ternary system NaF-KF-AlF are focused on the liquidus surface construction and phase formation [1]. The subsolidus part was studied insufficiently. The solid solutions except for the binary section Na3AlF6K3AlF6 are not mentioned in literature [13]. However, solid solutions in fluoride systems are typical phenomena [15,16]. Mention should be made of the existence of a narrow range of solid solutions in the system Na3AlF6-CaF2 based on β-Na3AlF6 at the eutectic temperature [17]. There is a wide range of solid solutions based on both Na3AlF6, and Li3AlF6 and their modifications in the system Na3AlF6-Li3AlF6 [18,19]. The available information allows concluding that the subsolidus areas in the NaF-KF-AlF3 system have not been studied in detail. Particularly, there are no data on the existence of solid solutions based on chiolite. The description of solid solutions based on elpasolite is superficial and lacks crystallographic details. However, the detailed information on the subsolidus region is important for solving applied problems. In particular, the phase composition data are significant for controlling the electrolyte composition by the X-ray diffraction technique in aluminum production [20]. The objective of this study has been to obtain data concerning the solid solution formation in the ternary system NaF-KF-AlF3. The study is focused on the characterization of solid solutions based on chiolite, cryolite and elpasolit, including the specification of solid solution boundaries, composition and crystal structure details. Some limitations of the concentration field (the KF content up to 20% (mol.)) are due to the actual compositions of the electrolytes used in low-temperature electrolysis. The investigation has been carried out using the laboratory-prepared samples. The crystal structure has been determined by X-ray diffraction full-profile analysis using multiphase polycrystalline samples.

2.2.2. Crystal structure analysis The crystal structure refinement in the multiphase samples was carried out using X-ray powder diffraction data and FullProf software [21]. The profile and cell parameters, atomic coordinates, atomic population were refined. Atomic thermal parameters were refined in isotropic approximation. 3. Results and discussion 3.1. Solid solutions based on the chiolite structure Na(5−x)KxAl3F14 According to the X-ray diffraction data the prepared samples of Series (2) and (3) were multiphase mixtures with chiolite (Na5Al3F14) as the main phase. The samples of Series (1) contained NaAlF4, AlF3, Na5Al3F14 and K2NaAl3F12. The phase NaAlF4 disappeared with the potassium concentration increasing. In the vicinity of K2NaAl3F12 high and low temperature forms of this phase were only observed. A higher concentration of KF in Series (2) and (3) resulted in disappearing Na5Al3F14, and increasing Na3AlF6. K2NaAl3F12 was the main potassium-containing phase. The phase K2NaAlF6 appeared with the higher KF content. The observed changes in the phase balance can be described by the following equilibrium: 2Na5Al3F14+2KF↔K2NaAl3F12+3Na3AlF6

(1)

and in a large excess of KF: Na5Al3F14+4KF↔2K2NaAlF6+Na3AlF6

(2)

The detailed analysis of the X-ray diffraction patterns revealed a specific shift of the line for the chiolite Na5Al3F14. The lines were shifted both in the low- and high angle areas with the variation in the KF content (Fig. 2.). The phenomenon could be interpreted as the lattice parameter variation within the sample series. It is reasonable to attribute the observed effect to the formation of solid solutions due to the replacement of sodium by potassium ions in the chiolite structure. The solid solution formation can induce two types of changes in the X-ray powder pattern. The first type is a shift of the diffraction lines and the second one is relative intensity variations. Both features have to be taken into account in the quantitative phase analysis. However, there may be a correlation between the mentioned effects. The crystal structure refinement was carried out using the X-ray

2. Experimental section 2.1. Synthesis of the samples The following reagents Na3AlF6 (99%), AlF3 (99%), KF (99.5%) (Reakhim, RF) of chemical grade purity were used for the sample synthesis. Before the synthesis all the reagents were calcinated at 400 °С for 1 h and milled. The sample weight was about 3 g. The synthesis was carried out in a vertical furnace in closed platinum crucibles. The temperature was increased up to melting (about 750– 2

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30.71 7000 6000

I (counts)

Table 1 Refined atomic coordinates for chiolite with the composition Na4.65K0.35Al3F14 from sample with initial ratio NaF:KF:AlF3=0.54:0.12:0.34 (mol) a=7.0479(2) Å, с=10.3732(3) Å, V=515.27(3) Å3, S.G. P4/mnc.

0% KF 2,5% KF

a) Fractional atomic coordinates.

