Materials Research Bulletin 49 (2014) 199–205

Preparation of barium monohydrofluoride BaF2�HF from nitrateaqueous solutions

Anna A. Luginina a, Alexander E. Baranchikov b, Arthur I. Popov a, Pavel P. Fedorov a,*a A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street, Moscow 119991, Russiab N. S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninsky Pr., Moscow 119991, Russia

A R T I C L E I N F O

Article history:

Received 17 June 2013

Received in revised form 8 August 2013

Accepted 31 August 2013

Available online 9 September 2013

Keywords:

A. Fluorides

A. Ceramics

A. Optical materials

B. Chemical synthesis

C. X-ray diffraction

A B S T R A C T

Barium ions react with HF in aqueous media, forming BaF2�HF for >0.7 wt.% aqueous HF instead of the

expected BaF2 precipitates. Optimal yields of BaF2�HF have been obtained with the use of concentrated

Ba(NO3)2 (0.8 ;) and HF (26.0 ;) aqueous solutions, using more than 2 equiv. of the latter. The

composition, morphology and particle size of the formed precipitates are dependent upon the synthetic

conditions, such as concentrations of starting materials, their stoichiometry and order of addition.

BaF2�HF is an excellent precursor for the preparation of optical fluoride ceramics having high

transparency in the 0.3–12 mm range, and scintillator ceramics (e.g., BaF2 or BaF2:Ce3+) as well.

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1. Introduction

Barium fluoride ceramics are some of the most important andcommon modern optical materials because of their uniqueproperties. For example, the luminescence of BaF2:Ce3+ ceramicsamples occurs with the higher quantum yield than for the singlecrystals of the same composition [1]. The syntheses of this andsimilar barium fluoride-containing specimens are based on theintrinsic features of the starting materials including BaF2. Ourpreliminary studies [2] have shown that BaF2�HF can be one of themost prospective precursors for the preparation of BaF2-basedmaterials. There are three known methods of BaF2�HF preparation:(1) reaction of BaF2 and aqueous HF; (2) reaction of BaF2 withanhydrous HF; and (3) thermal decomposition of BaF2�nHF ofhigher HF content than the target phase [3]. BaF2�HF syntheticprotocols using aqueous HF are preferable, as they are simple,efficient, inexpensive and more environmentally benign, especiallyif one starts with Ba(NO3)2, the least expensive barium salt. Thisvenue has its limitations, for according to the BaF2–HF–H2Osolubility isotherm [4], one can prepare only monohydrofluorideBaF2�HF by precipitation from aqueous solutions. Also despite thefact that BaF2�HF is known for more than 60 years [5,6], there is nodetailed study of chemical transformations in Ba2+–F�–NO3

�

aqueous system under acidic conditions described in literature to

* Corresponding author. Tel.: +7 4995038292; fax: +7 4991357744.

E-mail addresses: ppfedorov@yandex.ru, ppf@lst.gpi.ru (P.P. Fedorov).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.08.074

date. Therefore, the purpose of the present study was investigationof synthetic conditions and the properties of the aforementionedBaF2�HF as an important precursor for optical quality bariumfluoride.

2. Results and discussion

Considering results of our earlier experiments [2], we havechosen reaction with aqueous HF and varied the following fourparameters in our studies: (1) starting barium salt; (2) HF andBa(NO3)2 concentrations; (3) order of addition of the reactants (i.e.,what starting material will be in excess while the other is addedfrom dispenser); and (4) HF:Ba ratio.

In order to evaluate the nature of the starting materials, thepreparation of BaF2�HF has been carried out with different bariumsources, e.g., Ba(NO3)2, freshly precipitated BaF2 and BaCO3. Theaddition of a stoichiometric amount or 2 equiv. of aqueous HF to anaqueous BaCO3 suspension resulted in the expected formation ofBaF2 (Table 1; samples 1a and 2a). After drying under air at 30–35 8C, these samples are primarily BaF2 (a = 6.196 A; X-raydiffraction data; Fig. 1a) with HF content in them not exceeding1.5 wt.% (chemical analysis data).

The formed BaF2 particles were approximately the same size asthe starting BaCO3 size (Fig. 2a and 2b; samples BaCO3 and 1a).Further treatment of these cubic BaF2 micro-particles (samples 1aand 2a) produced by concentrated aqueous HF (26.0 M) resulted inmicro-sized monoclinic BaF2�HF powder with cell parametersa = 7.290, b = 4.302, c = 4.975 A, b = 115.488, coinciding with

Table 1Conditions of the samples preparation.

