Journal of Fluorine Chemistry 161 (2014) 95–101

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Journal of Fluorine Chemistry

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Phase formation in LaF3–NaGdF4, NaGdF4–NaLuF4, and NaLuF4–NaYF4

systems: Synthesis of powders by co-precipitation from aqueoussolutions

Sergei V. Kuznetsov a, Anna A. Ovsyannikova a, Ekaterina A. Tupitsyna a,Daria S. Yasyrkina a, Valery V. Voronov a, Nikolay I. Batyrev b, Liudmila D. Iskhakova c,Vyacheslav V. Osiko a, Pavel P. Fedorov a,*a A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street, Moscow 119991, Russiab M. V. Lomonosov Moscow State University of Fine Chemical Technologies Russia, 86 Vernadsky Prospect, Moscow 119571, Russiac Fiber Optics Research Center of Russian Academy of Sciences, 38 Vavilov Street, Moscow 119333, Russia

A R T I C L E I N F O

Article history:

Received 26 December 2013

Received in revised form 21 February 2014

Accepted 26 February 2014

Available online 6 March 2014

Keywords:

Rare earth fluorides

Sodium fluoride

Phase diagrams

Fluorite

Gagarinite

Tysonite

A B S T R A C T

Detailed studies of LaF3–NaGdF4, NaGdF4–NaLuF4, and NaLuF4–NaYF4 systems have revealed that LaF3–

NaGdF4 system precipitates, formed in aqueous media, contained gagarinite-type NaGd1�xLaxF4

(x � 0.0625) solid solution, tysonite-type La1�xGdxF3 (x � 0.50) phase, and cubic fluorite-type NaGdF4-

based phase, whereas NaGdF4–NaLuF4 precipitates contained hexagonal gagarinite-type (x � 0.25) and

cubic fluorite-type (x � 0.675) NaGd1�xLuxF4 solid solutions. Furthermore, there were continuous series

of single-phase cubic fluorite-type NaLuF4–NaYF4 solid solutions formed in the third investigated

system. Crystallization of Na0.5�xLu0.5+xF2+2x solid solutions from aqueous media occurred in an

incongruent manner; the use of 5-fold excess of NaF led to precipitation of NaLuF4 (x = 0) with almost

stoichiometric composition.

� 2014 Elsevier B.V. All rights reserved.

Introduction

Over the last 10–15 years, inorganic nanofluorides have activelyattracted the attention of scientists because of their quite wide andefficient use in modern optics, ceramics, catalysis, and medicine [1–16]. One of nanofluoride applications in the latter area is based ontheir up-conversion properties. Up-converters are necessary fortriggering photodynamically active agents capable of generatingreactive oxygen species in the vicinity of tumor cells under theinfluence of light of a particular wavelength. Most such photo-sensitizers require ca. 660 nm irradiation, but living tissues exhibitsufficient transparency for the 800–1000 nm region only, thusshielding targeted cancerous cells from radiation-initiated treat-ment [17]. The use of up-converting nanofluorides, such asNaYF4:Yb:R (R = Er, Tm, Ho), allows one to remedy this problem:970–980 nm light can easily reach deep tissue layers, where it isabsorbed by Yb3+ ions and, after transfer to another rare-earth

* Corresponding author. Tel.: +74995038292; fax: +74991357744.

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

http://dx.doi.org/10.1016/j.jfluchem.2014.02.011

0022-1139/� 2014 Elsevier B.V. All rights reserved.

cations, some part of absorbed radiation will be converted to thehigher frequency 660 nm radiation and trigger the delivery ofmedication. Whereas the aforementioned rare earth-doped NaYF4

matrix is one of the most efficient up-converter among knownnanofluorides, its efficiency depends on the selected rare earthdopants and phase composition: hexagonal NaYF4:Yb:R is severaltimes more efficient than its cubic polymorph [10]. Also preparationof hexagonal NaYF4:Yb:R (R = Er, Tm, Ho) nanofluorides byprecipitation from aqueous solutions is not so simple. It requiresa thorough choice for the concentration of the organic catalysts,solution pH, time and order of component addition [18–20]. Veryfrequently, metastable cubic NaYF4:Yb:R phase precipitates insteadof its hexagonal polymorph. One of the possibilities to bypass theseobstacles is the replacement of yttrium by different rare earthelements, such as La, Gd and Lu. The advantage of the latter metals isthat they lack absorption bands and luminescence lines in theaforementioned 800–1000 nm range of spectrum, but at the sametime multicomponent fluoride systems with these dopants have yetto be systematically described in the literature.

