CC BY
25DOI: 10.17516/1998-2836-0268 УДК 544.014
Solid Solutions in Aluminum Electrolytes with the Participation of LiF
Yulia N. Zaitseva*a, Sergei D. Kirika, b, Aleksandr S. Samoilob, Petr S. Dubininb, Oksana E. Bezrukovab, Aleksandr N. Zalogab, Sergei G. Ruzhnikovb and Igor S. Yakimovb
aInstitute of Chemistry and Chemical Technology FRC «Krasnoyarsk Scientific Center of the SB RAS» Krasnoyarsk, Russian Federation bSiberian Federal University Krasnoyarsk, Russian Federation
Received 21.09.2021, received in revised form 15.11.2021, accepted 13.01.2022
Abstract. The paper presents the investigation of the solid solutions formation in the fluoride system Na5Al3F14-Na3AlF6-K3AlF6 with the addition of LiF. Two local regions were under consideration. The first case concerns the possibility of potassium and sodium cations replacing with lithium in the elpasolite (K2NaAlF6) when LiF is added to the electrolyte. In the second, the effect of lithium fluoride on the chiolite (Na5Al3Fi4) was studied. The investigation was motivated by the need to control the composition of the electrolyte during the electrolytic production of aluminum. The electrolyte samples were obtained from the initial fluorides under laboratory conditions. Determination of crystal-chemical details of the structure of solid solutions was carried out by the method of full-profile analysis (Rietveld method) on multiphase polycrystalline samples. It was found that the addition of LiF in the structure of elpasolite is accompanied by a gradual substitution of lithium for sodium with the formation, ultimately, of the K2LiAlF6 phase with the lattice parameter a = 8.0779 Â. Potassium is not subject to substitution. No dissolution of LiF in the structure of cryolite (Na3AlF6) was found. It was shown that chiolite does not form solid solutions with LiF upon crystallization.
Keywords: system Na3AlF6-K3AlF6-AlF3-LiF, electrolyte for aluminium production, solid solutions, X-ray powder diffraction.
© Siberian Federal University. All rights reserved
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). Corresponding author E-mail address: [email protected]
Citation: Zaitseva, Yu. N., Kirik, S. D., Samoilo, A. S., Dubinin, P. S., Bezrukova, O. E., Zaloga, A. N., Ruzhnikov, S. G., Yakimov, I. S. Solid solutions in aluminum electrolytes with the participation of LiF. J. Sib. Fed. Univ. Chem., 2022, 15(1), 22-31. DOI: 10.17516/1998-2836-0268
Твердые растворы в алюминиевых электролитах с участием LiF
Ю. Н. Зайцева3, С. Д. Кирик3 б, А. С. Самойло6, П. С. Дубинин6, О. Е. Безрукова6, А. Н. Залога6, С. Г. Ружников6, И. С. Якимов6
аИнститут химии и химической технологии СО РАН обособленное подразделение ФИЦ КНЦ СО РАН Российская Федерация, Красноярск бСибирский федеральный университет Российская Федерация, Красноярск
Аннотация. Представлены исследования образования твердых растворов в двух локальных областях системы №5А13Г14-№3А№6-К3А№6 с добавлением LiF. В первом случае рассмотрена возможность замещения катионов калия и натрия в эльпасолите (К^ЫаАШ^ на литий при добавлении в электролит ЫЕ Во втором изучено влияние фторида лития на хиолит (№5А13Б14). Изучение фторидной системы обосновано необходимостью контроля состава электролита при электролизе алюминия. Исследования были выполнены на образцах, полученных из исходных фторидов в лабораторных условиях. Определение кристаллохимических деталей строения твердых растворов проведено методом полнопрофильного анализа (метод Ритвельда) на многофазных поликристаллических образцах.
Установлено, что при добавлении в структуре эльпасолита происходит постепенное замещение натрия на литий с образованием в конечном счете фазы К2ЫА№6 с параметром решетки а=8,0779 А. Калий не подвергается замещению. Растворения в структуре криолита (№3А1Г6) не обнаружено. Показано, что хиолит при кристаллизации не образует твердых растворов с ЫЕ
Ключевые слова: система №3А1Г6-К3А1Г6-А1Г3-ЫЕ электролиты для производства алюминия, твердые растворы, порошковая рентгеновская дифракция.