Sample

number

Starting materials concentration (M) or molar ratio Amount fluorinating reactant

in regard to BaF2 (equiv.)

HF content in matrix

solution (wt.%)

Ba(NO3)2 BaCO3: H2O HF NH4F

1a – 1:1 13.0 – 1.0 0.1

2a – 1:1 13.0 – 2.0 8.1

3a – 1:1 26.0 – 3.5 16.1

4a 0.2 – – 1.0 1.0 0

Fig. 1. X-Ray diffraction patters after drying at 308C: samples 1a (a), 1b (b), 3a (c), 4a

(d), 4b (e), and 5a (f), respectively.

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205200

literature data [7–9] (Table 2, samples 1b, 2b; Figs. 1b and 2c).Phase compositions of the dried samples 1b, 2b and 3a wereconfirmed by their IR spectra, which are in full agreement withprevious literature data [10] (Fig. 3a). During the single-stepsynthesis, BaF2�HF formed only when the BaCO3 suspension was

Fig. 2. Electron microscopy images of starting barium carbonate (a) and produc

treated with 3.5 equiv. of concentrated aqueous HF (26.0 M) andHF content in the matrix solution reached 16.1 wt.% level, whichcorresponds to BaF2–HF–H2O solubility isotherm (Table 2, sample3a containing 10.1 wt.% HF; Figs. 1c and 2d).

The addition of NH4F (instead of aqueous HF) to Ba(NO3)2

solution resulted in the precipitation of cubic 30–150 nm BaF2

particles with the same unit cell parameter a = 6.1961(2) A (Tables1 and 2; sample 4a; Figs. 1d, and 4a, b). When freshly prepared BaF2

was treated with excess concentrated HF (the excess of unreactedHF was quantified using BaF2–HF–H2O solubility isotherm data),the formation of monoclinic BaF2�HF occurred. Both chemical(Table 2; sample 4b) and X-ray diffraction (Fig. 1e) analysesconfirm this (a = 7.271, b = 4.289, c = 4.967 A, b = 115.468) [9].Treatment of BaF2 with HF resulted in a ca. 7-fold increase inparticle size and changed morphology from cubic to elongatedprismatic as compared to the starting material (Fig. 4c and d).

Ba(NO3)2 and HF aqueous solutions appear to be the mostconvenient starting materials for BaF2�HF preparation: ourexperiments have unequivocally shown that mixing aqueousBa(NO3)2 and HF resulted in the direct precipitation of BaF2�HF [9].No BaF2 precipitation was observed for >0.7 wt.% aqueous HF (seechemical analysis data in Tables 2 and 3, IR spectra in Fig. 3b, and X-ray diffraction data in Fig. 1f). All obtained BaF2�HF samples driedat 30–35 8C have the same monoclinic crystal lattice parameters(a = 7.306, b = 4.320, c = 4.993 A, b = 115.518). This is in direct

ts of its treatment with HF: samples 1a (b), 1b (c), and 3a (d), respectively.

Fig. 3. IR spectra of the samples dried at 308C: BaF2�HF samples 1b (a) and 5a (b), as

well as BaF2 sample 18a heated at 5008C (c).

Table 2BaF2�xHF samples composition after drying at 30–35 8C (chemical analysis data).

Sample

numbera

HF Content Sample

number

HF content

M, wt.% x, molar part M, wt.% x, molar part

1a 0 0 10a 10.0 0.4930

1b 10.3 0.5012 11a 9.5 0.4788

2a 1.5 0.1176 11b 10.3 0.5012

2b 10.3 0.5012 12a 10.0 0.4930

3a 10.1 0.4957 12b 10.3 0.5012

4a 0 0 13a 9.7 0.4845

4b 10.3 0.5012 13b 10.3 0.5012

5a 9.4 0.4759 14a 10.0 0.4930

5b 10.2 0.4985 14b 10.3 0.5012

6a 9.4 0.4759 15a 10.1 0.4957

6b 10.3 0.5012 15b 10.3 0.5012

7a 9.5 0.4788 16a 9.7 0.4845

7b 10.3 0.5012 16b 10.3 0.5012

8a 10.0 0.4930 17a 10.0 0.4930

8b 10.3 0.5012 17b 10.5 0.5066

9a 9.6 0.4817 18a 10.3 0.5012

9b 10.3 0.5012 18b 10.6 0.5092

a ‘‘b’’-Numbered samples were prepared via additional treatment of the formed

precipitates with excessive amount of concentrated HF.