It is also worth noting that despite known ability of rare earthfluorides to form metastable phases, especially, in aqueous

[(Fig._1)TD$FIG]

Fig. 1. Phase diagrams of NaF–LaF3 (a); NaF–GdF3 (b); NaF–YF3 (c) and NaF–LuF3 (d)

systems [21–25]. L—melt; G—hexagonal gagarinite-type phase, Na3xR2�xF6; T—

hexagonal tysonite-type phase, R1�JNayF3�2y; F—cubic fluorite-type phase,

Na0.5�xR0.5+xF2+2x; a-hexagonal a-YF3-type phase; b-orthorhombic b-YF3-type

phase; g-hexagonal KErF4-type phase.

Fig. 2. Typical X-ray diffraction pattern of a NaLuF4 sample (synthesized with a

5-fold excess of NaF).

S.V. Kuznetsov et al. / Journal of Fluorine Chemistry 161 (2014) 95–10196

systems [11], information about their thermodynamically stablephases is crucial for the selection of conditions and possibilities forthe preparation of various fluoride materials of the aforemen-tioned elements. For example, the high-temperature phaseequilibria in the NaF-RF3 (R = La, Gd, Y, Lu) systems [21–25](Fig. 1) indicate that, in the NaF–LaF3 system, the mixture ofcomponents – binary fluorides – is stable at relatively lowtemperature, i.e., below 330 8C. Similarly, hexagonal gagarinite-type G phases with the compositions close to NaRF4 are stable inthe NaF-GdF3, NaF-YF3 and NaF-LuF3 systems. Additionally, in thelatter system, the fluorite-like compound Na7Lu13F46 is stable, too[21].

Therefore, the goal of this paper was to investigate phaseformation in LaF3–NaGdF4, NaGdF4–NaLuF4, NaLuF4–NaYF4 sys-tems by co-precipitation of polycrystalline powders from aqueoussolutions.

Experimental

We utilized co-precipitation of nanofluorides from aqueoussolutions, used in this work, that has been described in details in[19,20,25–27]. We used commercially available 99.99 wt% pureY(NO3)3�6H2O, Gd(NO3)3�6H2O, Lu(NO3)3�6H2O, La(NO3)3�6H2O(Lanhit, Moscow, Russia), 99 wt% pure NaF, and doubly distilledwater as starting materials. Reagents were not subjected to furtherpurification. All experiments were carried out in polypropyleneequipment (such as lid-covered reactors) at ambient temperatureunder air unless otherwise specified. Prepared 0.30–0.35 M

aqueous solutions of rare earth nitrates (individual or mixturesof rare earth elements) were added dropwise under vigorousstirring to the appropriate amount of 0.30–0.35 M aqueous NaF(stirring continued for 2 h). The obtained precipitates weredecanted or centrifuged (when necessary), washed several timeswith doubly distilled water (the absence of nitrate ion impuritieswas determined by standard qualitative reaction with diphenyl-amine) and dried under air.

The phase composition of solid specimens was evaluated by X-ray diffraction (DRON-4 M diffractometer; Cu Ka radiation;graphite monochromator). Calculations of lattice parameters wereperformed using Powder 2.0 software, the error less than 10 takenDQ, where DQ = 104/d2

calc � 104/d2theory. We also used a JSM-

5910LV (JEOL) scanning electron microscope for microstructurestudy of the obtained precipitates and the same device for thesample X-ray microanalysis (energy dispersive X-ray-EDX).

