Low-temperature phase formation in the BаF2-CeF3 system

Accepted Manuscript Title: Low-temperature phase formation in the BɑF2 -CeF3 system Author: M.N. Mayakova V.V. Voronov L.D. Iskhakova S.V. Kuznetsov P.P. Fedorov PII: DOI: Reference:

S0022-1139(16)30120-8 http://dx.doi.org/doi:10.1016/j.jfluchem.2016.05.008 FLUOR 8778

To appear in:

FLUOR

Received date: Revised date: Accepted date:

18-2-2016 28-4-2016 14-5-2016

Please cite this article as: M.N.Mayakova, V.V.Voronov, L.D.Iskhakova, S.V.Kuznetsov, P.P.Fedorov, Low-temperature phase formation in the BɑF2-CeF3 system, Journal of Fluorine Chemistry http://dx.doi.org/10.1016/j.jfluchem.2016.05.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Low-temperature phase formation in the BаF2-CeF3 system M.N. Mayakova1, V.V. Voronov1, L.D. Iskhakova2, S.V. Kuznetsov1, P.P. Fedorov1

1

A. M. Prokhorov General Physics Institute, Russian Academy of Sciences

38 Vavilov Street, Moscow, 119991, Russia 2

Fiber Optics Research Center, Russian Academy of Sciences

38 Vavilov Street, Moscow, 119333, Russia e-mail: [email protected]

1

Graphical Abstract

Phase composition in the BaF2-CeF3 system at high temperature area (upper panel, equilibrium state) and after low temperature synthesis (low panel, non-equilibrium state).

Highlights  BaF2-CeF3 system was studied by co-precipitation from aqueous nitrate solutions  Fluorite-type Ba1-xCexF2+x (х = 0.32-0.58) was precipitated by HF  Tysonite-type Ce1-yBayF3-y (y = 0-0.20) was precipitated by HF and NH4F  Composition of the precipitates depends on the order of mixing of the reagents

2

ABSTRACT Phase formation in the BaF2-CeF3 system has been studied in the aqueous nitrate media by co-precipitation technique with the use of HF and NH4F as fluorinating agents. Formation and properties of fluorite-type Ba1-xCexF2+x (х = 0.32-0.58) and tysonite-type Ce1-yBayF3-y (y = 0-0.20) solid solutions are described in the present paper.

Keywords: barium fluoride, cerium trifluoride, solid solutions, coprecipitation, tysonite, fluorite

3

INTRODUCTION Heterovalent isomorphism is very common for fluorite-type M1-xRxF2+x and tysonite-type R1-yMyF3-y solid solutions in the MF2-RF3 (M = Ca, Sr, Ba, Cd, Pb; R= La-Lu, Y, Sc) systems [1-3]. Such solid solutions, formed in the BаF2-CeF3 system [4], possess high fluoride-ion ionic conductivity [5-9]. Also BaF2:Ce3+ solid solution is a very efficient scintillator [10-12] (optimal Ce concentration is about 0.1 mol. %). In addition, fluorite-type Ba0.75Ce0.25F2.25 solid solution is a prospective optical material: it melts congruently and, therefore, it is relatively simple to grow its high quality single crystals from the melt [13]. Phase diagram of the BaF2-CeF3 system, published by Sobolev and Tkachenko [4] (Fig. 1a), indicates that, between 870oC and corresponding melting temperatures, the only solid solutions existing in it are formed by its components’ phases, BaF2 and CeF3, respectively. The limiting concentration of the fluoritetype Ba1-xCexF2+x solid solution (x = 0.52) does not change within the aforementioned temperature interval. “BaCeF5” pseudo-compound, described by X-ray powder diffraction technique in [14], is actually one of many possible compositions from the area of homogeneity of the said solid solution (Banks [14] described a composition close to the saturation limit of the aforementioned solid solution, but, nevertheless, “BaCeF5” phase is not a Daltonide-type compound of exact composition). In turn, tysonite-type Ce1-yBayF3-y solid solution has its area of homogeneity limit at about 18 mol. % (y = 0.18). Kieser and Greis [15] were successful in their preparation of Ba4Ce3F17 individual compound by long-time annealing at 1000-400oC. Such stoichiometry compounds with narrow areas of homogeneity (Ba 4 x R 3 x F17 x ) are known for numerous rare earth elements (R = Ce-Lu, Y) [1, 4, 15-16], and their structures represent trigonal type-ordered fluorite crystal lattice [17]. For example, the BaF2-NdF3 phase diagram [18] with lowtemperature data [19], obtained at 600oC, contains such Ba4R3F17 (R = Nd) phase (Fig. 1b). Crystal lattice ordering, which results in phase R with trigonal distortion fluorite-type structure (Fig. 1b), is accompanied by sharp decrease of the span of 4

the homogeneity area of BaF2-based solid solution. Perhaps, similar crystal lattice ordering phenomenon exists for the BaF2-CeF3 system, too; but it might take place at lower temperatures than the ones studied by Sobolev and Tkachenko [4]. Traditional syntheses of specimens in the MF2-RF3 systems are carried out at the elevated temperature; they include melt crystallization, solid phase sintering, hot pressing, etc. [20-23]. Such methods are energy consuming, and they also require pyrohydrolysis prevention.

