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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2017, 8 (6), P. 830-834
The solubility of sodium and potassium fluorides in strontium fluoride
P.P. Fedorov1'3, M.N. Mayakova1, V. A. Maslov1, A.E. Baranchikov2, V.K. Ivanov2, A. A. Pynenkov3, M.A. Uslamina3, K.N. Nishchev3
1Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991, Russia 2Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991, Russia 3Ogarev Mordovia State University, Saransk, 430005, Russia
DOI 10.17586/2220-8054-2017-8-6-830-834
The phase diagram of the NaF-SrF2 system was studied by thermal analysis and X-ray powder diffraction analysis with the determination of the chemical composition. The system was found to be of the eutectic type. The eutectic co-ordinates are 853 °C, 32 mol % SrF2. A narrow range of the existence of solid solution Sri—xNaxF2—x was established. The NaF solubility reaches a maximal value of x = 0.035 at eutectic temperature. The solubility of KF in SrF2 is very low.
Keywords: strontium fluoride, sodium fluoride, potassium fluoride, nanofluorides, NaF-SrF2 phase diagram, solid solution.
Received: 10 November 2017 Revised: 15 November 2017
1. Introduction
The phase diagram of the NaF-CaF2 system was studied in [1] by thermal and X-ray phase analysis with control of the chemical composition. A narrow region of existence of Ca1-xNaxF2-x solid solution was revealed. The temperature dependence of NaF solubility in the strontium fluoride has a quirky character: it reaches a maximum value x = 0.035 at 1200 ± 50 °C (the temperature that corresponds to the diffuse phase transition in fluorite); it reaches a local minimum (< 0.4 mol %) at 920 ± 25 °C, and again increases up to 2.2 ± 0.2 mol % at the eutectic temperature (818 °C).
Investigating the solubilities of sodium and potassium fluorides in strontium fluoride is of interest. Strontium fluoride is an important matrix for solid-state photonics, it is used in the form of melt grown single crystals [2-5], optical ceramics [6,7], and in nanotechnology processes [6-15]. In many synthetic processes, the ions of sodium and/or potassium are in the protocols [16]. Multicomponent ionic melts, including fluorides of strontium and alkali metals, are promising for use in metallurgy and nuclear power engineering [17].
Bukhalova and Berezhnaya studied the low-temperature parts of phase diagrams of the NaF-SrF2 [18] and KF-SrF2 [19] systems by method of visual polythermal analysis under the air. Systems belong to the eutectic type, the coordinates of the eutectics are 33 mol % SrF2, 856 °C for the NaF-SrF2 system and 22 mol % of SrF2, 744 °C for the KF-SrF2 system. Regions near that of strontium fluoride have not been investigated. Precision measurement of NaF liquidus temperatures in the NaF-SrF2 system was performed by Cantor [20]. Cryoscopic data [21] give no clear evidence for the formation of solid solution of sodium fluoride in strontium fluoride (as opposed to, for example, systems CaF2:Na and BaF2:K). According to Bollman et al. [22] single crystals of melt grown strontium fluoride can contain up to 1.8 mol % NaF, which leads to a drastic increase of ionic conductivity.
The aim of this work was to study the solubility of sodium and potassium fluorides in strontium fluoride, as well as clarifying the phase diagram of the NaF-SrF2 system. In order to identify the region of solid solutions based on SrF2, in addition to the classic techniques of research, we used the technique of nanotechnology: previous investigations have shown that under low temperature syntheses, as a rule, non-equilibrium phases of variable composition are formed, with the width of the homogeneity region of solid solutions based on components often correspond to the limiting width of the high-temperature solid solutions, or even exceed it [1,6,10,23-25].
2. Experimental
Sodium fluoride NaF (OS. h. 9-3); potassium fluoride bihydrate KF2-2H2O (analytical grade 9-3); strontium chloride SrCl2 (Alfa Aesar); strontium nitrate Sr(NO3)2 purity 99.99% (LANHIT company) were used as initial reagents for the synthesis as well as double distilled water.