Results and discussion

As has been found earlier for NaF-RF3 systems [11,25], fluorite-type solid solutions crystallize/precipitate from aqueous solutionsin an incongruent manner, i.e., Na:R ratios (R = rare earth) in thesolid phase differs from the one in the aqueous solution. Therefore,the initial step in our study included the determination ofconditions when precipitate composition (Na:R ratio) would bethe closest to the stoichiometric NaRF4 (i.e., 1:1).

Therefore, we chose NaF-LuF3 system as the typical one amongother NaF-RF3 systems and studied interaction of 0.3 M NaF withLu(NO3)3 at Na:Lu = 1:1, 3:1, 5:1, 7.5:1, 10:1 molar ratios. Mixingthe aforementioned solutions resulted in precipitation of single-phase face-centered cubic Na0.5�xLu0.5+xF2+2x powders only (Figs. 2and 3 and Table 1). The chemical composition of obtained Na0.5-

xLu0.5+xF2+2x solid solution was evaluated with the use of data onconcentration dependence of cell parameters a(x) = 5.4308 +0.2318x [28]. The presented data indicate a non-linear correlationbetween the lattice parameter a and the amount of NaF used in thesynthesis (Fig. 3). It is worth noting that the use of 5 eq. NaF led tothe precipitation of Na1.006Lu0.994F3.988, i.e., almost stoichiometric‘‘NaLuF4’’ (sample F483; Figs. 2 and 3 and Table 1). Similar effectswere observed for the other rare earth precipitates, so furtherprecipitation experiments were carried out at 5 eq. NaF excess withthe same 0.3 M solutions.

Phase equilibria in the triple NaF–LuF3-solvent water–salt(s)system are depicted in Fig. 4, which shows an incongruentcrystallization of Na0.5�xLu0.5+xF2+2x various composition phase.[(Fig._2)TD$FIG]

[(Fig._3)TD$FIG]

Fig. 3. Na0.5�xLu0.5+xF2+2x solid solution lattice parameters vs. NaF excess. —

experimental data; black graph is approximation curve (a = 5.428 + 0.0114 �exp(�t/3.16), t—NaF excess).

[(Fig._4)TD$FIG]

Fig. 4. Metastable phase equilibria in the triple NaF–LuF3-solvent water–salt(s)

system with incongruently soluble variable composition Na0.5�xLu0.5+xF2+2x phase.

Bold dashed line is a solubility curve, and semi-bold rays are co-nodes joining

figurative points that correspond to the compositions of coexisting phases.

Table 1The lattice parameters for Na0.5�xLu0.5+xF2+2x samples.

Sample Excess

value NaF

Cubic lattice

parameter a, A

x Solid solution

compound

F524 1 5.436 (2) 0.021 Na0.479Lu0.521F2.042

F700 3 5.432(1) 0.005 Na0.495Lu0.505F2.010

F483 5 5.430 (2) �0.003 Na0.503Lu0.497F1.994

F701 7.5 5.429(1) �0.007 Na0.507Lu0.493F1.986

F525 10 5.428 (2) �0.011 Na0.511Lu0.489F1.978

[(Fig._5)TD$FIG]

Fig. 5. X-ray diffraction patterns of NaGd0.75Lu0.25F4 (a) and NaGd0.325Lu0.675F4 (b)

samples.

S.V. Kuznetsov et al. / Journal of Fluorine Chemistry 161 (2014) 95–101 97

Area of this phase homogeneity is marked according to the high-temperature data (Fig. 1d). Co-nodes link solid phase figurativepoints (for their chemical composition, please, see Table 1) andpoints of intersections of solubility curves by rays, that correspondto the assigned Na:Lu oversaturation. It is worth noting that thesolubility of the equilibrium hexagonal gagarinite-type phaseshould be lower than the solubility of metastable fluorite-typephase.

System NaGdF4–NaLuF4

Studies of precipitation from 0.3 M Gd- and Lu-containingnitrate aqueous solutions with 5 eq. NaF revealed that the solidproducts of chemical reaction were either hexagonal NaGdF4-based (Fig. 5a) or cubic NaLuF4-based solid solutions (Fig. 5b) ormixtures of both (Table 2). X-Ray diffraction patterns of thesamples, prepared for NaGdF4–NaLuF4 system, are sown in Fig. 6.