CeF3 is one of the least stable rare earth

trifluorides from pyrohydrolysis point of view [24], and it also easily undergoes Ce(III)  Ce(IV) oxidation in the presence of oxygen. The latter factors require strict synthetic conditions including fluorinating atmosphere. Thus,

low-temperature

syntheses

of

MF2-RF3

materials,

such

as

mechanochemical techniques [25-26], sol-gel protocols [27], solvothermal preparations [28], and some other methods [29-30], appear to be more and more lucrative. This is why we have developed our own co-precipitation approach to the syntheses of corresponding nanofluorides from aqueous solutions [30-33]. Using this co-precipitation technique, we have successfully studied phase formation in the BaF2-YF3 [34], BaF2-BiF3 [35], BaF2-ScF3 [36], and SrF2-YF3 [37] systems at lower temperatures. Earlier, we have synthesized cubic Ba4Се3F17 phase having the same molar volume as trigonal Ba4Се3F17 [38].

We have also studied preparation of the

precursor for BaF2:Ce3+ scintillation optical ceramics [39-40].

In the latter

experiments, we have found that co-precipitation of barium and cerium(III) fluorides by aqueous HF resulted in formation of separate BaF 2·HF and nanoCeF3-rich phases, and Ba-Ce fluoride solid solution forms later, in the course of the sample thermal treatment [39-40]. Our present study is focused at the further investigation of the coprecipitation processes in the BaF2-CeF3 system with the higher cerium content. Such development of the synthetic techniques is very important for manufacturing the solid state fluoride ion batteries, especially the ones operating at the elevated temperatures [41-42]. 5

EXPERIMENTAL We have used 99.99 wt. % pure Ce(NO3)3•6H2O and Ba(NO3)2 (manufactured by LANHIT, Moscow, Russia), and 99.9 wt. % pure HF (manufactured by TEK System) and NH4F along with double distilled water as our starting materials. We used two methods of solution addition: direct method included dropwise addition of aqueous nitrates to the solutions of fluorinated agents, i.e., HF or NH4F; and “reversed” method included dropwise addition of the fluorinating compounds to the corresponding aqueous nitrate solutions. All experiments were carried out in the polypropylene reactors under stirring at ambient temperature.

Formed

precipitates were decanted, thoroughly washed with double distilled water, and dried at 40oC. We maintained in our experiments 0.2 mol/l concentrations for aqueous metal nitrates (Vwater= 127-132 ml), 5 vol. % HF concentration (VHF= 1320 ml, Vwater= 100-156 ml), and 0.6 mol. % NH4F concentration (Vwater= 100-150 ml), respectively (total volume of matrix solution = 240-300 ml). General equations for the precipitation reactions are as follows: (1-x)Ba(NO3)2 + xCe(NO3)3 + (2+x)HF = Ba1-xCexF2+x↓+(2+x)HNO3 (1) yBa(NO3)2 + (1-y)Ce(NO3)3 + (3-y)NH4F = Ce1-yBayF3-y↓+(3-y)NH4NO3 (2). We utilized 7-fold excess of HF and 5-fold excess of NH4F (compared to the stoichiometry of equations (1) and (2)) in our direct precipitation experiments, and 4-fold excess of HF in our reversed order experiments. Actual chemical composition of the samples was determined by X-ray energy dispersion spectroscopy (AZtecENERGY analytical system, Oxford Instruments; JSM-5910LV scanning electron microscope, JEOL, 20 kV accelerating voltage). Phase composition of the prepared specimens was studied by X-ray diffraction analysis (Bruker D8 diffractometer; CuKα radiation). The TOPAS 4.0.2 software package was used for treatment of experimental data, including refinement of crystal lattice parameters and determination of areas of coherent 6

