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Synthesis of CaF2-YF3 nanopowders by co-precipitation from aqueos solutions
P.P. Fedorov1, M.N. Mayakova1, S. V. Kuznetsov1, V. V. Voronov1, Yu.A. Ermakova1, A.E. Baranchikov2
XA. M. Prokhorov General Physics Institute, Russian Academy of Sciences 38 Vavilov Street, Moscow, 119991, Russia 2N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991, Russia [email protected]
DOI 10.17586/2220-8054-2017-8-4-462-470
Study of the CaF2-YF3 system by co-precipitation from aqueous nitrate solutions revealed the formation of Cai—xYxF2+x solid solution precipitate containing up to 20 mol. % yttrium fluoride (x < 0.2). A higher yttrium to calcium ratio in the starting solutions caused additional precipitation of orthorhombic ,3-YF3 nanophase elongated along the (b) axis. Cubic (H3O)Y3Fio phase was also formed (SSG Fm3m, a = 11.60 A, KY3F10 structural type).
Keywords: calcium fluoride, yttrium fluoride, nanopowders.
Received: 19 July 2017 Revised: 2 August 2017
1. Introduction
The CaF2-YF3 system, along with the NaF-YF3 system [1-4], plays a particularly important role among binary fluoride systems. Solid solution of yttrium fluoride in calcium fluoride is a classic example of heterovalent isomorphism [5]. Its study was initially discussed by Vogt in treatises on yttrofluorite [6], and has continued for more than a hundred years [7-24] (for a more detailed history of this study, please see [18]): the said CaF2-YF3 system has become a model for describing the interaction of calcium fluoride with the rare earth fluorides from yttrium group of elements (see Fig. 1). The CaF2-YF3 system is also the basis for several natural fluoride minerals [6,15,25-27].
Yttrium cation substitutes calcium ions in the fluorite structure, and supplementary fluoride anions, penetrating the formed crystal lattice, compensate for the corresponding changes in electrostatic charges for the sake of electrical neutrality of the system. The formed cationic and anionic defects associate among themselves, thus forming defect clusters [28,29]. Ca1-xYxF2+x solid solution maintains its original fluorite-type structure within the 0 < x < 0.38 interval limits. A smooth maximum in the melting curves of Ca1-xYxF2+x at x = 0.11 (Fig. 1) allows the growth of high-quality Ca1-xYxF2+x single crystals from its melts with x < 0.15.
Such synthetic Ca1-xYxF2+x yttrofluorite crystals have become widely used photonics materials, including solid state laser matrices [13]. Also, introducing yttrium fluoride into the calcium fluoride crystal lattice causes dramatic changes in its physical properties, including fluoride-ion ionic conductivity, hardness, cleavage and heat conductivity (the latter two parameters decrease significantly) [23, 30]. Relatively high yttrium concentrations complicate Ca1-xYxF2+x single crystal growth from the melts due its incongruent melting, and the formation of a cellular substructure [31,32], and local ordering of the formed solid solution [7,12]. Additional increase in the YF3 content in the CaF2-YF3 system leads to the formation of another berthollide-type variable-composition solid solution at 65-75 mol. % YF3 with hexagonal LaF3 tysonite-type structure [10,11]. This phase undergoes metastable ordering under cooling. Another solid solution, based on high-temperature a-YF3 polymorph, is also formed in the CaF2-YF3 system [33].
Fedorov [21] reported the lower temperature part of the phase diagram of the CaF2-YF3 system, taking into account the results for Kuntz's [15] hydrothermal studies and Bergstol et al. [25] investigation of tveitite mineral formation under natural conditions (tveitite is an ordered fluorite-type phase), and considering that fluorite-type solid solutions undergo ordering with the formation of a series of fluorite-type phases when cooled [4,12]. The latter fluorite-type phases contain Y6F37 clusters in their crystal lattices with Thompson antiprism coordination yttrium polyhedra. Such clusters fit in naturally in the fluorite crystal lattice (Fig. 2) and appear to be the dominant type of structural defects in Ca1-xYxF2+x solid solutions at higher yttrium concentrations [14,16,28,29]. The heterovalent substitution mechanism for the formation of the aforementioned solid solutions (Fig. 2) can be described by the following equation:
(M6F32)20- ^ (YeF37)19- + F-nt. (1)
T, K
1400
1000
600
L
< y F+Z\ —■ ■ 'J'"*'^
\ \ \ \ a-F F+T 1', . T+a \j
T+p YF^
1 \ 1 ----:— w F l (3YF3 ----1
-1— 0-Fi1 i i
X 1
r
a
CaF2 20 40 60 80 YF3 mol.% YF,
Fig. 1. Phase diagram of the CaF2-YF3 system [17]. L - melt, F -Ca1-xYxF2+x fluorite-type solid solution, T - tysonite (LaF3 ) type berthollide phase
Fig. 2. Insertion of R6F37 clusters into the fluorite matrix
Nanofluorides are another rapidly developing area of the modern science [34-39], for nanofluorides are widely implemented as luminophores, catalysts, biomedical and electrochemical materials; the CaF2-YF3 system has also become crucially important in this area, as well. Low-temperature and soft chemistry syntheses of nanofluorides (e.g, mechanochemical [40,41], sol-gel [24], solvothermal [42] methods and some other techniques [35]) are especially prominent because of their technological advantages. Recently, we have successfully used our co-precipitation from aqueous solutions methods for nanofluoride preparations [34,35,43-48], including our systematic studies of phase formations in the BaF2-YF3 [49], BaF2-BiF3 [50], BaF2-ScF3 [51], BaF2-CeF3 [52], SrF2-YF3 [53] and CaF2-HoF3 [54] systems. We have observed varieties of phase fields in the studied MF2-YF3 and NaF-RF3 systems, including non-equilibrium phases with wide areas of homogeneity; we have also observed the absence of the ordered phases that exist under higher temperature equilibrium conditions [4].