Цитирование: Зайцева, Ю. Н. Твердые растворы в алюминиевых электролитах с участием ЫБ / Ю. Н. Зайцева, С. Д. Кирик, А. С. Самойло и др. // Журн. Сиб. федер. унта. Химия, 2022, 15(1). С. 22-31. DOI: 10.17516/1998-2836-0268
Introduction
The Heroult-Hall electrolytic process [1, 2], used for the industrial production of primary aluminum, is possible due to the dissolution of alumina in a fluoride-salt melt at a temperature of about 960 °C. For more than 100 years, the compositions of the electrolytes used have been invariably based on the (NaF-
AlF3) system in the crystallization area of cryolite (Na3AlF6) and chiolite (Na5Al3F14). Improvement and optimization of electrolyte properties is carried out through the addition of various fluoride salts: CaF2, MgF2, BaF2, KF, LiF [3, 4]. Some fluorides, such as lithium and potassium, can enter the bath with alumina [5, 6]. In the last decade, there has been an increase in the number of publications devoted to a detailed study of potassium-containing electrolyte [7-11]. For the efficiency of the electrolytic process, the composition of the multicomponent melt requires rather fine control. Practice shows that the most objective and accurate analysis of the electrolyte can be obtained on cooled samples [12]. It is possible that this is one of the most motivating factors for a thorough study of the subsolidus region of multicomponent diagrams of the type (AlF3-NaF-Me1F1 -Me2Fj ...).
Solid solutions are common in fluoride salt systems. Craig, D. F. and Brown, J. J. [13], describing the equilibria in the CaF2-AlF3-Na3AlF6 system, have indicated the presence of p-cryolite-based solid solutions along the Na3AlF6-NaCaAlF6 quasi-binary section. The homogeneity region of the solid solution increased with temperature. Later, these data were confirmed by considering the quasi-binary section CaF2-Na3AlF6 [14].
According to [15], there were formed two regions of solid solutions in the Na3AlF6-Li3AlF6 system in the temperature range 540-720 °C. They were both on the basis of Na3AlF6 and Li3AlF6. Below a temperature of 540 °C, they decomposed into two compounds of constant composition Na3AlF6 and LiNa2AlF6. The formation of a solid solution region based on Na3AlF6 was described in [16].
There was reported in the paper [17], that the binary section Na3AlF6-K3AlF6 was divided into two subsystems by elpasolite (K2NaAlF6) with a melting point of 954 °C. Above the line of polymorphic transformation of cryolite (542 °C), there were regions of solid solutions based on K2NaAlF6 and the high-temperature form of Na3AlF6. The polymorphic transition temperature decreased to 340 °C at the composition shifted towards K2NaAlF6. The mixture of two individual phases Na3AlF6 and K2NaAlF6 was observed at room temperature.
The study of the NaF-KF-AlF3 system in [18, 19] have shown small region of solid solutions based on Na5Al3Fi4. Potassium ions replaced sodium only in one of two crystallographic positions.
There were many studies, where only references or suspicions about the existence of solid solutions were formulated. Comprehensive studies have not been undertaken. The presence of solid solutions was often hidden by the temperature dependence and the width of the region of existence. As the result there were no detailed descriptions of the solid solutions compositions. At the same time, detailed information on the subsolidus area is important for solving applied problems. Data on the composition of phases, including phases with a variable composition, are used to provide X-ray diffraction analysis of the electrolyte composition during technological control [12]. Sampling for analysis is carried out directly from the operating electrolysis bath at a high temperature. The sample cooling mode is practically not controlled. It is obvious that the composition of the solid solution can vary.