scattering D. Error magnitudes presented in this paper were automatically calculated by TOPAS (e.g., 6.0653(2) means 6.0653±0.0002 Å, etc.). Thermal analysis studies were performed using a MOM Q-1000D (PaulikPaulik-Erdey, Hungary) derivatograph (Pt crucibles, air, 10 K/min heating rate, ca. 300 mg samples). RESULTS Addition of aqueous nitrates to HF solution (direct order) resulted in the formation of fluorite-type, tysonite-type and two-phase precipitates (Tables 1 - 3, Fig. 2). Single phase tysonite specimens were formed if Ba/(Ba+Ce) ratios for the starting solutions did not exceed 20% (0.20) (it is worth mentioning that the real metal ratios in tysonite precipitates were very close to the same ratios in starting nitrate solutions). Crystal lattice parameters of such Ce1-yBayF3-y tysonite samples (Fig. 3) are in a good agreement with the similar data for the sintered specimens [43]. However, real chemical compositions of fluorite-type precipitates sometimes were quite different from the originally taken Ba/(Ba+Ce) ratios. Therefore, we had to take this factor into account; and after considering such corrections for the real chemical compositions, we have found that crystal lattice parameters of such fluorite-type nano-precipitates were in a good agreement with known linear correlation between cubic lattice parameter а(х) and Ba/(Ba+Ce) ratio [4, 44] as well as with data [14]. Increase of cerium concentration in Ba1-xCexF2+x solid solution decreases the crystal lattice parameter according to the following correlation: а = 6.200 – 0.36 х, Å ,

(3)

where 6.200 Å is the crystal lattice parameter for intrinsic BaF2. Taking into account the borderline maximum (а = 6.0853(2) Å obtained for the Ce/(Ce+Ba) = 0.04 metal ratio in starting nitrate solutions) and minimum (а = 5.9912(4) Å; Ce/(Ce+Ba) = 0.6; two-phase specimen) values, we calculated the minimal (32 mol.% CeF3, x = 0.32) and maximal (58 mol.% CeF3, x = 0.58) concentrations of cerium fluoride in fluorite-type Ba1-xCexF2+x solid solutions. 7

When we replaced HF as precipitating agent by NH4F, no fluorite-type Ba1xCexF2+x

precipitates were obtained (Figs. 4-5). X-Ray diffraction patterns for the

samples with 0.01-1 mol. % Ce contained only lines of barium fluoride (а = 6.2012(1) Å). The sample with 10 mol. % Ce contained two phases, BaF2 (а = 6.2018(1) Å) and tysonite-type solid solution (а = 7.229(1), с = 7.355(2) Å). The further increase of cerium content in the starting solutions resulted in the increase of intensities of tysonite-type phase lines in comparison with the intensities of BaF2 lines in the corresponding X-ray diffraction patterns. Single phase tysonitetype precipitates were obtained only for the samples with 70 mol. % Ce starting solutions. Their real chemical composition was quite different from the starting metal ratio. Unusual results were obtained for BaF2-CeF3 precipitates when we used reversed order of reagent addition (dropwise addition of HF to starting aqueous nitrates) (Fig. 6).

When we used lower cerium concentrations, we obtained

tysonite-type single-phase precipitates instead of expected fluorite-type products. Moreover, lattice parameters of these tysonite-type precipitates corresponded to the maximum saturation of the corresponding solid solution, about 20 mol. % barium fluoride. Study of the thermal stabilities of fluorite-type F874, tysonite-type F880 and F893 specimens at 25-600oC showed that their heating was accompanied by insignificant (1.6, 3.6, and 3.9 wt%, respectively) weight losses.

The latter,

perhaps, was caused by the loss of adsorbed water that had been steadily desorbed up to 500oC. X-Ray diffraction patterns of the aforementioned samples after their heating (Fig. 7) confirmed that F874 specimen retained its fluorite-type structure with insignificant decrease of the lattice unit parameter from 6.0653(2) to 6.0629(1) Å (according to the equation (3), this corresponds to 38 mol% CeF 3 in Ba1-xCexF2+x solid solution. Narrowing of the lines in the aforementioned F874 sample X-ray diffraction pattern indicated a significant increase in the size of the coherent scattering domains. However, the heated F874 specimen did not exhibit any ordering of the said solid solution. In turn, tysonite-type samples F880 and 8