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205 201

contradiction with the known BaF2-HF-H2O phase diagram [4],which suggests that BaF2�HF can only be formed under equilibriumif the HF concentration varies from 15.60 to 49.83 wt.%. Reactionsof HF solutions with BaF2, Ba(OH)2 and BaCO3 obey this rule, soprecipitation of BaF2�HF instead of BaF2 from lower concentrationHF media points at either a shift of equilibrium by the presence ofHNO3 or the non-equilibrium character of the observed chemicaltransformation in case of the reaction between aqueous Ba(NO3)2

and HF.We suggest that this phenomenon may be explained by the role

of the strong nitric acid salt – starting Ba(NO3)2 – that can formmixed anion derivatives. Unfortunately, we did not find directevidence for the presence of barium nitrate-hydrofluoride ion pairsin the studied aqueous systems, but similar effects of F–H� � �Ohydrogen bond formation between hydrofluoride ions and oxygen-containing anions have been described for nitrates, phosphatesand phosphites of potassium, cesium and ammonium [11].

Also we would like to mention that the presence of HNO3 by-product in the matrix solution did not allow determination of HFcontent by conventional titration. We could not employ thepotentiometric method using the fluoride-selective electrode, for itdetects both fluoride and hydrofluoride ions (i.e., F� and HF2

�)from dissolved barium fluoride and hydrofluoride. For comparison,the calculated values of HF content, if one simply dilutes the used

Table 3Conditions of the synthetic experiments BaF2�HF.

Sample

number

Concentration of the

starting solutions, M

Excess of fluorinating reactant in

regard to BaF2 stoichiometry

(equiv.)

Ba(NO3)2 HF

5a 0.2 3.0 0

6a 0.2 3.0 0

7a 0.3 3.0 0

8a 0.2 3.0 2

9a 0.3 3.0 0

10a 0.3 3.0 2

11a 0.2 26.0 0

12a 0.2 26.0 2

13a 0.3 26.0 0

14a 0.3 26.0 1

15a 0.3 26.0 2

16a 0.8 26.0 0

17a 0.8 26.0 1

18a 0.8 26.0 2

amount of HF up to the total volume of the starting solutions, aregiven in Table 3.

With regard to the influence of starting materials concentration,it is worth noting that the yield of BaF2�HF is strongly dependentupon the concentration of starting solutions and the stoichiometryof the excess of aqueous HF. The BaF2�HF yield was only 10% whendiluted solutions of starting components were used with 1 equiv.of HF (i.e., BaF2 stoichiometry). However, the yield increased to 60%when 3 equiv. of concentrated HF were used. Similarly, increasingthe Ba(NO3)2 concentration from 0.2 to 0.8 M increased the yield ofBaF2�HF up to 78% (in order to achieve 0.8 M Ba(NO3)2 concentra-tion, the experiment was performed at 80 8C).

Varying the concentration of the starting materials solutionsaffected the size of the formed BaF2�HF crystals; increasing the HFconcentration from 3 to 26 M and the Ba(NO3)2 concentration from0.2 to 0.8 M increased the BaF2�HF crystal lengths from 15 to100 mm (Fig. 5a and b); and prepared BaF2�HF samples containelongated prismatic particles. These same concentration changesincreased the product yield from 10% to 35% (stoichiometric ratios)and from 40% to78% (3 equiv. HF) (Table 3).

The order of reactant addition did not affect the yield (Table 3),however, it did alter the size of the precipitated BaF2�HF particles.Addition of aqueous HF to Ba(NO3)2 solutions resulted in BaF2�HF

Order of supplying the

reagents (1 – Ba(NO3)2 to

HF, 2 – HF to Ba(NO3)2

Calculated HF content

in the combined

matrix solution (wt.%)

Precipitate

yield (%)

1 0.7 11

2 0.7 10

1 1.0 12

2 1.8 25

2 1.0 12

2 2.3 40

1 0.9 12

1 2.3 40

2 1.3 14

1 3.1 41

2 3.4 60

2 3.0 35

2 6.1 67

2 8.2 78

Fig. 4. Electron microscopy images of the sample 4a after 308C drying (a) and 5008C heating (b) as well as sample 4b before (c) and after 5008C heating (d). Sample 4a was prepared

from Ba(NO3)2 and NH4F solutions. Sample 4b was synthesized with the use of additional treatment of the formed precipitate with excess of concentrated aqueous HF.