Thus, according to the background given above, the purpose of the present study was the investigation of nanophase formation in the CaF2-YF3 system under co-precipitation from aqueous solutions at lower temperatures.
2. Experimental
We used 99.99 wt.% pure Y(NO3)3•6H2O and Ca(NO3M^O (manufactured by OOO Lanchit), as well as 99.9 % pure 40 wt.% aqueous HF (manufactured by TECH System) and double distilled water as starting materials without any further purification.
Specimens in the CaF2-YF3 system were prepared by co-precipitation from aqueous solutions in polypropylene reactors according to previously-described procedures [4,49,53,54]. 0.2 Mol/L aqueous nitrate solutions in double distilled water were vigorously mixed with magnetic stirring bar and then added dropwise under continued stirring to a 2-fold excess of 5 vol.% aqueous HF. The formed precipitates were decanted, rinsed with double distilled water until a pH of 5-6 was obtained. In some experiments, precipitates were additionally neutralized with aqueous ammonia (99.9 % pure) and then rinsed again with double distilled water until a pH of 5-6 was maintained. All precipitates were air-dried at 40 0 C.
Phase composition of the synthesized samples was characterized by X-ray powder diffraction (Bruker D8 diffractometer; CuKa radiation; TOPAS software package for experimental data treatment and coherent scattering domain and microdeformation size calculations). Particle dispersity and morphology were controlled by scanning electron microscopy (SEM) (NVision 40 microscope). The same NVision 40 microscope was also used for the sample chemical analyses (X-ray spectroscopy). Specimen chemical composition was also studied by atomic emission spectroscopy (AES) with the use of LEA-S500 analyzer (OOO SOL Instruments, Minsk, Belarus) (see Supporting Information for the further details). MOM Q-1500D PaulikPaulikErdey derivatograph has been utilized for the thermal analysis investigations (Pt crucibles, air).
3. Results and Discussion
Colloid solutions in the CaF2-YF3 system were obtained during the synthesis in which the precipitate formed very slowly (couple of weeks) (Figs. 3-7; Tables 1, 2). SEM data (Fig. 3) confirmed that the precipitated nanoparticles were actually of the small sizes. Chemical analyses of both types, X-ray spectroscopy and AES (Table 1, also see Supplemental Information), have shown that the metal ratios in the formed solid precipitates were close to the corresponding ratios in the starting aqueous solutions/mixtures even if the observed ratio differences were a little bit larger than in the case of the previously studied SrF2-YF3 [53] and BaF2-YF3 [49] systems.
X-Ray diffraction data indicated that precipitates formed from solutions with 20 mol. % YF3 or less were CaF2-based fluorite-type solid solutions (cubic system, Fm3m SSG). SEM image of the 10 mol. % YF3 specimen (i.e., precipitated from the 10 % Y3+ and 90 % Ca2+ solution) contained readily-visible/resolved agglomerates of the same phase particles 30-50 nm in diameter. Experimental data for the unit lattice parameters of precipitated Ca1-xYxF2+x solid solutions coincided within the 0.004 A range with the a(x) concentration dependency function for Ca1-xYxF2+x solid solutions, synthesized at higher temperatures [55] (Fig. 7). However, in addition to the increasing crystal lattice parameter, X-ray diffraction patterns of precipitates Ca1-xYxF2+x contained weak (200) lines at about 32.5 0 2в (this line is absent in the X-ray diffraction pattern of the pure face-centered CaF2). The latter observation was an additional evidence of the solid solution formation.