F893 underwent similar phase transformations with generation of the second fluorite-type phases (as per X-ray diffraction data). An exothermic effect for the F893 specimen observed at about 365oC perhaps corresponds to this transformation (solid solution decomposition with the second phase generation). In addition, X-ray diffraction patterns of heated F880 and F893 samples contain weak lines at 28.7, 33.1, and 47.5 2 degrees which we could not definitively identify. Fluorite-type phases in F880 and F893 specimens had 6.024(1) and 6.029(1) Å unit cell parameters, respectively (this corresponds to 48-49 mol% CeF3 in Ba1-xCexF2+x solid solution as per equation (3); see also Fig. 3). Tysonitetype phase parameters for F880 decreased after heating from a = 7.212(1), c = 7.374(1) Å to a = 7.184(3), c = 7.332(1) Å, respectively (this corresponds to a decrease of BaF2 content in the CeF3-based solid solution from 20 mol% to about 10 mol%, i.e., Ce0.9Ba0.1F2.9; see Fig. 3). However, the crystal lattice parameters for F893 sample changed insignificantly from a = 7.190(1); 7.330(1) Å to 7.1816(1); 7.330(1) Å, respectively; and its final composition was also about Ce0.9Ba0.1F2.9 (Fig. 3). DISCUSSION Comparison of our results with the corresponding phase diagram data (Fig. 1) allows to conclude that ambient temperature co-precipitation of Ba/Ce fluorides from aqueous solutions results in the formation of the high-temperature phases, Ce1-yBayF3-y tysonite- and Ba1-xCexF2+x fluorite-type solid solutions. Moreover, their maximal concentrations slightly exceed the corresponding limits for the hightemperature samples: 20 mol. % vs. 18 mol. % for BaF2, and 58 mol. % vs. 52 mol. % for CeF3, respectively. Such phases cannot be thermodynamically stable at ambient temperature. According to the existing data [19] and the third law of thermodynamics [18], only almost pure components and ordered Ba4Ce3F17 phase with minimal deviation from its stoichiometry should be thermodynamically stable at room temperature. However, such formation of non-equilibrium phases is very common phenomenon for nanotechnology [30, 45], and studied by us fluoride systems [34-37] with their non-equilibrium phases, prepared by co-precipitation 9

with HF (directed synthesis), also fall into the same category. The behavior of the BaF2-СеF3 samples with tysonite-type structure confirmed their non-equilibrium character. Our present results for the BaF2-СеF3 system studying correlate in the best possible manner with our earlier data for the low-temperature phase formation in the BaF2-BiF3 system [35], where we have observed formation of fluorite-type solid solution at 35-45 mol% BiF3 (the latter corresponds to the high-temperature trigonal Ba4Bi3F17-based phase). In the interval of 0-35 mol% BiF3, we have observed formation of the aforementioned fluorite-type phase along with practically pure BaF2, which is quite similar to the studied BaF2-СеF3 system. However, the difference between the BaF2-BiF3 and BaF2-СеF3 systems is that, in the former case, co-precipitation in the composition interval with the higher content of bismuth fluoride was accompanied by hydrolysis and BiOF oxyfluoride co-precipitation (and to hydrolyze voluminous particles of bismuth fluoride samples entirely, one had to heat them). The cubic phase in the BaF2-YF3 system [34] also formed at 35-45 mol% YF3 (this corresponds to Ba4Y3F17 phase) and, for the lower YF3 content, this phase was formed along with BaF2. The known BaY2F8 was never obtained by coprecipitation from aqueous solutions.

Instead, another fluorite-type phase of

various compositions were precipitated.

Yttrium fluoride precipitated as

hydroxonium-ion compound, (H3O)Y3F10. (It is worth mentioning that our results for the BaF2-YF3 system [34] have been independently confirmed and expanded later in [46]). In contrast, co-precipitation in the BaF2-ScF3 system [36] is accompanied by the formation of the Ba3Sc2F12 compound which is present in the corresponding phase diagram [36]. Discontinuity of the miscibility area for the Ba1-xCexF2+x solid solution at x<0.3 (as well as a similar effect observed in the BaF2 - YF3 [34] and BaF2 - BiF3 [35] systems) was caused by formation of BaF2•HF hydrofluoride [39]. In contrast with barium fluoride, BaF2•HF did not possess the usual isomorphic capacity for 10

rare earth cations. However, washing of the precipitated samples with water resulted in BaF2•HF decomposition and formation of pure BaF2 phase, which was observed in the corresponding X-ray diffraction patterns. SrF2 did not form the corresponding hydrofluoride and, as a result, a wide area of homogeneous Sr1-xYxF2+x (х < 0.6) solid-solution was formed as a result of co-precipitation from aqueous nitrate solutions by HF. This area of homogeneity overlaps concentration interval of existence for the Sr1-xYxF2+x solid solution as well as the intervals of existence for the series of ordered fluorite-type phases that are present in the SrF2-YF3 system phase diagram [37]. The fact that the single-phase cubic (Ba 4 x R 3 x F17 x ) specimens were synthesized by co-precipitation techniques for all rare earth elements [38], as well as for bismuth [35], have apparently shown their low solubilities in aqueous media. We did not observe crystal lattice ordering for Ba1-xCexF2+x fluorite-type solid solution with 40-50 mol. % Ce that would correspond to the formation of

Ba 4 x Ce3 x F17 x phase. This is in the agreement with data of Kuznetsov et al. [38] and Lei et al. results [28] (the latter authors synthesized cubic “BaCeF5” nanopowder by solvothermal technique with the use of oleic acid). Our results unequivocally indicate that order of reactant addition drastically change the phase composition of precipitates.