Fig. 5. Electron microscopy images of BaF2�HF powder obtained by mixing 3.0 M HF and 0.2 M Ba(NO3)2 (a), by mixing 26.0 M HF and 0.8 M Ba(NO3)2 (b), by adding 3.0 M HF to

0.2 M Ba(NO3)2 (c), and by adding 0.2 M Ba(NO3)2 to 3.0 M HF (d) in aqueous media.

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205202

crystals that were 4 times smaller than if Ba(NO3)2 solution wasadded to aqueous HF (Fig. 5c and d).

The addition of a stoichiometric amount of Ba(NO3)2 aqueoussolution to the bulk aqueous HF caused an immediate precipitation

of BaF2�HF (samples 6a and 9a), whereas reversing the order ofreactant addition, i.e., HF was added to aqueous Ba(NO3)2, resultedin delayed onset of precipitation for more than 20 min (samples 5aand 7a).

Fig. 6. Thermogravigramm of BaF2�HF sample 18a (sample weight – 1200 mg, heating rate – 10 K/min).

Fig. 7. Electron microscopy images of BaF2�HF particles synthesized by adding 0.2 M Ba(NO3)2 to 3.0 M HF (a) and by adding 26.0 M HF to 0.3 M Ba(NO3)2 (b). The same

samples (a, b) after the thermal treatment at 5008C (c, d), respectively.

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205 203

Further experimentation showed that the use of the loweramount of HF resulted in the lower HF content in the synthesizedBaF2�HF (Table 2, samples 5a, 6a, 9a, 11a, and 13a) when comparedto specimens synthesized using an excessive amount of HF(2+ equiv.) (Table 2, samples 8a, 10a, 15a and 18a). HFstoichiometry affects the yield of BaF2�HF similarly to theaforementioned influence of HF concentration; increasing theamount of HF used resulted in the higher yielded syntheses (e.g.,3 equiv. HF provided a yield of 78% BaF2�HF) (Table 3, sample 18a).

Rinsing formed BaF2�HF with water resulted in substoichio-metric levels of HF in these samples (10.26 wt.%, or 50.00 mol.%).Because solubility data for BaF2�HF are absent in the literature, wehad to perform supplementary experiments dissolving our

single-phase BaF2�HF samples in water at 20 8C for 20 h. BaF2�HFwas found to have a solubility of 0.424 g in 100 g water at thistemperature, but its dissolution had an incongruent character withunsolved BaF2 left as a precipitate (X-ray diffraction data;a = 6.1904(9) A).

Thermal decomposition of monoclinic elongated BaF2�HFcrystals proceeded in a one-step manner at 189–254 8C (endo-thermic effect maximum at 233 8C; 10.3 wt.% mass loss; Fig. 6) andproduced cubic BaF2 crystals.

Comparison of the starting BaF2�HF to the thermally formedBaF2 crystals clearly demonstrated a change from elongatedmonoclinic BaF2�HF prisms to barium fluoride crystals of variousshapes damaged by nano-cracks, that were easily visible,

Fig. 8. Optical ceramics samples prepared by hot-pressing of thermally generated BaF2 from starting BaF2�HF specimens synthesized by treating solid BaCO3 (a) and aqueous

Ba(NO3)2 (b) with 26.0 M HF.

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205204

especially under increased magnification (Fig. 7). BaF2 powderprepared under thermolytic conditions possessed highly devel-oped surface area with loose, incoherent and strained structure.This is a very chemically active powder for use in solid phasesynthesis, making it an attractive starting material for opticalceramics preparation [12].

IR spectra of dried BaF2�HF precipitates were fairly consistent,agreeing with literature data [10], containing typical absorptionbands of the HF2

¯ion at 1158 and 1183 cm�1 (doublet correspond-

ing to the deformation vibration) as well as at 1630–1780 cm�1

(valent asymmetric vibration), thus confirming the formation ofBaF2�HF.

IR spectra of the thermolytically synthesized BaF2 did not haveabsorption bands at 1640 and 3400 cm�1, which would indicatethe presence of water, thus showing that these samples wereanhydrous.