Synthesis of samples was performed by coprecipitation from aqueous solutions at room temperature on the installation of periodic action. The main units of the set up for the synthesis are: the polypropylene reactor with a magnetic stirrer, and chemical feeder. The solutions of strontium nitrate (0.15 mol/l) and a fluorinating agent
The solubility of sodium and potassium fluorides in strontium fluoride
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(0.25 mol/l) in doubly distilled water was prepared initially. To obtain precipitate, the aqueous solution of strontium nitrate was added dropwise to a solution of a fluorinating agent (NaF or KF). The equation of synthesis reactions is the following:
(1 - x)Sr(NO3)2 + (2 - x)MF = Sri-XMxF2-* I +(2 - 2x)MNO3. (1)
Mixing of solutions was carried out under permanent stirring with a magnetic stirrer. A suspension formed as a result of chemical reactions in the mother solution. The resulting precipitate was defended, decanted, washed with doubly distilled water until negative reaction of diphenylamine on the nitrate ions and dried under air at a temperature of 40 - 50 0C.
To determine the limit of solubility for sodium and potassium fluorides in the SrF2 at elevated temperatures, we used the synthesis from chloride flux by the reaction
(1 - x)SrCl2 + (2 - x)MF = Sri-xMxF2-X I +(2 - 2x)MCl. (2)
The reagents were pre-dried, thoroughly mixed in agate mortar. Samples weight was 5.3 g. The mixture was loaded into platinum crucibles (10-20 cm3), and then were dipped in shaft furnace, heated to 900 - 1100 0C, kept for 10 minutes. The reaction product was washed with distilled water.
The morphology of the powders, prepared in accordance with reaction (2), is shown on Fig. 1. Habitus of powder particles suggests that the formation of microcrystals occurs by a non-classical crystal growth mechanism by oriented attachment of nanoparticles [26].
Fig. 1. Scanning electron microscopy image of a SrF2-based samples, obtained by chloride flux method: a - SrF2 :Na, 950 0C; b - SrF2 :K, 950 0C; c - SrF2 :K, 950 0C
Differential scanning calorimetry (DSC) was performed on thermoanalyzer NETZSCH DSC 404 F1 Pegasus. The measurements were carried out in platinum crucibles under constant argon flow (20 ml/min); the heating rate was 10 K/min, and the cooling rate was 30 K/min. We analyzed carefully ground mixture of components, with weight of ~ 30 mg. X-ray phase analysis was performed with the Bruker D8 Advanced diffractometer, CuKa radiation. Data processing was performed by software TOPAS. Electron microscope Carl Zeiss NVision 40 was used for carrying out scanning electron microscopy (SEM) and X-ray microanalysis (RSMA).
3. Results and discussion
Nanosized powders Sr1-xNaxF2-x and Sr1-xKxF2-x were precipitated by the reactions (1). X-ray powder diffraction showed that the samples were single phase and had a cubic (fluorite-type) structure. The lattice parameters were slightly different from the lattice parameter of pure SrF2 (5.800 A, JCPDS card # 06-0262). According to XRD, the size of coherent scattering regions D = 15 ± 1 nm (see Table 1). RSMA showed the presence in the samples of 3.1 mol % NaF and 1.2 mol % KF, respectively. Thus, wide areas of solid solutions on the basis of strontium fluoride in the systems NaF-SrF2 and KF-SrF2 are small.
After synthesis from a flux according to reaction (2), we obtained powders of fluorite structure with lattice parameters similar to those of SrF2 (Table 1). A small amount of sodium fluoride found only in the sample synthesized at 950 0C. At 1000 0C sodium fluoride is not included in appreciable quantities in the lattice of SrF2. Admixture of potassium fluoride was not detected at all.