Table 2The lattice parameters for samples in the NaGdF4–NaLuF4 system.

Sample NaLuF4(mol%) Composition Th

a (

F476 0 NaGdF4 6.0

F482 25 NaGd0.75Lu0.25F4 6.0

F699 28.2 NaGd0.718Lu0.282F4 5.9

F583 31.3 NaGd0.687Lu0.313F4 5.9

F536 37.5 NaGd0.625Lu0.375F4 5.9

F480 50 NaGd0.5Lu0.5F4 5.9

F535 62.5 NaGd0.375Lu0.625F4 5.9

F645 65 NaGd0.35Lu0.65F4 No

F692 67.5 NaGd0.325Lu0.675F4 –

F479 75 NaGd0.25Lu0.75F4 –

F483 100 NaLuF4 –

Compositions of the single-phase samples F482 (hexagonalNa0.82Gd0.86Lu0.32F4.36) and F692 (cubic Na0.84Gd0.38Lu0.78F4.32) havebeen confirmed by X-ray microanalysis (EDX; Table 3). Typicalscanning microscopy data for single-phase cubic NaGd0.25Lu0.75F4

powder (lattice parameter a = 5.464(2) A) are presented in Fig. 7:

e hexagonal phase The cubic phase

A) F (A) a (A)

36(1) 3.590(1) –

04(2) 3.554(2) –

97(2) 3.548(5) 5.540(5)

95(2) 3.537(2) 5.543(3)

83(4) 3.526(3) 5.525(4)

52(2) 3.508(4) 5.502(2)

29(5) 3.530(9) 5.485(2)

t enough peaks for indexing 5.489(1)

– 5.478(1)

– 5.464(2)

– 5.430(2)

[(Fig._8)TD$FIG]

Fig. 8. Lattice parameters of NaGdF4–NaLuF4 samples vs. Lu(NO3)3 content in the

initial nitrate solutions. Hexagonal NaGdF4-based G phase: —parameter a, —

parameter c. Cubic NaLuF4-based F phase: —parameter a. [TD$INLINE] —area of two-phase

sample compositions.

[(Fig._6)TD$FIG]

Fig. 6. Typical X-ray diffraction patterns of the samples in the NaGdF4–NaLuF4

system at different Gd:Lu ratio: (a) 100% NaLuF4–0% NaGdF4, (b) 75% NaLuF4–25%

NaGdF4, (c) 65% NaLuF4–35% NaGdF4, (d) 50% NaLuF4–50% NaGdF4, (e) 37.5%

NaLuF4–62.5% NaGdF4, (f) 25% NaLuF4–75% NaGdF4, (g) 0% NaLuF4–100% NaGdF4.

Table 4Lattice parameters for samples in the NaYF4-NaLuF4 system.

Sample NaYF4 (mol%) Composition Cubic lattice

parameters a (A)

F483 0 NaLuF4 5.430(2)

F526 25 NaY0.25Lu0.75F4 5.4497(7)

F531 50 NaY0.5Lu0.5F4 5.466(1)

F580 75 NaY0.75Lu0.25F4 5.482(1)

F581 100 NaYF4 5.4933(8)

Table 3The results of X-ray microanalysis for samples in the NaGdF4–NaLuF4 system.

Sample The Gd:Lu ratio

in original nitrate

solution

The composition of the

solid solution, adjusted

according to the EDX

Oxygen content

in the sample

(wt%)

F482 0.75:0.25 Na0.82Gd0.86Lu0.32F4.36 2.52

F692 0.325:0.675 Na0.84Gd0.38Lu0.78F4.32 1.37

S.V. Kuznetsov et al. / Journal of Fluorine Chemistry 161 (2014) 95–10198

primary 10 nm nanoparticles form spherical 100–200 nm agglom-erates. Image, obtained by diffraction of reflected electrons (EBSD)(Fig. 7b), has confirmed chemical homogeneity of the preparednanopowder.[(Fig._7)TD$FIG]

Fig. 7. Typical SEM image of NaGd0.25Lu0.75F4 sample (total composition).