This work presents the results of investigation of the solid solution formation in two local regions of Na5Al3F14-Na3AlF6-K3AlF6 system when LiF was added to the electrolyte. Research topics arose directly from the industrial practice of the electrolyte control by quantitative X-ray diffraction analysis. The first topic is related to the possibility of potassium and sodium cations in the elpasolite to be replaced with lithium when LiF have been added to the electrolyte. In the second topic, the effect of lithium fluoride on chiolite is discussed. Earlier it was found [18, 19] that chiolite forms solid solutions with the substitution of sodium for potassium. There was no information on the change in chiolite in
the presence of LiF. The studies were carried out on samples obtained from the initial fluorides under laboratory conditions. Determination of crystal-chemical details of the structure of solid solutions was carried out by the method of full-profile analysis (Rietveld method) on multiphase polycrystalline samples.
Experimental
Sample preparation. Initial substances NaF, AlF3, KF, LiF of reagent grade were used for the synthesis of samples. Before synthesis, all the starting materials were calcined in a furnace at a temperature of 400-450 °C for at least 1 hour. Stoichiometric weighed portions of NaF, AlF3, KF, LiF were ground in an agate mortar, placed in the closed platinum crucible in the furnace at appropriate temperatures and kept in the furnace for 30-50 min. before melting. Crystallization of the melts was carried out by pouring the melt from the crucible into a massive metal cooled mold. The weight of the samples was about 3 g. After complete cooling, the sample was removed and ground in an agate mortar. Then, to achieve an equilibrium state, each sample was thermally treated at different temperatures. The obtained materials were analyzed by X-Ray diffraction control at all stages of preparation. Weight loss during heat treatment was in the range of 1.3-1.8 wt.%.
X-ray diffraction (XRD). X-ray powder diffraction data were obtained using CuKa radiation on an X'Pert PRO diffractometer with a PIXcel detector equipped with a graphite monochromator (PANalytical). The samples were ground in an agate mortar and prepared by direct filling of the cuvette. Scans were carried out at room temperature in the range from 5 to 80 ° on the 20 scale, with a step of 0.026 °, At - 50 s/step.
X-ray diffraction data processing. The crystal structures were refined from powder diffraction data by full-profile analysis (Rietveld method) using the FulProf program [20] in the multiphase sample mode. The coordinates and the occupancies of atomic positions were refined. The thermal parameters of the atoms were refined in the isotropic approximation.
Results and discussion
1. Formation of solid solutions in the NaAF -
K2NaAlF6 system upon doping with LiF.
The presence of cryolite is the feature of electrolytes containing elpasolite after crystallization. This is consistent with the data obtained by of thermal analysis [17], where a continuous region of solid solutions were established above 500 °C. The solid solution decomposed into cryolite and elpasolite below the indicated temperature.
The observed transformations can be considered from a crystal-chemistry point of view. There two layers can be distinguished in the crystal structure of cryolite (Fig. 1a). The first is a mixed cation-anionic layer and the second is a cationic one. Sodium cations in the first anionic layer (position 2b) have a distorted octahedral environment with Na-F distances in the range of 2.227-2.282 A. Sodium atoms (position 4e) in the second cationic layer, have a tetrahedral environment with Na-F distances in the range of 2.29-2.35 A. From the point of view of the interatomic distances it seems unlikely to expect the substitution of potassium for sodium in the indicated positions.
There is a characteristic detail of the low-temperature cryolite structure. The axis of AlF6 octahedron is inclined to the vertical unit cell axis (c) at the angle is 19.7°. As the temperature rises, the
axes of the octahedron are oriented along the axes of the cell. Above 500 °C, the octahedron axis has a vertical direction. The transition to the high-temperature P-form is carried out in a continuous manner. The symmetry group of the P2j/n changes to Fm3m (Fig. 1b). The nearest environment of sodium in position 4a forms an octahedron with Na-F distances of 2.283 Â. The environment of the sodium 8c position from a tetrahedron is transformed into a cuboctahedron with Na-F distances of 2.85 Â. The resulting structure repeats the structure of elpasolite (Fig. 1c). The dimensions of the cationic cavities are prone to isomorphic substitution of sodium for potassium. It explains the formation of a continuous region of solid solutions.
The crystal structure of LiNa2AlF6 (Fig. 2d) repeats the structure of the low-temperature cryolite with a slight displacement of the second cationic layer, in which sodium is located. The similarity of the structures encourages the expectation of a possible mixing of LiNa2AlF6 with the structure of elpasolite at high temperatures.