Perhaps, this was caused by

different types of oversaturation in the area of chemical reaction.

When HF

solution was added to the metal nitrate solution, indeed, there was a high HF concentration in the drop at the beginning. However, the HF drop immediately underwent dilution, and this resulted in an excess of metal cations. When metal nitrate was added to HF solution (direct order), there was always an extreme excess of HF. However, HF excess is always larger in the case of direct addition experiments rather than for the reversed order cases. It is known that the solubility of the substances strongly depends on the medium’s acidity.

For example,

according to Weaver and Purdy [47], the solubility of CeF3 decreases very gradually with a decrease in pH from 5 to 2 and then increases sharply from 2•10-5 mol/l to 10-3 mol/l as pH is further changes from 2 to 0. Taking into account BaF2 11

solubility product (SP) at 25°С (SP = 1.7·10-6 mol/l [48]), we have estimated BaF2 solubility in 3.5 M HF (which is approximately equal to the concentration of vol% HF for the direct order experiments) and the solubility of BaF2 in 0.2 M Ba(NO3)2 solution. The obtained values of 1.4·10-7 mol/l and 1.5·10-3 mol/l confirmed that BaF2 is much more soluble in 0.2 M aqueous Ba(NO3)2 than in 3.5 M HF solution. This estimate is quite approximate, for BaF2•HF has even lower solubility that BaF2 in concentrated HF and, thus, BaF2•HF was more stable under the aforementioned conditions. Nevertheless, it has become obvious that an increase in the solution’s acidity results in an inversion of the solubilities for BaF2 and CeF3: cerium fluoride has the higher solubility than barium fluoride in strongly acidic media, whereas BaF2 becomes more soluble than CeF3 under moderate pH values. Perhaps, this factor hindered the formation of the fluorite-type phase in the course of the reverse addition experiments (dropwise addition of HF to aqueous nitrates). Unfortunately, we do not have any solubility data for Ba1-xCexF2+x and Ce1-yBayF3-y solid solutions as well as for Ba 4 x R 3 x F17

x

phase to carry out similar

estimates or speculation. Difference between phase compositions of the specimens prepared with NH4F instead of HF may be attributed to the formation of ammonium-cerium fluorocomplexes in aqueous solution. Such complexes, including NH 4CeF4 [4950], are well-known and described in the literature. Also it is worth noting that our results are in a good agreement with [51], where authors synthesized Ce0.8Ba0.2F2.8 tysonite sample. Concluding our discussion, we would like to recommend direct HF coprecipitation technique for the preparation of fluoride-ion ionic conductors with addition of aqueous nitrates to HF solution. As per our results, such method allows very simple preparation of fluorite-type Ba1-xCexF2+x (x = 0.4-0.58) and tysonitetype Ce1-yBayF3-y (y ≤ 0.20) solid solutions and guarantees that actual composition of precipitates would not deviate too much from the metal ratios in the starting nitrate solutions.

12

CONCLUSIONS Fluorite-type Ba1-xCexF2+x (x = 0.32-0.58) and tysonite-type Ce1-yBayF3-y (y ≤ 0.20) solid solutions form in the ВaF2-CeF3 system. Crystal lattice parameters of such specimens, formed at ambient temperature, vary similarly to the same parameters obtained for the samples obtained by high-temperature solid-state syntheses. Chemical composition of the precipitates depends on the order of addition of the starting materials.

ACKNOWLEDGEMENTS Authors thank A. I. Popov and R. Simoneaux for their help in the preparation of this manuscript.

This work was partially supported by RFBR grant No. 15-08-02481 and by President of Russian Federation Stipend to M. N. Mayakova.

13

References. 1. B.P. Sobolev, The rare earth trifluorides. Part 1. The high temperature chemistry of the rare earth trifluorides, Barcelona: Institut d`Estudis Catalans, 2000. 2. P.P. Fedorov, Russ. J. Inorg. Chem. 45 (2000) S268-S291. 3. I.I. Buchinskaya, P.P. Fedorov, Russ. Chemical Rev. 73 (2004) 371-400. 4. B.P. Sobolev, N.L. Tkachenko, J. Less-common metals 85 (1982) 155-170. 5. P.P. Fedorov, T.M. Turkina, B.P. Sobolev, E. Mariani, M. Šhvantner, Solid State Ionics 6 (1982) 331-335. 6. A.K. Ivanov-Shits, N.I. Sorokin, P.P. Fedorov, B.P. Sobolev, Solid State Ionics 31 (1989) 269-280. 7. N.I. Sorokin, B.P. Sobolev, Russ. J. Electrochim. 43 (2007) 398-409. 8.

N.I. Sorokin, B.P. Sobolev, Phys. Sol. State 50 (2008) 416-421.

