Soft chemistry synthesis of powders in the BaF2-ScF3 system

Article abstract of DOI:10.1134/S003602361407016X

The BaF₂-ScF₃ system is studied using coprecipitation from aqueous solutions. Compound Ba₃Sc₂F₁₂ is found to form over a wide range (10-60 mol %) of scandium concentrations in the initial solution. At higher scandium concentrations in solution, a new phase of provisional composition BaSc₂F₈ · 2H₂O (hexagonal system, a = 9.6346 A?, c = 4.0483 A?) was obtained. Scandium fluoride hydrolysis is not observed by energy-dispersive X-ray analysis.

Full text of DOI:10.1134/S003602361407016X

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2014, Vol. 59, No. 7, pp. 773–777. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © M.N. Mayakova, S.V. Kuznetsov, V.V. Voronov, A.E. Baranchikov, V.K. Ivanov, P.P. Fedorov, 2014, published in Zhurnal Neorganicheskoi Khimii, 2014,
Vol. 59, No. 7, pp. 988–992.
Soft Chemistry Synthesis of Powders in the BaF2–ScF3 System
M. N. Mayakovaa, S. V. Kuznetsova, V. V. Voronova,
A. E. Baranchikovb, V. K. Ivanovb, c, and P. P. Fedorova
aProkhorov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia
b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
c National Research Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia
eꢀmail: ppfedorov@yandex.ru
Received January 22, 2013
Abstract—The BaF2–ScF3 system is studied using coprecipitation from aqueous solutions. Compound
Ba3Sc2F12 is found to form over a wide range (10–60 mol %) of scandium concentrations in the initial soluꢀ
tion. At higher scandium concentrations in solution, a new phase of provisional composition BaSc2F8
(hexagonal system, a = 9.6346 c = 4.0483 Å) was obtained. Scandium fluoride hydrolysis is not observed
by energyꢀdispersive Xꢀray analysis.
⋅
2H2O
Å,
DOI: 10.1134/S003602361407016X
Fluoride nanopowders in recent years have received
The most reliable data on the BaF2–ScF3 system
appreciable attention from researchers due to the speꢀ are those published in [14]. This system forms two
cific properties of nanosized materials and the unique compounds with close compositions: Ba3Sc2F12 (space
set of physical and chemical properties of fluorides [1– group
6]. This combination opens a way to design a new genꢀ 9.482
eration of scintillators and white light sources and to use the highꢀtemperature congruently melting Ba5Sc3F19
P
Å,
4/mbm, tetragonal unit cell parameters:
a =
c
= 5.587 Å), which is stable below 740 , and
°C
fluorides in biomedical applications and so on.
(space group 4/mbm, unit cell parameters: = 21.07
Р
a
Å
,
c
= 15.38 Å). Ba1 – xScxF2 + x solid solution ( < 0.09)
x
Scandium fluoride is the highest melting inorganic
fluoride (its melting point is 1550°C [7]). The high
volatility of scandium fluoride near the melting point
creates difficulties in syntheses of its based materials
and makes it pertinent to search for lowꢀtemperature
synthetic methods. ScF3 crystallizes in a ReO3 type
was found to exist at high temperatures; this solid soluꢀ
tion has high fluorideꢀion conductivity [15] and serves
as the basis for creating a promising scintillator [16].
The scandium distribution coefficient during barium
fluoride crystallization appreciably differs from unity
[17], so that it is difficult to obtain samples with a uniꢀ
form distribution of the dopant by melt crystallization.
Ceramic synthesis using softꢀchemistry methods is
preferred in this context.
structure, space group Pm3m (with only very small
trigonal distortions of the cubic lattice) [8, 9]. This
constitutes a strong distinction of scandium fluoride
from the other rareꢀearth fluorides, while formally
scandium fluoride is a member of this family [10]. In
terms of crystal chemistry, scandium in fluorides is
close to indium [11], but its compounds are distinꢀ
guished by greater chemical stability. Scandium fluoꢀ
ride compounds are of interest as components of antiꢀ
reflecting coatings with low refractive indices and as
matrices to be doped with dopants having small ionic
Bariumꢀcontaining nanosized fluorides are promꢀ
ising theranostics due to their high Xꢀray contrast [5].
In continuation of our studies into the preparation
of nanosized fluorides [18–29], here we study the
preparation of powders in the BaF2–ScF3 binary sysꢀ
tem using coprecipitation from aqueous solutions.
radii, in particular Cr3+
.
SYNTHESES
Noteworthy, scandium fluoride nanomaterials
remain virtually unstudied. There are only few releꢀ
vant publications [12, 13], in contrast to numerous
publications concerning, for example, nanosized
yttrium fluorides.
Syntheses were accomplished by coprecipitation
from aqueous solutions. The initial chemicals used
were barium nitrate (high purity grade 10ꢀ2), scanꢀ
dium nitrate (chemically pure grade), hydrofluoric
acid (high purity grade 27ꢀ5), and aqueous ammonia
Barium fluoride crystallizes in a fluorite type strucꢀ (pure for analysis grade). Precipitation was accomꢀ
ture (space group Fm ). Its highꢀpressure orthorꢀ plished in a polypropylene reactor. First, initial soluꢀ
hombic phase is known to be formed during the synꢀ tions of the nitrates were prepared in bidistilled water.
thesis of BaF2 powders (space group Pnam) [4]. The nitrate solutions were mixed and added dropwise
3m
773