Results of phase composition studies of precipitates in NaGdF4–NaLuF4 system are presented in Fig. 8 as function of Gd:Lu ratio.

Sample F645 (NaGd0.35Lu0.65F4.00) corresponded to the borderbetween one- and two-phase areas. Hexagonal NaGdF4-based solidsolutions could incorporate up to 25.0 mol% NaLuF4, and cubicNaLuF4-based solid solutions could incorporate up to 32.5 mol%NaGdF4.

Variations of unit cell parameters for both phases in the area of2-phase compositions show that the prepared samples weremetastable and not under equilibrium (Fig. 8).[(Fig._9)TD$FIG]

Fig. 9. Lattice parameters of NaYF4–NaLuF4 samples vs. Y and Lu content in the

original nitrate solutions.—experimental data; black line—the approximation curve.

Table 6The results of X-ray microanalysis for samples in the NaGdF4–LaF3 system.

Sample

code

The La:Gd ratio in

the original nitrate

solution

The composition of

the solid solution

adjusted according

to the EDX data

Oxygen content

in the sample

(wt%)

F662 0.0625:0.9375 Na0.88Gd1.04La0.06F4.18 2.08

F411 0.50:0.50 Na0.20Gd0.90La0.90F5.60 –

F195 1.0:0.0 Na0.18La1.82F5.64 –

Table 5The lattice parameters for samples in the LaF3–NaGdF4 system.

Sample La (mol%) Composition LaxGd1�xF3 solid solution NaGd1�xLaxF4

cubic phase

NaGd1�xLaxF4 hexagonal

phase

a (A) F (A) a (A) a (A) F (A)

F489 0 – – – 6.018(3) 3.581(2)

F567A 3 – – – 6.033(3) 3.583(3)

F662 6.25 NaGd0.9375La0.0625F4 – – – 6.041(1) 3.595(2)

F705 7.5 NaGd0.925Lu0.075F4 Not enough peaks

for indexing

6.021(4) 3.588(5)

F688 9.4 NaGd0.906La0.094F4 6.909(2) 7.082(3) – 6.041(2) 3.606(2)

F625 12.5 NaGd0.875La0.125F4 6.926(2) 7.109(3) – 6.063(2) 3.611(6)

F614 15.5 NaGd0.845La0.155F4 6.913(4) 7.123(2) – 6.054(3) 3.610(2)

F659 18.75 NaGd0.8125La0.1875F4 6.910(3) 7.108(4) – 6.040(2) 3.611(4)

F613 21.5 NaGd0.785La0.215F4 6.934(2) 7.133(5) 5.590(2) 6.042(2) 3.627(4)

F687 25.0 NaGd0.75La0.25F4 6.941(2) 7.142(4) 5.616(9) 6.064(3) 3.628(2)

F459 37.5 6.980(2) 7.206(4) 5.646(1) – –

F486 45.0 7.009(3) 7.183(6) 5.659(2) – –

F570 47.5 7.016(4) 7.200(7) 5.660(1)

F411 50.0 7.023(2) 7.208(6) – – –

F410 75.0 7.072(2) 7.299(2) – – –

F195 100.0 7.160(3) 7.390(1) – – –

S.V. Kuznetsov et al. / Journal of Fluorine Chemistry 161 (2014) 95–101 99

The system NaLuF4–NaYF4

Precipitation from 0.3 M Y–Lu nitrate solutions with 5-fold NaFexcess resulted in the formation of continuous series of single-phase fluorite-type cubic solid solutions with almost linearcorrelation between their lattice parameters and Y/(Y + Lu) ratio,or Y mol% (Table 4, Fig. 9): a = 5.432 + 0.000636x with R2 = 0.9957(x—yttrium content relative to the total content of yttrium and

[(Fig._10)TD$FIG]