The constructed phase diagram of the NaF-SrF2 system is shown in Fig. 2. Our data on this system matches well with previous studies of [18-20] in terms of the temperature of the eutectic equilibrium (our DSC measurements gave 853 0C), the curve of primary solidification (liqiudus) of the NaF, and the composition of the eutectic
P.P. Fedorov, M.N. Mayakova, V.A. Maslov, A.E. Baranchikov, V.K. Ivanov, et al. Table 1. Characterization of the synthesized samples
Composition Preparation method Lattice parameter, A MF content, mol % D, nm
SrF2 :NaF Coprecipitation 5.7991(3) 3.1 15± 1
SrF2 :NaF Flux synthesis, 950 0C 5.8001(5) 1.4 > 150
SrF2 :NaF Flux synthesis, 1000 0C 5.7995(9) Not found > 150
SrF2 :NaF Flux synthesis, 1050 0C 5.7988(4) Not found > 150
SrF2 :KF Coprecipitation 5.8031(2) 1.2 15± 1
SrF2 :KF Flux synthesis, 950 0C 5.8009(4) Not found > 150
SrF2 :KF Flux synthesis, 1000 0C 5.8001(5) Not found > 150
SrF2 :KF Flux synthesis, 1050 0C 5.8008(4) Not found > 150
Fig. 2. Phase diagram of the NaF-SrF2 system. 1 - data [18,19]; 2 - data [20]; 3 - our data, DSC heating curves; 4 - our data, DSC cooling curves; 5 - our data, chemical analysis. L -melt, a - solid solution on the basis of SrF2
point (32 mol % SrF2). As for the SrF2 liquidus curve, there is a difference. This is due to the fact that the study of Bukhalova and Berezhnaya were carried out under air, and the process of partial pyrohydrolysis resulted in a lowering the apparent of SrF2 melting temperature.
The thermal effect of a eutectic is observed on the thermograms of samples containing 7 and 5 mol NaF, but it is not fixed for the composition with 3 % NaF. For the last sample, the cooling curve after the second heating is fixed effect beginning with 760 °C, which can be interpreted as the decomposition of the solid solution, see Fig. 2. Solid solution decomposition during cooling of the system is a consequence of the third law of thermodynamics [27].
Thus, we can conclude that the solubility limit of NaF in the SrF2 is 3.5 ± 0.5 mol % at the eutectic temperature. The general formula of the solid solution can be written as Sr1-xNaxF2-x Then, the maximum solubility of sodium fluoride corresponds to x = 0.035.
The solubility of sodium and potassium fluorides in strontium fluoride
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In general, there was a negligible amount of sodium fluoride entry into the SrF2 lattice. The solid solution revealed was heterovalent, with a variable number of ions in the unit cell [28].
It is noteworthy that the chemical analysis showed the complete absence of KF in flux prepared samples of SrF2 phase (see Table 1).
The fluorite structure is loose; in it, cations occupy sites of an fcc cubic close-packed lattice, anions are located at the centers of tetrahedral voids, and octahedral voids remain vacant. In all the known cases of isomorphism in the fluorite structure, the cation sublattice remains fully occupied. Substitution of a trivalent cation for a divalent cation by the reaction:
M2+ <—> R3+ + F-, (3)
is accompanied by the introduction of additional fluorine ions to the existing octahedral voids in the lattice, which leads to the formation of clusters of defects and to stabilization of the lattice. Conversely, the formation of anion vacancies during substitution of sodium for strontium by the reaction:
Sr2+ + F- <—> M+ + VF, (4)
causes loosening of the lattice and is undesirable from the crystal chemical standpoint. Note that simultaneous introduction of both trivalent and monovalent ions to the fluorite lattice by the reaction:
2Sr2+ <—> R3+ + M+, (5)
which results in abrupt widening of the homogeneity ranges of fluorite solid solutions. This fact was revealed for the NaF-CaF2-RF3, NaF-PbF2-BiF3, NaF-BaF2-GdF3 systems [1]. For strontium fluoride, there are only data for solid solutions of the type NH4F-SrF2-RF3 [11,29].
The formation of solid solution must be considered in thermodynamic modeling of phase equilibria in the system NaF-SrF2 [17].
Acknowledgements
The authors express their appreciation V. V. Voronov for his kind assistance in the X-ray diffraction experiments. The authors also wish to thank E. V. Chernova for her help in the preparation of the present manuscript. This work was partially supported by RFBR 15-08-02481-a grant.
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