774
MAYAKOVA et al.
I/I
0
1
2
3
4
5
6
7
8
9
30
50
70
40
60
20
2θ, deg
Fig. 1. Xꢀray diffraction patterns for samples precipitated from solutions in the BaF –ScF system. Scandium percentage in the
2
3
initial solution, mol %: (1) 0, (2) 5, (3) 10, (4) 50, (5) 60, (6) 65, (7) 80, (8) 95, and (9) 100.
to the magnetically stirred hydrofluoric acid solution. no. 04ꢀ0452]), but well corresponds with the results
The white slurry formed in the course of mixing was obtained for barium fluoride prepared by precipitation
separated from the mother solution, washed with bidꢀ with hydrofluoric acid [29]. When the scandium conꢀ
istilled water, neutralized with aqueous ammonia, and tent in the solution was 5 mol %, a singleꢀphase sample
dried at a temperature below <100
of the nitrate solutions were varied in the range 0.1– eter
0.2 mol/L; the hydrofluoric acid solution concentraꢀ percentage in the initial solution was 10 mol %, Xꢀray
°С
. Concentrations of Ba1 – xScxF2 + x cubic solid solution with the paramꢀ
a
= 6.0421 Å was prepared. When the scandium
tion was 11 vol %.
The phase compositions of asꢀsynthesized samples
were determined by Xꢀray powder diffraction on
diffraction patterns featured distinct reflections from a
second phase; these reflections corresponded to the
compound Ba3Sc2F12 (with the unit cell parameters
а
= 9.4923
Å, с = 5.6001 Å).
DRONꢀ4M and Bruker D8 diffractometers (CuK 1 radiꢀ
α
ation). Unit cell parameters were calculated from
Xꢀray diffraction data using Powder 2.0 and TOPAS
software. Thermogravimetric studies were carried out
on a Q 1500D (MOM) derivatograph under heating to
The compound Ba3Sc2F12 was precipitated over a
wide range of concentrations, when scandium perꢀ
centages in the initial solution were 10–60 mol %. An
Xꢀray diffraction pattern from a sample prepared when
scandium percentage in the initial solution was 50 mol %,
corresponded to the compound Ba3Sc2F12 in a pure
form. Indexing of the Xꢀray diffraction pattern is
shown in Table 1. Calculated unit cell parameters
600°C at 10 K/min. Scanning electron microscopy
(SEM) studies were performed on a Carl Zeiss NViꢀ
sion 40 microscope. Compositions of samples were
estimated from energyꢀdispersive Xꢀray analysis in a
scanning electron microscope. IR spectra were
recorded on an INFRALUM FT08 spectrometer
equipped with an ATR (attenuated total internal
reflectance) attachment.
(space group P4/mbm, a = 9.489 Å, c = 5.568 Å) corꢀ
respond with the published values. Figure 2a displays
SEM data for this sample. Micronꢀsized agglomeraꢀ
tions composed of particles with sizes of from 30 to
500 nm can be distinguished in the SEM images. A
singleꢀphase sample was also prepared when the scanꢀ
dium percentage in the solution was increased to
60 mol %; the Xꢀray diffraction pattern for this sample
RESULTS AND DISCUSSION
Samples were prepared with different barium and
scandium contents in the initial solutions. Relevant
Xꢀray diffraction patterns are shown in Fig. 1.
The unit cell parameter of an undoped fluorite barꢀ
ium fluoride sample prepared by the aboveꢀdescribed
corresponded to the compound Ba3Sc2F12 (a = 9.469 Å,
c
= 5.559 , but particle morphology was quite differꢀ
Å)
ent. Particles with sizes of ~30 nm are distinguishable
in the SEM image (Fig. 2b).
procedure was
fers from the literature data (
а
= 6.1903 Å. This value noticeably difꢀ
As the scandium percentage in the initial solution
a
= 6.200 Å [PCPDFWIN increased further, Ba3Sc2F12 was no longer formed.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 7 2014