X-Ray diffraction patterns of Ca1-xYxF2+x samples with 30 mol. % or more YF3 contained broadened lines of e-YF3 nanoparticles (orthorhombic system, Pnma SSG [18]) (Figs. 4-5). However, relative clarity of (020) e-YF3 line at about 26 0 26 indicated that crystal lattices of the said ,0-YF3 nanoparticles were stretched along (Ъ) axis.
All precipitated fluorides were hydrated and contained about 5.5 ± 0.3 wt. % water (DTA data, see Supplemental Information). Heating of these specimens was accompanied with mass losses that continued to 450-500 0C. X-Ray diffraction patterns of such samples annealed at 450-500 0C contained only narrowed lines, and the latter phenomenon was an unequivocal evidence of the nanoparticle enlargement.
Synthesis of the 90 % YF3 - 10 % CaF2 solid solution resulted in the formation of the novel phase (Fig. 4) with the X-ray diffraction pattern indexed in the P-cubic system with a = 5.800(2) A parameter (Table 2) or in the F-cubic system with a = 11.60 A (calculated size of the coherent scattering domain D = 25 nm). The SEM image of this specimen contained joined together plate-type nanocrystals (Figs. 3c and 3d).
The X-Ray diffraction pattern of this phase was similar to the one of KY3F10 (Fm3m SSG, Z = 8), so one could assume that it was (H3O)Y3F10 compound with hydroxonium ions occupying potassium sites in the crystal lattice. Heating the specimen resulted in about 11.3 wt. % mass loss (Supplemental Information), which might
C d
Fig. 3. SEM images of the CaF2-YF3 specimens: 10 mol. % YF3 (a), 50 % mol. YF3 (b),
90 % mol.% YF3 (c, d)
TABLE 1. Chemical analysis of CaF2-YF3 specimens
Sample composition Refined YF3 content, mol. %
X-Ray spectroscopy (electron microscopy) Atomic emission spectroscopy (AES)
290 nm excitation wavelength 320 nm excitation wavelength
Ca0.70Y0.30F2.30 (30.0 mol. % YF3) - 26.06±0.75 25.85±0.92
Ca0.50Y0.50F2.50 (50.0 mol. % YF3) 52.4 52.01 ±1.69 54.14±1.85
ca0.30 y0.70f2.70 (70.0 mol. % YF3) - 68.83±2.16 73.31±1.41
Ca0.i0Y0.90F2.90 (90.0 mol. % YF3) 90.8 - -
1 , .1
1 T ■ 1 1 ; 1 1 1 1 1 1 1 I1a.2
, . ....... . 1 k ,f L i A A !
—1-1-----r-----'— __~jJ 1 ' 1 '--!-1---1 t-r-' * v—.„_ —vv— —A - --
1<1<1<J<1<1<1 K* A
-1-"-1--' I -'— J '-!-'— 1 1 1 1 . g
1 1 1 1 10 20 1 1 1 1 1 1 1 1 1 1 30 40 50 60 70 ■ J 111 J- 1 '<•■■,'I,
10 2D 30 40 50 60 70
20, grad.
Fig. 4. X-Ray powder diffraction patterns for the specimens obtained by co-precipitation of calcium and yttrium fluorides from aqueous nitrate solutions: 10 mol.% (1); 20 mol.% (2); 30 mol.% (3); 50 mol.% (4); 70 mol.% (5); 90 mol.% (6) YF3 (nominal compositions), and JCPDS Card No. 27-0465 for KY3F10 phase (7)
Table 2. X-Ray diffraction pattern of the Ca0.10Y0.90F2.90 specimen (Q = 104/d2. P-cubic lattice, a = 5.800(2) A, F(14) = 11.9, M(14) = 25.8)
N 2O(obs) d(obs), A Q(obs) ///0,% h k l Q(calc) AQ
1 15.320 5.7789 299.44 15 1 0 0 297.25 2.19
2 21.730 4.0866 598.79 39 1 1 0 594.50 4.29
3 26.740 3.3312 901.15 100 1 1 1 891.75 9.40
4 30.900 2.8915 1196.06 15 2 0 0 1189.00 7.06
5 38.020 2.3648 1788.18 5 2 1 1 1783.50 4.68
6 44.240 2.0457 2389.55 75 2 2 0 2378.00 11.55
7 47.000 1.9318 2679.64 30 2 2 1 2675.25 4.38
8 52.350 1.7463 3279.16 50 3 1 1 3269.75 9.40
9 54.740 1.6755 3562.14 10 2 2 2 3567.00 -4.86
10 59.530 1.5516 4153.75 7 3 2 1 4161.50 -7.75
11 64.170 1.4502 4754.93 3 4 0 0 4756.01 -1.07
12 70.820 1.3294 5658.34 8 3 3 1 5647.76 10.58
13 72.810 1.2979 5936.32 5 4 2 0 5945.01 -8.68
- 1
l l l l l l
- 1 1 1 1 1 1 j 1 1 1 1 1
- 1 r | 1 1 1 1 1 1 r | 1
1 r 20 II 1 3C 1 I . T . [ r 1 1 ) 40 50 60 70 i - - * i mil m li . . — _____
1 1 l 1 i 1 1 1 ] 1 Ï 1
20 30 40 50 60 70
2©, grad
Fig. 5. X-Ray powder diffraction patterns for the specimens obtained by co-precipitation of calcium and yttrium fluorides from aqueous nitrate solutions (second set of experiments): 80 mol.% (1), 85 mol. % (2), 90 mol. % (3), 95 mol.% (4), and JCPDS Card No. 74-0911 for P-YF3 phase (5)
i-1-1-1-1-1-1-1-1-1-r
20 30 40 50 60 70
20, grad.