17.03

5000

5% KF

4000

7,5% KF

3000 2000 1000 0 16,8 17,0 17,2 17,4

30,6 30,8 31,0 31,2 2 Theta

Atom

Wyck.

Occ.

x/a

y/b

z/c

B (iso)

Al1 Al2 Na1 K1 Na2 F1 F2 F3

2a 4c 2b 2b 8g 4e 8h 16i

2 4 1.31(3) 0.69(3) 8 4 8 16

0 0 0 0 0.2759(4) 0 0.0620(6) 0.1774(3)

0 ½ 0 0 0.7759(4) 0 0.2489(5) 0.5361(4)

0 0 ½ ½ ¼ 0.1693(5) 0 0.1202(2)

1.2(2) 1.2(2) 1.9(2) 1.9(2) 2.2(2) 2.4(2) 1.2(2) 1.4(2)

b) Some important interatomic distances. Al1–F1 1.756(5) Al1–F2i 1.808(4) Al2–F3 1.784(2) Na1–Na2ii 3.606(2) Na2–F1iii 2.641(3) iv Na2–F3 2.299(3)

Fig. 2. Illustration of the chiolite X-ray diffraction line shifts for the samples of Series (2) against the standard positions (ICDD PDF2 #01–74–755).

diffraction data for the multiphase samples. The synthesis conditions, namely, crystallization from the melt, allow assuming the full occupation of atomic positions, both anionic and cationic, which is due to the absence of diffusion hindrances during crystallization. The aluminum atoms do not occupy other cationic positions in the structure, and viceversa, sodium or potassium does not occupy the aluminum positions. There are two types of alternating layers in the chiolite structure (Fig. 3a,b) [22]. The first layer is a square network of octahedra [AlF6] connected by vertices. The interatomic Al-F distances are in a narrow range of 1.78–1.82 Å. The sodium ions are in the middle of the square cells. There are two different types of [AlF6], which differ in orientation and in the number of the shared vertices [AlF2F4/2] and [AlF4F2/2] and lie in the grid nodes as the node linkers. The coordination sphere of the sodium ion in the layer is a square prism formed by fluorine ions with the equal Na(1)-F distances - 2.583 Å [21]. The second layer in the structure consists of sodium cations with the nearest distances Na(2)Na(2) equal to 3.527 Å. The distances are slightly shorter than between the cations in the adjacent layers Na(1)-Na(2) (3.603 Å). The sodium cations in the second layer have a distorted octahedral surrounding formed by fluorine. The distances Na(2)-F in the equatorial plane are in the range of 2.268–2.290 Å and 2.626 Å up to the vertical vertices. The layered framework formed by the [AlF6] octahedra seems to be the

Al2–F3 Al2–F2v Na2–Na2vi Na1–F3vii Na2–F3

1.784(2) 1.823(4) 3.543(4) 2.606(2) 2.270(3)

(i) y, -x, z; (ii) 0.5-y, 0.5-x; (iii) -y, 1+x, z; (iv) 0.5-x, 0.5+y, 0.5-z; (v) -x, 1-y, z; (vi) 0.5-x, −0.5+y, 0.5+z; (vii) 1-y, x, z.

most robust molecular construction in the chiolite structure, since its elements are connected by covalent bonds. However, a possible small octahedra rotation within the layer can provide certain elasticity of the framework and be responsible for some tension and compression of the lattice in the basal plane. The second layer of the sodium ions is submitted to the dense packing law for minimizing the electrostatic energy. Despite the apparent liability of sodium in the Na(2) position for the substitution with potassium, the process does not proceed. The short distances d(Na(2)-F)=2.268 Å make this process impossible. At the same time there are no spatial hindrances for the substitution of Na(1) with rather spacious positions, with the shortest being d(Na(1)F)=2.583 Å. The crystal structure refinement confirmed these expectations. The solid solution formation proceeds upon sodium substitution in position Na(1) in the anionic-cationic layer. The ultimate composition corresponds to the formula Na4KAl3F14. However, it was never observed in our experiments. The most saturated solid solutions were experimentally obtained in the multiphase samples with the KF content

Fig. 3. Crystal structure of chiolite; (a) the general view; (b) the structure of the cation-anion layer (position Na(1)) and cation layers (position Na(2); (c) the crystal structure of the solid solution Na5−xKxAl3F14 and polyhedra for the positions Na(1) and Na(2). The partial substitutions take place in the position Na-K(1).