The latter fact is very important, as oxygen traces in bariumfluoride diminish its optical and scintillation properties. Tocircumvent this, heating must be done under fluorinatingatmosphere as the fluorides can be easily contaminated withoxygen traces because of their susceptibility to pyrohydrolysis (thereaction with water vapor at elevated temperature) [13]. The use ofbarium hydrofluoride as a fluoride ceramics precursor efficientlyaddressed the problem of synthesizing oxygen-free BaF2, for thethermal decomposition of BaF2�HF provided in situ generation ofan HF atmosphere, ridding BaF2 of oxygen traces.

Highly dispersed BaF2 powders obtained via the thermolysis ofBaF2�HF derived from different barium sources were evaluated asprecursors for the preparation of ceramics by hot-pressingmethods [14,15].

Intrinsic BaF2 and BaF2:Ce3+ ceramics samples derived fromBaCO3 that was sequentially converted to BaF2�HF then BaF2, hadgrayish coloration. The samples contained black inclusionsbecause of carbon contamination, thus scattering the light(Fig. 8a). Obviously, these ceramic samples were poor candidatesfor optical and/or laser applications.

In contrast, similar BaF2 and BaF2:Ce3+ ceramic specimens,prepared from Ba(NO3)2 solutions via BaF2�HF intermediate did notcontain any inclusions (BaF2:Ce3+ samples were synthesized fromCe3+-doped aqueous Ba(NO3)2). The obtained BaF2 ceramicsamples demonstrated excellent optical quality in the 0.3–12 mm region of the electromagnetic spectrum (their typicaltransmission coefficients were 86% at 0.3 mm, 92% at 0.4 mm, 93%at 3.0 mm, 94% at 7.0 mm, 94% at 10.0 mm, and 87% at 11.0 mm,respectively), lacking opalescence and inclusions (Fig. 8b). Thelevel of oxygen impurities in these specimens was evaluated bymethod [16]. It was found that oxygen content in the preparedceramics did not exceed 5 � 10�4 wt.% (maximum capacity offluorite type crystal lattice to accommodate oxygen atoms), for

otherwise the synthesized samples would contain the secondphase and exhibit much higher light scattering. This confirmedthat the obtained material was not contaminated with oxygenimpurities and showed that BaF2�HF, as a precursor, may play acrucial role in the manufacture of BaF2-based photonic materials,including both intrinsic and doped ceramics. Further investigationof the use of BaF2�HF as a precursor for barium fluoride ceramicsare ongoing and will be reported.

3. Experimental

We utilized 99.9 wt.% pure Ba(NO3)2, BaCO3, NH4F, and aqueousHF as well as double distilled water as starting materials. Allexperiments were carried out in a polypropylene reactor withpolypropylene stirring bar and polypropylene solution dispensersat room temperature unless otherwise specified.

Conditions of BaF2�HF thermolysis were studied thermo-gravimetrically with the use of Mettler derivatograph assembly(platinum crucibles; 10 8C/min). HF content in the specimenswas determined titrimetrically using 0.1 M aqueous KOH(phenolphthalein indicator; 6.2% and 4.3% accuracies for 0.5–5.0 wt.% and 5.0–30.0 wt.% intervals, respectively). Phase com-position of the samples was controlled by X-ray powderdiffraction (DRON-4M diffractometer, CuKa radiation, Bregg–Brentano design, continuous scanning with scintillation detec-tor, 28 2u per minute). Lattice parameters were calculated withthe use of POWDER 2.0 software. We used Infralum FT-08 FTIRspectrophotometer (400–3400 cm�1; KBr pellets) for infraredspectra recording, and Carl Zeiss NVision 40-38-50 electronmicroscope for scanning microscopic studies of the size andshape of the formed particles.