Fig. 6. X-Ray powder diffraction patterns for the YF3 specimens: freshly prepared, a = 6.294(3), b = 6.867(4), c = 4.528(3) A (1), annealed at 200 0C, a = 6.317(2), b = 6.875(2), c = 4.469(2) A (2), YF3 annealed at 300 0C, a = 6.346(1), b = 6.861(1), c = 4.419(1) A (3)
Fig. 7. Unit cell parameters for Ca1-xYxF2+x fluorite-type solid solutions as per Fedorov et al. [7] (1) and Gettmann and Greis [12] (2), (all specimens were synthesized by solid phase synthesis) along with the present work data for Ca1-xYxF2+x nanopowders (3). Straight-line dependence according to Fedorov and Sobolev [55]
have come from 8 wt. % loss from decomposition:
(H3 O)Y3F10 ^ 3YF3 + H2 O + HF (2)
along with additional evaporation of hydration water from the solid sample.
Structure of KY3F10 type is derived from fluorite. It consists of a 3D framework formed by (Y6F35)18-clusters (Fig. 2) interconnected by their vertices. Monovalent cations occupy cavities of the aforementioned framework [14, 55, 56]. KR3F10 (R = Dy-Lu, Y) and RbR3F10 (R = Sm-Tb) [3] crystallize in this structure type. However, the structure for KY3F10 can also can be described as a 3D framework of (Y6F32 )14- clusters (constructed from Thompson antiprism coordination yttrium polyhedra in another way) [57] that are also present in the structure of (H3O)Y3F10nH2O phase (Fd3m SSG, Z =16) [36]. However, the latter clusters are packed in a different manner, and the previously described (H3O)Y3F10nH2O phase (Fd3m SSG, Z = 16) is not a fluorite-type phase, in contrast with the phase obtained in our experiments for 90 % YF3-10% CaF2 solid solution specimen (Table 2).
The above results for the CaF2-YF3 system, obtained by co-precipitation method, are similar to our data for the CaF2-HoF3 system [54]. They are also in a good agreement with data [43] regarding Ca1-xRxF2+x solid solutions. Weak additional lines in the X-ray diffraction patterns, corresponding to /-YF3 nanoparticles, were also observed for Ca0.6Y0 4F2 4, also synthesized by the aforementioned co-precipitation in [45].
It is also worth mentioning that we did not observe the formation of tysonite-type phase(s) in the co-precipitated specimens at the lower temperatures. This should not be surprising if one takes into account that such a tysonite-type phase is stable at higher temperatures only (Fig. 1). Nevertheless, it is quite strange that there was no fluorite-type solid solutions formed in the CaF2-RF3 systems (R = rare earth element) that contain a relatively high concentration of the rare earth metals (35-40 mol. % RF3) and possess ordered fluorite-type structures (such ordered phases are usually thermodynamically stable at the lower temperatures). The other previously studied MF2-RF3 systems (e.g., SrF2-YF3 [53], BaF2-YF3 [49], BaF2-CeF3 [52]) with M = Sr and Ba have exhibited different features: each of these systems had the concentration ranges, where ordered fluorite-type phases were observed under equilibrium conditions at the higher temperatures, the unordered solid solutions were formed. Currently, it is hard to find a reasonable explanation for the different results for calcium fluoride systems.
In conclusion, results of our study demonstrate that in the course of the synthesis of Ca1-xYxF2+x solid solutions by co-precipitation from aqueous media, the single phase specimens have been formed for the relatively low yttrium content only (up to 20 mol. % YF3). These samples, apparently, are not under equilibrium, but they
are fairly stable and do not undergo any detectable changes over the course of a few years. The latter is crucially important for the preparation of materials of practical value [58].
Acknowledgments
The authors express their appreciation to R. Ermakov for his kind assistance in the X-ray diffraction experiments as well as to V. K. Ivanov for his valuable discussion of the obtained results. The authors also wish to thank E. V. Chernova and A. I. Popov for their help in the preparation of the present manuscript. This work was partially supported by RFBR 15-08-02481-a grant.
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