SOFT CHEMISTRY SYNTHESIS OF POWDERS
775
100 nm
100 nm
(а)
(а)
200 nm
100 nm
(b)
(b)
Fig. 2. SEM images of powders with asꢀbatch composiꢀ
tions: (a) 0.50BaF –0.50ScF and (b) 0.40BaF –
2
3
2
0.60ScF .
3
The major phase became an earlier unknown phase X.
The sample obtained from the solution where the
scandium percentage was 65 mol %, was a single phase;
its Xꢀray diffraction pattern was indexed in terms of a
hexagonal space group with the unit cell parameters: a =
9.6346 Å, c = 4.0483 Å (Table 2). Its IR spectra featured
absorption bands (at 1430, 1630, 3580 cm–1) that correꢀ
sponded to the vibrations of water molecules and OH–
groups. Thermogravimetric studies showed that the
sample lose 9.1% of its weight in the range 100–
30 nm
(c)
285
BaSc2F8
the samples after DTG were comprised of two phases:
compound Ba3Sc2F12 ( = 9.4932 = 5.9447 and
scandium fluoride ( = 4.014 Å). The corresponding
decomposition reaction is
°С
. Calculations imply the chemical formula
⋅
2H2O. According to Xꢀray powder diffraction,
Fig. 3. SEM images of powders with asꢀbatch composiꢀ
tions: (a) 0.50BaF –0.95ScF and (b, c) 0.35BaF –
2
3
2
a
Å,
c
Å)
0.65ScF .
3
a
As the scandium percentage in the initial solution
increased 95 mol %, twoꢀphase samples were precipiꢀ
tated; they were composed of BaSc2F8 2H2O and
ScF3 in various proportions. Figure 3a displays a SEM
image of a twoꢀphase sample where the scandium perꢀ
centage in the initial solution was 95 mol %. Large
3BaSc2F8 ⋅ 2H2O = Ba3Sc2F12 + 4ScF3 + 6H2O.
⋅
Figures 3b and 3c display SEM images of this
phase. Particles having extended shapes (“rods”) with
diameters of 30–50 nm and lengths of up to 200 nm
are distinguished in the micrograph.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 7 2014