According to our standard protocol for the preparation ofBaF2�HF, 500 ml of 0.8 M aqueous Ba(NO3)2 was heated to 80 8Cand transferred to a polypropylene reactor placed in a preheatedwater bath (80 8C). Concentrated aqueous HF 87 ml (26.0 M) wasadded under continuous stirring. After complete HF addition, heatwas turned off, and the formed suspension was allowed to cool toambient temperature under the constant stirring over 3 h. Oncestirring was stopped, the formed precipitate was allowed to settle.The supernatant was decanted. The precipitate was washed with1.0 M aqueous HF (1 L � 3 times). The absence of nitrate ions in thewash was ascertained by the diphenylamine test. Finally, 45 ml26.0 M aqueous HF was added to the washed precipitate and thesuspension was stirred for an additional 3 h. After the secondfiltering, BaF2�HF was placed in a Teflon cup and dried under air at30–35 8C (100 W IKZ incandescence lamps). Thermal decomposi-tion of some BaF2�HF samples was carried out in platinumcrucibles at 500 8C for 1 h under dry air in the presence of in situgenerated HF. The sample in the platinum crucible was placed in an

A.A. Luginina et al. / Materials Research Bulletin 49 (2014) 199–205 205

oven at ambient temperature and then heated at 5 8C/min up to250 8C; then heating rate was increased up to 10 8C/min, and afterthe temperature reached 500–550 8C, it was kept at this level for anadditional 1 h period (78% yield).

4. Conclusions

Our study has shown that barium ions react with HF in aqueousmedia, forming BaF2�HF for >0.7 wt.% aqueous HF instead of theexpected BaF2 precipitates. Optimal yields of BaF2�HF wereobtained with the use of concentrated Ba(NO3)2 (0.8 ;) and HF(26.0 ;) aqueous solutions, using more than 2 equiv. of the latter.We have demonstrated how the composition, morphology andparticle size of the formed precipitates were dependent upon thesynthetic conditions, such as concentrations of starting materials,their stoichiometry and order of addition. Also, we have found thatBaF2�HF is an excellent precursor for the preparation of opticalfluoride ceramics having high transparency in the 0.3–12 mmrange, and scintillator ceramics (e.g., BaF2 or BaF2:Ce3+) as well.

Acknowledgements

Authors are very grateful to V.V. Voronov, A.E. Garibin, V.K.Ivanov, P.E. Gusev, and S.V. Kuznetsov for their help in theexperimental; E.V. Chernova and R. Simoneaux for their help in thepreparation of this manuscript. This work was supported by theFederal Targeted Program ‘‘Priority Directions of Research and

Development of Russian Science and Technology in 2007–2012’’(Project No. 16.523.11.305 from July 12, 2011 and Agreement No.8029 from July 11, 2012).

References

[1] 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.

[2] A.A. Luginina, P.P. Fedorov, S.V. Kouznetsov, M.N. Mayakova, V.V. Voronov, A.E.Baranchikov, Nanosyst.: Phys. Chem. Math. 3 (2012) 125–137.

[3] A.A. Opalovsky, T.D. Fedotova, Hydrofluorides, Nauka (Science), Novosibirsk,1973, 147 pp.

[4] D.D. Ikrami, N.S. Nikolaev, Zh. Neorg. Khim. 16 (1971) 804–808.[5] A.W. Jache, G.H. Cady, J. Phys. Chem. 56 (1952) 1106–1109.[6] A.F. Clifford, A.G. Morris, J. Inorg. Nucl. Chem. 5 (1957) 71–75.[7] H. Prevost, J. Moret, L. Anderson, O. Lindqvist, J. Fluor. Chem. 21 (1982) 31.[8] W. Massa, E. Herdtweck, Acta Cryst. C39 (1983) 509–512.[9] JCPDS Card No. 76-1301.

[10] D.D. Ikrami, E.D. Ruchkin, N.S. Nikolaev, Russ. J. Struct. Chem. 8 (1967) 354–356.[11] S.I. Troyanov, I.V. Morozov, E.M. Snigireva, C.D. Ling, E. Kemnitz, J. Solid State

Chem. 177 (2004) 3224–3228.[12] P.P. Fedorov, Russ. J. Inorg. Chem. 55 (2010) 1722–1739.[13] S.V. Kuznetzov, V.V. Osiko, E.A. Tkatchenko, P.P. Fedorov, Russ. Chem. Rev. 75

(2006) 1065–1082 (Uspekhi Khimii, 75 (2006) 1193–1211).[14] P.P. Fedorov, A.A. Luginina, S.V. Kuznetsov, V.V. Osiko, J. Fluor. Chem. 132 (2011)

1012–1039.[15] 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 Phys. 53 (2008)485–488.

[16] A. Apinov, Study of processes of formation of oxygen-containing impurities inalkaline earth fluoride crystals and their influence on the spectral transparency ofthe latter, 1984 (Ph.D. Thesis), Dushanbe.