9. V. Trnovcova, L.S. Garashina, A. Skubla, P.P. Fedorov, R. Cička, E.A. Krivandina, B.P. Sobolev, Solid State Ionics 157 (2003) 195-201. 10. R. Visser, P. Dorenbos, C.W.E. van Eijk, A. Meijerink, G. Blasse, H.W. den Hartog, J. Phys. Condens. Matter 5 (1993) 1659-1680. 11. Kh.S. Batygov, L.S. Bolyasnikova, A.E. Garibin, V.A. Demidenko, M.E. Doroshenko, K.V. Dykelskii, A.A. Lugingina, V.V. Osiko, P.P. Fedorov, Doklady Physics 53 (2008) 485-488. 12. A.A. Demidenko, E.A. Garibin, S.D. Gain, Yu.I. Gusev, P.P. Fedorov, I.A. Mironov, S.B. Michrin, V.V. Osiko, P.A. Rodnyi, D.M. Seliverstov, A.N. Smirnov, Opt. Mater. 32 (2010) 1291-1293. 13. P.P. Fedorov, I.I. Buchinskaya, Russ. Chem. Rev. 81 (2012) 1-20. 14. E. Banks, JCPDS card # 43-0394 (1992). 15. M. Kieser, O. Greis, Z. anorg. allg. chem. 469 (1980) 164-171. 16. V. Grover, S.N. Achary, S.J. Patwe, A.K. Tyagi, Mat. Res. Bull. 38 (2003) 1101-1111.

14

17. B.A. Maksimov, Kh. Solans, A.P. Dudka, E.A. Genkina, M. Font-Badria, I.I. Buchinskaya, A.A. Loshmanov, A.M. Golubev, V.I. Simonov, M. Font-Altaba, B.P. Sobolev, Crystallography Reports 41 (1996) 50-57. 18. P.P. Fedorov, Russ. J. Inorg. Chem. 55 (2010) 1722-1739. 19. K.N. Zolotova, I.V. Kolbanev, E.I. Ardashnikova, A.M. Abakumov, V.A. Dolgikh, Russ. J. Inorg. Chem. 56 (2011) 1623-1631. 20. B.P. Sobolev, The rare earth trifluorides, Part 2. Barcelona: Institut d`Estudis Catalans, 2001. 21. C.E. Bamberger, Experimantal Techniques in Molten Fluoride Chemistry. In Advances in Molten Salt Chemistry, V.3, Ed. J. Braunsstein, G.Mamontov, G.P.Smith, Plenum Press: New York-London, 1975, p. 177-247. 22. P.P. Fedorov, V.V. Osiko, Crystal Growth of Fluorides. In: P. Capper. (Ed.), Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials. Wiley Series in Materials for Electronic and Optoelectronic Applications, John Wiley & Son, Ltd., 2005, p. 339-356. 23. P.P. Fedorov, Fluoride laser ceramics. In: Handbook on solid-state lasers: materials, systems and applications. Ed. by B. Denker and E. Shklovsky. Oxford Cambridge Philadelphia New Delhi, Woodhead Publishing Limited, UK, 2013, Is. 35, p. 82-109. 24. C.V. Banks, K.E. Burke, J.W. O’Laudhlin, Anal. Chim. Acta. 19 (1958) 239243. 25. A. Düvel, J. Bednarcik, V. Šepelák, P. Heitjans, J. Phys. Chem. C 118 (2014) 7117-7129. 26. G. Scholz, S. Breitfeld, T. Krahl, A. Düvel, P. Heitjans, E. Kemnitz, Solid State Sciences 50 (2015) 32–41. 27. Th. Krahl, G. Scholz, E. Kemnitz, J. Phys. Chem. C 118 (2014) 21066−21074. 28. Lei Lei, Daqin Chen, Feng Huang, Yunlong Yu, Yuansheng Wang, J. Alloys Comp. 540 (2012) 27-31. 29. S.V. Kuznetzov, V.V. Osiko, E.A. Tkatchenko, P.P. Fedorov, Russ. Chem. Rev. 75 (2006) 1065-1082. 15

30. Р.P. Fedorov, A.A. Luginina, S.V. Kuznetsov, V.V. Osiko, J. Fluorine Chem. 132 (2011) 1012-1039. 31. P.P. Fedorov, S.V. Kuznetsov, M.N. Mayakova, V.V. Voronov, R.P. Ermakov, A.E. Baranchikov, V.V. Osiko, Russ. J. Inorg. Chem. 56 (2011) 1525-1531. 32. A.A. Luginina, P.P. Fedorov, S.V. Kuznetsov, M.N. Mayakova, V.V. Osiko, V.K. Ivanov, A.E. Baranchikov, Inorg. Mater. 48 (2012) 531-538. 33. S.V. Kuznetsov, A.A. Ovsyannikova, E.A. Tupitsyna, D.S. Yasyrkina, V.V. Voronov, P.P. Fedorov, N.I. Batyrev, L.D. Iskhakova, V.V. Osiko, J. Fluorine Chem. 161 (2014) 95-101. 34. P.P. Fedorov, M.N. Mayakova, S.V. Kuznetsov, V.V. Voronov, R.P. Ermakov, K.S. Samarina, A.I. Popov, V.V. Osiko, Mat. Res. Bull. 47 (2012) 1794–1799. 35. P.P. Fedorov, M.N. Mayakova, S.V. Kuznetsov, V.V. Voronov, V.V. Osiko, R.P.