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6000

in the X-ray powder patterns (Fig. 2). The unit cell parameters of chiolite vary in a rather limited range (Fig. 5a,b). The cell parameter(a) increases, whereas (c) decreases with the increase in the KF concentration. The antibatic variation is due to the [AlF6] octahedra arrangement, which leads to the convergence of the layers in the structure. The highest KF concentration in the chiolite structure is about 5% (wt.). It corresponds to the substitution of 40% sodium with potassium ions in the Na(1) position. The values were achieved after homogenizing annealing of the samples at 540 °С. The annealing temperature can slightly affect the ultimate content of potassium in the structure. However, it does not influence the trend linearity between the lattice parameters and KF content. Thus, chiolite forms a solid solution when potassium fluoride is added to the NaF-AlF3 system. The concentration limit is about 5% KF. The solid solutions are stable from 20 °C up to the melting temperature (~730 °C). An additional amount of KF leads to the crystallization of K2NaAl3F12 and K2NaAlF6, which indicates a limited range of solid solutions. Concluding the discussion on the chiolite solid solutions, it is worth mentioning some related experimental observations. The attempts to obtain solid solutions following the heterovalent substitution scheme involving magnesium and calcium ions did not yield any positive results. At the same time, lithium ions partially enter into the chiolite structure, replacing Na(2) in the eight-fold positions. Valuable results for the problem were obtained in [23] where FeF3 was dissolved in cryolite, forming a solid solution on the basis of the chiolite structure.

I(counts)

5000 4000 3000 2000 1000 0

Na K Al F K NaAlF

-1000 10 10

20

30

20

40

30

50

40

60

50

60

70

2 Theta

70

Fig. 4. Example of the agreement between the experimental (black dots) and calculated (black line) X-ray powder patterns (red line – difference) achieved during the crystal structure refinement (Rwp=9.8%, Rp=7.8%, Rexp=7.4%). The experimental sample with initial ratio NaF:KF:AlF3=0.54:0.12:0.34 consists of two phases: chiolite with the composition Na4.65K0.35Al3F14 and elpasolite K2NaAlF6 The reflection positions of the presented phases are indicated at the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of about 12% (wt.). The chiolite structure corresponded to the approximate composition Na4.60K0.40Al3F14. Fig. 3c depicts the crystal structure of the solid solution. Only the Na(1) position is subjected to cationic exchange. The example of the crystal structure coordinates after refinement is given in Table 1. The achieved agreement between the experimental and calculated X-ray diffraction patterns is presented in Fig. 4. The refinements were performed for the two-phase sample. The quality of the structure refinement was provided by the correctness of the obtained interatomic distances. The minor changes of the atomic coordinates are associated with a small change in the octahedra orientation caused by the potassium location in the Na(1) positions. Analogous calculations were performed for all the synthesized samples. The obtained data allows one to draw a trend revealing the dependence between the cell parameters and calculated potassium fluoride concentration in chiolite (Fig. 5a,b). Although the accuracy of the KF concentration could be influenced by the data distortion caused by a slight texture in the sample, there is an apparent linear dependence. It is interesting to note that the parameter (a) follows a positive, whereas (c) - a negative trend. The reason is the arrangement of some [AlF6] octahedra causing the convergence of the layers. The consequence of this phenomenon is a specific shift of the line positions

3.2. Solid solutions based on elpasolite K2NaAlF6 The formation of the solid solutions in the system Na3AlF6 K2NaAlF6 was investigated on the samples of Series (4) (Fig. 1). Since the solid solutions existed at temperatures above 542 °С [13] the samples of each composition were divided into two parts and annealed at temperatures 450 °С and 750 °C to reach the equilibrium. The crystal structure of cryolite is presented in Fig. 6(a). Cryolyte as well as chiolite consists of two distinct layers: a mixed cationic-anionic and a cationic one. Na(1) in the first layer (2b position) has the distorted octahedral surrounding, with the Na(1)-F distances being in the range 2.227–2.282 Å. Another sodium, Na(2) (4e position), has tetrahedral coordination surrounding with the distances of Na(2)-F being 2.29–2.35 Å. [23]. Considering the short interatomic distances, the ionic exchange in these positions seems doubtful. A small slope of about 19.7° of the [AlF6] octahedron axis to the cell axis is a characteristic feature of the cryolite structure. When the temperature increases above 500 °C, the octahedron axes gradually orient along the cell axes. The phase transition of the second type occurs. The space group changes from P21/n to Fm-3m (Fig. 6(b)). The Na(1) position transforms from (2b) into (4a) with the octahedral

10,41 Cell parameter c, Α

Cell parameter a, A

7,07 7,06 7,05 7,04 7,03 7,02 7,01

10,40 10,39 10,38 10,37 10,36

0

1

2

3

0

4 5 C(KF), %(wt)

1

2

3

4 5 C(KF), % (wt)

Fig. 5. Relationship between the chiolite unit cell parameters calculated during the structure refinement (a and c) and the calculated KF concentration.