In this work, attention was paid to the changes in the crystal structure of cryolite and elpasolite at high temperatures which could happened in a result of the subsequent addition of 5, 10, 15, 20, 25 wt.% LiF to the electrolyte sample. The study was carried out on a series of samples lying on the Na3AlF6 -KNaAlF6, annealed at 750 0C and at 450 0C.
The Figure 2 shows X-ray diffraction patterns of samples with the composition Na3AlF6 / K2NaAlF6 = = 60/40. After annealing at 750 °C, a cubic elpasolite phase with the unit cell parameter of 8.015 Â was dominated. The right-sided shoulders in diffraction lines (Fig.2) can be associated with the formation of a small amount of cryolite, as a result of the partial decomposition of the solid solution during the quenching of the sample. The X-ray diffraction pattern shows well-crystallized phases of elpasolite and cryolite after annealing the sample at 450 °C. The lattice parameter of elpasolite essentially increases to 8.086 Â. However it is slightly lower than that of the stoichiometric phase (8.118 Â). Refinement of the occupancy of the eight-fold cationic position leads to the composition 7.46K + 0.54Na.
After annealing at 750 °C elpasolite shown a tendency to increase the lattice parameter in the range of 7.990-8.098 Â upon transition in composition from Na3AlF6 to K2NaAlF6. Annealing at 450 °C gave the change in the lattice parameter in a narrow range of 8.0840-8.098 Â, which indicated the absence of a continuous region of solid solutions at this temperature. Potassium in the eight-fold cationic position was substituted for sodium by only 6 %, which was ~ 2 % in the structure as a whole. Repetition of this value for other samples indicated the limiting composition of the solid solution. The cryolite unit cell parameters were close to the parameters of a standard cryolite sample [21]. Refinement
ssl8
ssl8t2 ^AJL^J
20 30 40 50 60
Position [°2Theta] (Copper (Cu))
Fig. 2. X-ray diffraction patterns of the sample with composition Na3AlF6/K2NaAlF6 = 60/40 at 450 °C (lower) and 750 °C (upper)
of the cryolite structure did not reveal changes in the occupancy of cation positions for all samples. The observed insignificant changes in the unit cell parameters, within ± 0.02 A, should be attributed to a change in the rotation of the AlF6 octahedron. Above 500 °C, the cryolite structure turned out to be adapted to replace potassium ions in sodium positions. As a result the wide area of a solid solution was formed. At cooling, the solid solution area shrunk. Cryolite did not support the structure of elpasolite without significant substitution of sodium for potassium. Because cryolite structure was rearranged into a low-temperature form.
When LiF was added to elpasolite, there was observed a small regular shift of all lines on the X-ray powder diffraction pattern to the far region with some change in intensity. With an increase in the addition of LiF, small lines related to NaF appeared on the X-ray diffraction patterns. It was difficult to judge about the presence of LiF, since the positions of the LiF lines were overlapped by the elpasolite lines. In the considered limit of concentration changes, the appearance of new multicomponent phases does not occur. Refinement of the lattice parameters was carried out by the method of full-profile fitting of X-ray diffraction patterns (LeBail method). Table 1 and the graph in Fig. 3 show the refined lattice parameters.
Table 1. Lattice parameters of the main phase in the samples of the series K^NaAlFg + 0,5,10,15,20,25 wt.% LiF
LiF, wt.,% Unit cell parameter a, À
0 8.1179
5 8.1057
10 8.0934
15 8.0841
20 8.0794
25 8.0779
a(A)-C(%)
■Pflfll
0 5 10 15 20 25 30
C(+LiF,%)
Fig. 3. Graph of changes in the lattice parameter of elpasolite versus the concentration of LiF additive
The lattice parameter decreases with an increased with the amount of lithium fluoride. Two zones could be distinguished on the graph. The first zone with a LiF contented of up to 12-15 wt.% represented a linear decrease in the lattice parameter. The boundaries of the zone corresponded to the complete substitution of lithium for sodium: K2NaAlF6 ^ K2LiAlF6. The second zone was flat and indicated the completion of the replacement process. It can be concluded that elpasolite interacts with LiF with the substitution of lithium for sodium. The depth of substitution corresponds to one sodium atom, that is, no further substitution is observed.