Ermakov,

I.V.

Gontar’,

A.A.

Timofeev,

L.D.

Iskhakova,

Nanotechnologies in Russia 6 (2011) 203–210. 36. M.N. Mayakova, S.V. Kuznetsov, V.V. Voronov, A.E. Baranchikov, V.K. Ivanov, P.P. Fedorov, Russ. J. Inorg. Chem. 59 (2014) 773-777. 37. M.N. Mayakova, A.A. Luginina, S.V. Kuznetsov, V.V. Voronov, R.P. Ermakov, A.E. Baranchikov, V.K. Ivanov, O.V. Karban’, P.P. Fedorov, Mendeleev Communications 24 (2014) 360-362. 38. S.V. Kuznetsov, P.P. Fedorov, V.V. Voronov, K.S. Samarina, R.P. Ermakov, V.V. Osiko, Russ. J. Inorg. Chem. 55 (2010) 484–493. 39. A.A. Luginina, A.E. Baranchikov, A.I. Popov, P.P. Fedorov, Mat. Res. Bull. 49 (2014) 199-205. 40. S.Kh. Batygov, M.N. Mayakova, S.V. Kuznetsov, P.P. Fedorov, Nanosystems: Physics, Chemistry, Mathematics 5 (2014) 752-756. 41. F. Gschwind, G. Rodriguez-Garcia, D.J.S. Sandbeck, A. Gross, M. Weil, M. Fichtner, N.Hormann, J. Fluorine Chem. 182 (2016) 76-90. 42. L. Zhang, M. Anji Reddy, M. Fichtner, Solid State Ionics 272 (2015) 39-44. 43. B.P. Sobolev, V.B. Aleksandrov, P.P. Fedorov, K.B. Seiranyan, N.L. Tkachenko, Sov. Phys. Crystallogr. 21 (1976) 49-54. 16

44. P.P. Fedorov, B.P. Sobolev, Sov. Phys. Crystallogr. 37 (1992) 651-656. 45. V.K. Ivanov, P.P. Fedorov, A.Y. Baranchikov, V.V. Osiko, Russ. Chem. Rev. 83 (2014) 1204–1222. 46. M. Karbowiak, J. Cichos, J. Alloys Comp. 673 (2016) 258-264. 47. J.L. Weaver, W.C. Purdy, Anal. Chim. Acta 20 (1959) 376-385. 48. S. Makishima, Z. Elektrochem. 41 (1939) 697-712. 49. E.G. Rakov, E.I. Mel'nichenko, Russ. Chem. Rev. 53 (1984) 851-869. 50. P.P. Fedorov, R.M. Zakaliukin, Russ. J. Inorg. Chem. 45 (2000) 1581-1585. 51. D. Chen, Y. Yu, F. Hang, Y. Wang, Chem. Commun. 47 (2011) 2601-2603.

17

Fig. 1. Phase diagrams of the BaF2-CeF3 [1, 4] (а) and BaF2-NdF3 [17] (b) systems. L is melt, F is fluorite-type Ba1-xRxF2+x solid solution, T is tysonite-type R1-yBayF3-y solid solution, and R is Ba4R3F17 –type phase. Dots represent DTA (differential thermal analysis) data; unfilled, or white, circles represent singlephase specimens as per X-ray diffraction analysis data, whereas half-filled circles correspond to two-phase annealed and quenched specimens (X-ray diffraction data).

18

Fig. 2. X-Ray diffraction patterns (CuKα radiation) of the specimens prepared in the BaF2-CeF3 system by direct precipitation with the use of HF as fluorinating agent (addition of aqueous nitrates to HF solution). Sample composition symbols correspond to the starting metal ratios in nitrate solutions.

19

Fig. 3. Crystal lattice parameters of the specimens synthesized in the BaF2-CeF3 system: data [4, 43-44] for the solid-state syntheses (1), uncorrected starting compositions (2), data after chemical composition corrections (3), data for twophase specimens (4). F field corresponds to the variable composition fluorite-type phase; T filed corresponds to the variable composition tysonite-type phase.