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Fig. 6. Crystal structure of cryolite Na3AlF6: (a) alpha – low-temperature form, (b) beta - high-temperature form. Crystal structure of elpasolite K2NaAlF6 (c) (the potassium position is highlighted in red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

#

Rel. Int.

#

* - Na3AlF6 # - K2NaAlF6

#

I(counts)

4000

3000 o

*

#

* * *# *

420 C

*

#

#

2000

* **

* ****

1000

Solid Solution 0

o

750 C

10

20

30

40

50

-1000 10

60 70 2 Theta

30

8,0 Cell parameter, A

8,09 8,08 8,07 8,06 o

750 C

7,2 6,8 6,4 6,0

5,2 5

10

15

(3)

7,6

(b) (a)

5,6

8,03 0

70

(ñ)

o

8,04

60

The structural transformation leads to dramatic changes in the Xray powder patterns. Fig. 7a,b presents two patterns obtained from the

450 C

8,05

50

(x)Na3AlF6+(y)K2NaAlF6↔K2yNa(1+x)AlF6

8,11 8,10

40

Fig. 9. Agreement of the experimental (black dots) and calculated (black line) X-ray powder patterns (Rwp=8.6%, Rp=6.4%, Rexp=6.7%) obtained for the sample with initial ratio NaF:KF:AlF3=0.57:0.18:0.25 (mol) containing Na3AlF6, K2NaAlF6, NaF. The line positions of the presented phases are indicated in the bottom of the graph as well as the difference of the X-ray powder patterns (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

coordination and Na(1)-F distances of 2.283 Å. The coordination polyhedron for Na(2) (4e) transforms from tetrahedron to cubooctahedron (8c position) with the Na(2)-F distances equal to 2.85 Å [25]. The resulting structure corresponds to elpasolite (Fig. 6c) [26]. A new size of cationic voids (8c) indicates the possibility of isomorphic substitution of sodium with potassium ions. The process of the solid solution formation can be described by the following equation with (2y+x)=2:

Cell parameter, A

20

2 Theta

Fig. 7. XRD patterns from the sample with the composition Na3AlF6/K2NaAlF6=60/40.

8,02

Na AlF K NaAlF NaF

20 25 30 35 40 C(KF) in sample, %(wt)

8

10

12

14

16 18 20 22 24 C(KF) in sample, %(wt)

Fig. 8. a) Cell parameters of elpasolite in the samples annealed at 450 °C and 750 °C, depending on the KF concentration. b) Cell parameters (a,b,c) of cryolite after annealing at 450 °C, depending on the KF concentration.

5

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Fig. 10. Crystal structure and structurе of the layer motif of K2NaAl3F12(1) (a, c); the crystal structure of K2NaAl3F12(2) (b, d).

potassium ions to the sodium positions. The obtained results allow concluding that cryolite does not change its composition when heated to a temperature lower than 450 °C. At a higher temperature the second-order phase transition occurs. It consists in a continuous change of the [AlF6] octahedron orientation. The transformation completes with the formation of β-cryolite where sodium can be substituted in the 8c positions with potassium ions. The process is limited by the diffusion rate, which is noticeable at temperatures above 700 °С. As a result, there occurs the formation of a continuous field of solid solutions with the elpasolite structure. The stoichiometry of the solid solutions corresponds to KхNa3-хAlF6, where x < 2. A rapidly cooled sample of the solid solution decomposes into αcryolite and a solid solution of elpasolite with the sodium content limited to 2%.