Thus, the composition of a multiphase mixture with the participation of cryolite and elpasolite upon heating obeys the following regularities. At temperatures below 500 °C, cryolite does not change its composition. Above 500 °C, a second-order phase transition occurs in its crystal structure, which consists in a change in the orientation of the AlF6 octahedra along the c axis. The restructuring is completed by the high-temperature P-form of cryolite. At temperatures above 700 °C in the presence of KF or an elpasolite phase in the system, the process of dissolution or equalization of KF concentration occurs according to the diffusion mechanism. Ultimately, a continuous region of solid solution appears in the system based on the elpasolite structure. Upon rapid cooling, p-cryolite transforms into a-cryolite, the solid solution decomposes into a-cryolite and a solid solution based on elpasolite with an excess of sodium content of about 2 %. With the addition of LiF, sodium in the elpasolite structure is gradually replaced by lithium with the formation, ultimately, of the K2LiAlF6 phase with the lattice parameter a = 8.0779 Â. Dissolution of LiF in the structure of cryolite Na3AlF6 was not detected.
2. Interaction of chiolite with LiF
Chiolite Na5Al3F14 is the main component in cold sample of acidic electrolyte. It is likely that chiolite should be active with respect to the «basic» additives of the LiF type introduced into the electrolyte. Two variants of interaction of chiolite with LiF can be assumed. The first is the formation of a chiolite-based solid solution. The second is the interaction with LiF and the formation of new
Na5AI3F14_00LiF_1 J I. J .1
- Na5AI3F14_05LiF_1 lL A jJL.jJ
Na5AI3F14_10LiF_ llL 1 Liiji Jul L/JU
_ Na5AI3F14_15LiF_ . A . 1 L. Liai u I^aaÂAJI_tK—^_ivÀ^_*_
Na5AI3F14_20LiF_ ! 1 , JI_.. I. Wi_oIILAJJÂ.^vaaa^rv ^___
10 20 30 40 50 60 70 80
Position [ 2Theta] (Copper (Cu))
Fig. 4. X-ray powder diffraction patterns of the products of Na5Al3F14 + 5, 10, 15, 20 wt.% LiF interaction. Chiolite reacted with LiF to form new phases
phases. The formation of a solid solution based on chiolite was previously observed when part of sodium was replaced by potassium [18, 19]. Refinement of the crystal structure using X-ray powder diffraction data showed that the substitution of sodium for potassium occurs only in the 2-fold position of sodium by ~ 40 %. The limiting dissolution does not exceed 5 % (wt.) KF. The solid solution is stable in the range from melting temperature to room temperature.
To clarify the nature of the interaction, a series of samples of chiolite with LiF was synthesized. Figure 4 shows X-ray diffraction patterns of the synthesis products.
Samples of the series with an increasing amount of LiF have shown that chiolite did not form solid solutions. The interaction even with a small amount of LiF led to the appearance of the LiNa2AlF6 phase. It could be observed on the X-ray diffraction patterns that chiolite disappeared (maximum peak at 30.70o), a group of peaks (20-21o), corresponding to LiNa2AlF6 appeared. «Unreacted» chiolite retained its unit cell parameters. Further increasing LiF amount, resulted with LiNa2AlF6, Li3Na(AlF6)2 crystallization. When the amount of LiF is more than 20 % (wt.), LiNa2AlF6, Li3Na3(AlF6)2 and traces of Li3AlF6 were observed in the reaction products.
Conclusion
Thus, crystallization of K2NaAlF6 from melted electrolyte in the presence of LiF in the melt finishes with the formation of a solid solution of the composition K2Na1-xLixAlF6. The boundaries of the
region of solid solutions allow complete replacement of sodium by lithium. Potassium is not subjected to substitution. Na5Al3Fi4 during crystallization does not form solid solutions with LiF. LiNa2AlF6 initially crystallizes from acidic melts (KO < 2.3) in the presence of LiF. At a high concentration of LiF, along with LiNa2AlF6, Li3Na3(AlF6)2 and Li3AlF6 crystallize.
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