Fig. 10. Lattice parameters LaF3–NaGdF4 samples vs. La and Gd content in the

original nitrate solutions. Hexagonal tysonite-type LaF3—based solid solutions T:—

parameter a,—parameter c. Cubic fluorite-type NaGdF4-based solid solutions, F:—

parameter a. Hexagonal gagarinite-type NaGdF4-based solid solutions, G:—

parameter a,—parameter c. [TD$INLINE] —three-phase sample area. Two-phase sample

areas: [TD$INLINE] —G and T phases, [TD$INLINE] —T and F phases.

lutetium in the initial solution, expressed in mole fraction).However, Fig. 1c diagram [22] indicates that cubic fluorite-typeNaYF4 was a high-temperature phase, which was unstable atambient temperature–temperature of our experiments. It isknown [20] that heating of cubic NaYF4 leads to its exothermaltransformation to the hexagonal polymorph. Thus, the aforemen-tioned observation unequivocally indicates a non-equilibriumcharacter for the synthesized specimens.

The system LaF3–NaGdF4

Results of precipitation experiments for LaF3–NaGdF4 systemare presented in Tables 5 and 6 as well as in Figs. 10 and 11. Thesedata indicate that there were 3 phases existing in this system:hexagonal tysonite-type LaF3-based phase (La1�xGdxF3 x < 0.525),hexagonal gagarinite-type NaGdF4-based phase (NaGd1�xLuxF4

with x < 0.075), and cubic fluorite-type NaGdF4-based phase(please see Fig. 11 for the typical X-ray diffraction patterns of thesynthesized samples).

Compositions of the borderline specimens (gagarinite sampleF662 Na0.88Gd1.04La0.06F4.18 and tysonite sample F411 Na0.20G-d0.90La0.90F5.60, Table 6) were confirmed by EDX.

Typical scanning electron microscopy data for the single-phasehexagonal NaGd0.9375La0.0625F4 sample (a = 6.035(3), F = 3.598(3) A)are presented in Fig. 12, where one can easily see elongatednanoparticles with 100 nm length and ca. 30–40 nm diameter. Our[(Fig._11)TD$FIG]

Fig. 11. Typical X-ray diffraction patterns of the samples in the LaF3–NaGdF4 system

at different La:Gd ratios: (a) 7.5% LaF3–92.5% NaGdF4, (b) 9.4% LaF3–90.6% NaGdF4,

(c) 15.5% LaF3–84.5% NaGdF4, (d) 18.75% LaF3–81.25% NaGdF4, (e) 21.5% LaF3–78.5%

NaGdF4, (f) 25% LaF3–75% NaGdF4.

[(Fig._12)TD$FIG]

Fig. 12. Typical SEM image of NaGd0.9375La0.0625F4 sample (total composition).

[(Fig._13)TD$FIG]

Fig. 13. Relative stability of different phases under NaRF4 precipitation conditions.

F—cubic fluorite-type phase, Na0.5�xR0.5+xF2+2x; G—hexagonal gagarinite-type

phase, Na3xR2�xF6; T—LaF3-based hexagonal phase; g-NaLuF4 middle-

temperature polymorph with hexagonal KErF4-type structure.

S.V. Kuznetsov et al. / Journal of Fluorine Chemistry 161 (2014) 95–101100

results also demonstrated that there was a wide range (up to50 mol%) for the existence of single-phase tysonite-type LaF3-basedsolid solutions and a narrow range (up to 6.25 mol%) for theexistence of hexagonal gagarinite-type NaGdF4-based solid solu-tions. There were several multi-phase transitional areas (includingones with cubic fluorite-type NaGdF4 structure) between thesesingle-phase areas. Similar to Fig. 8, changes in lattice parametersof both phases in the area of 2-phase compositions indicatemetastable (non-equilibrium) character for the obtained precipi-tates (Fig. 10).