samples annealed at 750 °C and 450 °C and then, quenched with the composition corresponding to Na3AlF6/K2NaAlF6=60/40. After annealing at 750оС the cubic phase with the lattice parameter 8.015(2) Å dominates in the sample. The well-crystallized elpasolite and cryolite is present in the sample treated at 450 °C. The lattice parameter of elpasolite increases up to 8.086(2)Å, which is slightly lower than that for stoichiometric elpasolite (8.118(2) Å). The crystal structures of elpasolite and cryolite were refined. The occupation of the 8c cationic position appear to be filled as 7.46 K+0.54 Na. Fig. 9 shows the resulting X-ray patterns after the structure refinement of the sample containing 3 phases. The graphs presented in Fig. 8a,b summarise the data on the solid solutions in the Na3AlF6 - K2NaAlF6 system. The elpasolite cell parameter increases in the range from 7.990 to 8.098 Å, with the samples being annealed at 750 °С (Fig. 8a). The potassium concentration increased simultaneously. The additional annealing procedure at 450 °C causes the cell parameter variation in a narrow range of 8.084– 8.098 Å, indicating the existence of solid solutions in a limited region at this temperature. The substitution in the (8c) position with sodium has a limited range and does not exceed 6% (Fig. 9). It was revealed that the cryolite cell parameters were close to those presented in literature [24] after annealing at 450 °C (Fig. 8b). The observed minor variations within ± 0.02 Å should be attributed to the [AlF6] octahedra rotations. The occupation of the cation positions did not change in all the investigated samples. At heating above 500 °C the cryolite structure transformed continuously to accommodate the

3.3. Solid solutions based on the ternary phase K2NaAl3F12 Recently two polymorphic forms of K2NaAl3F12 and phase transition have been investigated [11,12]. Both mentioned phases constantly contribute to the cooled samples of electrolysis electrolytes in aluminum production. Thus, it is worth considering in the context of the solid solution formation. The crystal structure of two polymorphic forms of K2NaAl3F12 are depicted in Fig. 10a,b,c,d. In both cases, the structure is based on the alternation of the wavy anionic layers composed of the [AlF6] octahedrons connected via equatorial vertices. The anionic framework consists of triangle and hexagonal rings known 6

Journal of Solid State Chemistry 252 (2017) 1–7

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Federation, program "Management of Scientific Research", Task #4.8731.2017/BY. Scientific facility for investigation was provided by SFU Multiple-access centre.

as the "kagome" net, while the cations are located between the nets in the cavities of two types: an octahedral and a truncated hexagonal pyramid [11,12]. The phases are different in the cation arrangement in the existing cavities (Fig. 10c,d). In the high-temperature form of K2NaAl3F12(1) there is some disordering of the cations in the cavities. In the other form of K2NaAl3F12(2) the sodium cations are located exclusively in the octahedral cavities and the potassium cations in the truncated pyramids. The phase transition consists in shifting each of the second cationic layer along the wave-like anionic net. After the shift the cations occupy the positions more suitable in size and the cell volume decreases. Considering the crystal structure of both forms and some experimental observations allows assuming the presence of a specific region of solid solutions in the vicinity of the K2NaAl3F12 composition. It was noted that a small excess of KF during the synthesis resulted in the improved stability of the high temperature form. The deficiency of KF or excess of NaF provided the stability of the low temperature form [12]. So, the small excess of KF or NaF shifted the temperature of the phase transition. These features suggest the existence of a narrow region of cationic disordering in the vicinity of K2NaAl3F12 composition, which could be considered as a specific type of the solid solution range.

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4. Conclusion The obtained results show that the expansion of the binary NaFAlF3 into the ternary NaF-KF-AlF3 system leads to the appearance of new phases and phases with variable composition. Solid solutions are formed due to the exchange of sodium and potassium cations. There are some crystallographic features limiting the solid solution range. The study was focused on four phases Na3AlF6, Na5AlF14, K2NaAlF6, K2NaAl3F12, which usually contribute to cooled samples of the electrolyte in aluminum production. Three different mechanisms of the solid solution formation were revealed in the system. In the case of chiolite only one Na(1) of two independent crystallographic positions is subjected to the substitution of sodium with potassium ions. The substitution is limited and does not exceed 40%. Due to sterical hindrances the second position Na(2) is not involved in the exchange. In the case of the Na3AlF6 - K2NaAlF6 section the solid solution formation results in a wide region of solid solutions. The process proceeds above 500 °C where the cryolite crystal structure adapts to elpasolite during the second-order phase transition. Again, only Na(2) of two cationic positions is involved in the cationic exchange. At temperatures below 500 °C the solid solutions decay to cryolite and elpasolite enriched with sodium. The solid solution range in the vicinity of K2NaAl3F12 seems to be due to small disordering in the cationic positions. A slight excess of potassium or sodium fluoride over the exact stoichiometry influences the experimentally observed shift of the phase transition. The investigated solid solution regions in the NaFKF-AlF3 system are shown in Fig. 1. Acknowledgement This work was financially supported by RUSAL ETC (Inert anode project) and by the Ministry of Education and Science of the Russian

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