20

Fig. 4. X-Ray diffraction pattern for the tysonite-type Ce/(Ce+Ba) = 0.70 specimen precipitated with NH4F (trigonal lattice; а = 7.1891; c = 7.3301 А). Chart includes experimental data, calculated X-ray diffraction pattern and difference curve.

21

Fig. 5. X-Ray diffraction patterns (CuKα radiation) of the specimens prepared in the BaF2-CeF3 system by direct precipitation with the use of NH4F as fluorinating agent (addition of aqueous nitrates to NH4F solution). Sample composition symbols correspond to the starting metal ratios in nitrate solutions.

22

Fig. 6. X-Ray diffraction patterns (CuKα radiation) of the specimens prepared in the BaF2-CeF3 system by reversed precipitation with the use of HF as fluorinating agent (addition of HF to aqueous nitrates): starting Ce/(Ce+Ba) ratios equal to 0.2 (1), 0.3 (2), and 0.4 (3), respectively.

23

Fig. 7. X-Ray diffraction patterns (CuKα radiation) of specimens before (F874, F880, F893) and after (F874a, F880a, F893a) DTA.

24

Table 1. Phase composition, lattice parameters and domains of coherent scattering D for specimens prepared by precipitation with HF (series 1, direct order of addition of the reagents) Сe/(Ce+Ba)

Sample

Fluorite (F)

starting ratio

number

lattice

D,

lattice parameters

parameters

Nm

(а and c, Å)

Tysonite (T)

References D, nm

(а, Å) 0

-

6.200

[44]

0.04

F882

6.0853(2)

34±1

-

-

0.07

F883

6.0826(2)

31±1

-

-

0.1

F872

6.0787(2)

32±1

-

-

0.2

F874

6.0653(2)

30±1

-

-

0.3

F875

6.0501(3)

29±1

-

-

0.4

F876

6.0298(3)

31±1

-

-

0.5

F877

6.0128(2)

22±2

7.242(2); 7.369(3)

-

0.6

F878

5.9912(4)

23±2

7.2168(9); 7.371(2)

36±2

0.7

F879

5.988(2)

17±2

7.226(1); 7.387(2)

21±1

0.8

F880

-

-

7.212(1); 7.374(1)

21±11

0.9

F881

7.176(1); 7.341(1)

21±1

1.0

-

7.125; 7.292

[43 ]

25

Table 2. Phase composition, lattice parameters and domains of coherent scattering D for specimens prepared by precipitation with NH4F (series 2, direct order of addition of the reagents) Сe/(Ce+Ba)

Sample

Fluorite (F)

starting ratio

number

lattice

D,

lattice parameters

D,

parameters

nm

(а and c, Å)

nm

Tysonite (T)

(а, Å) 0.0001

F887

6.2012(1)

-

-

-

0.001

F884

6.2023(1)

-

-

-

0.01

F885

6.2016(1)

45±1

-

-

0.1

F886

6.2018(1)

44±1

7.229(1); 7.355(2)

17±2

0.2

F888

6.2024(1)

42±1

7.2193(8); 7.343(1)

23±2

0.3

F889

6.2006(2)

48±1

7.2223(6); 7.348(1)

25±2

0.4

F890

6.2023(1)

45±1

7.2102(4); 7.344(1)

27±2

0.5

F891

6.203(1)

38±2

7.220(1); 7.346(1)

17±1

0.6

F892

-

-

7.212(1); 7.343(1)

29±2

0.7

F893

-

-

7.190(1); 7.330(1)

29±2

26

Table 3. Chemical compositions, lattice parameters and domains of coherent scattering D for prepared specimens (series 3) Sample

Сe/(Ce+Ba) starting

Сe/(Ce+Ba)

Synthesis

number

ratio

analyzed ratio

method

Phase and

Lattice

D, nm

parameter,

fluorinating

Å

agent F 1014

0.20

0.368

Direct HF

fluorite

6.0631(1)

54±2

F 1015

0.30

0.380

Direct HF

fluorite

6.0579(1)

F 1016

0.40

0.434

Direct HF

fluorite

6.0437(1)

F 1017

0.20

0.375

Direct HF

fluorite

6.0636(1)

44±1

F 1018

0.30

0.391

Direct HF

fluorite

6.0580(2)

44±2

F 1019

0.40

0.423

Direct HF

fluorite

6.0480(2)

34±1

F 1020

0.20

no data

Reversed HF

tysonite

7.208(1);

29±2

7.375(1) F 1021

0.30

0.807

Reversed HF

tysonite

7.1974(3);

31±2

7.363(1) F 1022

0.40

0.836

Reversed HF

tysonite

7.205(1);

29±1

7.371(1) F 881

0.90

0.897

Direct HF

tysonite

7.1760(1); 7.3410(1)

27