776
MAYAKOVA et al.
Table 1. Indexing an Xꢀray diffraction pattern for a Table 2. Indexing an Xꢀray diffraction pattern for a
Ba3Sc2F12 phase BaSc2F8 2H2O phase, which was precipitated from a soluꢀ
tion where the scandium percentage was 65 mol %
⋅
hkl
d
e, Å
2
θ
, deg
I
/
I
0, %
Δ
Q
hkl
d
e, Å
2
θ
, deg
I
/
I
0, %
Δ
Q
2 0 0
1 1 1
2 1 0
2 0 1
2 1 1
2 2 0
3 1 0
2 2 1
0 0 2
3 1 1
2 0 2
4 0 0
2 1 2
4 1 0
3 3 0
4 0 1
2 2 2
4 2 0
3 3 1
3 1 2
3 2 2
1 1 3
5 1 1
2 0 3
2 1 3
5 2 1
5 3 0
6 0 0
6 2 1
4 1 3
5 3 2
6 0 2
1 1 4
7 0 1
4.736
4.287
4.236
3.607
3.373
3.367
3.000
2.873
2.781
2.638
2.401
2.370
2.328
2.302
2.238
2.182
2.144
2.124
2.074
2.043
1.913
1.790
1.766
1.730
1.702
1.680
1.628
1.581
1.450
1.445
1.406
1.376
1.363
1.317
18.72
20.70
20.95
24.66
26.39
26.45
29.75
31.10
32.15
33.95
37.42
37.93
38.64
39.10
40.26
41.34
42.10
42.52
43.61
44.30
47.49
50.96
51.73
52.88
53.82
54.57
56.48
58.32
64.18
64.41
66.41
68.08
68.82
71.60
6
1.38
–0.19
1.72
1 0 0
1 1 0
2 0 0
1 0 1
2 1 0
1 1 1
0 3 0
2 1 1
3 1 0
3 0 1
4 0 0
2 2 1
0 0 2
3 1 1
3 2 0
1 1 2
4 0 1
1 4 0
4 1 0
3 2 1
2 1 2
8.344
4.817
4.172
3.642
3.154
3.099
2.781
2.488
2.314
2.292
2.086
2.070
2.024
2.009
1.914
1.866
1.854
1.821
1.821
1.731
1.703
10.61
18.40
21.28
24.42
28.28
28.78
32.16
36.07
38.89
39.27
43.34
43.69
44.74
45.09
47.46
48.76
49.09
50.05
50.06
52.86
53.77
10
17
15
32
20
55
21
14
12
84
5
0.01
–0.05
0.03
30
21
89
37
2.15
–0.10
0.21
0.96
100
19
5
–6.88
–0.04
0.68
–0.16
–0.25
0.17
15
7
4.54
–0.25
–0.66
0.11
3.94
5
1.92
7
2.65
100
71
10
19
30
46
16
49
5
0.06
30
11
15
15
33
36
25
54
13
8
0.95
–0.36
–0.17
–0.57
–0.33
–0.85
0.77
–1.39
–3.70
0.31
–2.27
–6.52
3.61
0.77
–3.04
0.07
1.93
28
–1.86
a = 9.6346 Å,
–0.62
–3.50
–0.83
–1.08
–2.10
–4.41
2.08
A primitive hexagonal lattice with the parameters:
= 4.0483 Å.
25
13
13
13
15
11
18
19
14
12
11
11
c
group Pm3m). Noteworthy, the samples were not
hydrolyzed as probed by energyꢀdispersive Xꢀray analꢀ
ysis, while in the BaF2–RF3 (R = Bi or Y) systems,
hydrolysis was observed for low barium concentraꢀ
tions.
–9.71
0.28
Lowꢀtemperature syntheses in this system yielded
virtually the same phases as were identified in
approaching equilibrium “from above.”
–10.92
–7.34
8.58
In summary, a method to prepare singleꢀphase
Ba3Sc2F12 powders in approaching equilibrium “from
above” has been found. Noteworthy, inasmuch as this
compound is stable only in a lowꢀtemperature range,
its solidꢀphase synthesis is very difficult and would
require longꢀterm anneals, especially in view of the
high melting temperatures of the constituents. A small
scatter in unit cell parameters of samples may be indicꢀ
ative of the existence of a homogeneity range in this
phase. Ba3Sc2F12 served us to demonstrate the feasibilꢀ
ity to control the dispersion of precipitations via altering
the component ratio in the initial solution. The disperꢀ
0.50
The test sample was precipitated from a solution where the scanꢀ
dium and barium percentages each were 50 mol %. A primitive
tetragonal lattice with the parameters:
a = 9.489(1) Å, c =
= 10 (1/d2e – 1/d2e ).
4
5.5768(2) Å.
Δ
Q
cubic particles having sizes of 100–500 nm are distinꢀ
guished in the micrograph.
We also prepared scandium fluoride. The calcuꢀ
lated unit cell parameter was
a
= 4.0035 Å (space sion of the precipitate is of importance both for biomedꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 7 2014

SOFT CHEMISTRY SYNTHESIS OF POWDERS
777
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We have been the first to carry out systematic synꢀ
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of coprecipitation from aqueous solutions. We have
prepared Ba1 – xScxF2 + solid solution, compounds
x
Ba3Sc2F12 and ScF3, and an earlier unknown phase
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as BaSc2F8 ⋅ 2H2O and which has been indexed in
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