Conclusions

Interpreting our above-presented results, we took into accountthe current information about existing phases in NaF-RF3 systemsand their transformations under equilibrium conditions. NaF–LaF3,NaF–GdF3, NaF–YF3, NaF–LuF3 as well as some other NaF–MF3

phase diagrams were studied at the higher temperature (400–1200 8C) by Thoma et al. [29] and Fedorov et al. [21–25] with theuse of precise differential thermal analysis (DTA) and X-raydiffraction (Fig. 1). Comparison of our experimental data withaforementioned results unequivocally indicated that the precipi-tated cubic fluorite-type solid solutions, prepared in this work,were unstable from a thermodynamics point of view (only co-precipitated hexagonal NaGdF4-based phases were stable):according to the NaF–YF3 phase diagram (Fig. 1c), NaYF4 cubicphase is stable at the high temperature only, and it is unstable atthe ambient temperature of our syntheses.

In general, hexagonal phases in NaF–RF3 series are more stablethan cubic phases at lower temperatures (Fig. 1). However, anincrease of the R3+ ionic radius in Lu–Y–Gd–La row leads to thedecrease of stability of both cubic and hexagonal phase [21]. Therelative stability scheme for different phases under NaRF4

precipitation conditions is shown in Fig. 13. Using the latterscheme, it is worth remembering that according to Ostwald’s steprule [30], the least stable (metastable) polymorph crystallizes first,and – later – it can undergo transformation to more stablepolymorph (whether it will happen or not—this is a separatequestion).

In addition, the changes of the lattice parameters in the two-phase region also pointed to the non-equilibrium nature of thesynthesized samples. It is necessary to keep in mind this kind ofphase metastability when considering the possible application ofthese polycrystalline materials.

Given the nature of incongruent crystallization of solidsolutions Na0.5�xR0.5+xF2+2x by coprecipitation from aqueoussolutions, the optimal ratio of NaF to Lu(NO3)3 in the startingsolution of 5:1 has been determined. Using this ratio, we havesynthesized the closest to the stoichiometry solid solution,

Na0.503Lu0.497F1.994. This ratio should be and will be used infurther detailed studies.

Our study of phase formation in the system NaGdF4–NaLuF4

indicated the existence of single-phase hexagonal NaGdF4-basedsolid solutions (contained up to 25.0 mol% NaLuF4) and single-phase cubic NaLuF4-based solid solutions (contained up to32.5 mol% NaGdF4). According to the results of EDX, the composi-tions of the extreme points of the single-phase regions have beenfound to be Na0.82Gd0.86Lu0.32F4.36 for the hexagonal phase andNa0.84Gd0.38Lu0.78F4.32 for the cubic phase, respectively.

We found that, in the continuous series of solid solutions basedon the fluorite-type cubic phase in the NaYF4–NaLuF4 system, thelattice parameters of this cubic phase increased, as expected, with anincrease in the Y:Lu ratio in the starting nitrate solutions and couldbe approximated by a linear function a0 = 5.432 + 0.000636� x

with R2 = 0.9957 (x—yttrium content relative to the total content ofyttrium and lutetium in the initial solution, expressed in molefraction, i.e., x = Y/Y + Lu).

The three phases were found to exist in NaGdF4–LaF3 system:LaF3-based tysonite-type phase, NaGdF4-based gagarinite-typephase and NaGdF4-based fluorite-type phase. The areas ofexistence for single-phase solid solutions were refined: NaGd1�x-

LuxF4 (gagarinite) x < 0.075 mol fraction, La1�xGdxF3 (tysonite)x < 0.525 fraction. The single-phase compositions of samples at theboundary points have been clarified by X-ray microanalysis: forthe gagarinite phase, Na0.88Gd1.04La0.06F4.18, and for tysonite phase,Na0.20Gd0.90La0.90F5.60. The field of compositions consisting of theabove three phases is present in the phase diagram of the NaF–GdF3–LaF3 system.

Acknowledgements

This work was supported by the grant of the Russian FederationProgram ‘‘Scientific and scientific-pedagogical personnel of inno-vative Russia’’ (State contract N 14.740.12.1343), RFBR grants 12-02-00851-a and 13-02-12162 ofi_m., grant Russian President foryoung PhD scientist MK-3133.2014.2. Authors are very grateful toA. Baranchikov for SEM studies, A. I. Popov, R. Simoneaux and E.Chernova for their help in the manuscript preparation.

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