Preparation of nanopowdered M1-x R x F2+x (M = Ca, Sr, Ba; R = Ce, Nd, Er, Yb) solid solutions

Article abstract of DOI:10.1134/S0036023607030035

The synthesis procedure has been worked out, and nanopowders of fluoride solid solutions (ss) Ca_1-x R _x F_2+x (R = Er, Yb), Sr_1-x Nd _x F_2+x , and Ba_1-x Ce _x F_2+x

Full text of DOI:10.1134/S0036023607030035

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 3, pp. 315–320. © Pleiades Publishing, Inc., 2007.
Original Russian Text © S.V. Kuznetsov, I.V. Yarotskaya, P.P. Fedorov, V.V. Voronov, S.V. Lavrishchev, T.T. Basiev, V.V. Osiko, 2007, published in Zhurnal Neorganicheskoi
Khimii, 2007, Vol. 52, No. 3, pp. 364–369.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Preparation of Nanopowdered M1 – xRxF2 + x (M = Ca, Sr, Ba;
R = Ce, Nd, Er, Yb) Solid Solutions
S. V. Kuznetsov, I. V. Yarotskaya, P. P. Fedorov, V. V. Voronov, S. V. Lavrishchev,
T. T. Basiev, and V. V. Osiko
Research Center for Laser Materials and Technologies, Prokhorov Institute of General Physics,
Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia
E-mail: tez@rambler.ru
Received September 25, 2006
Abstract—The synthesis procedure has been worked out, and nanopowders of fluoride solid solutions (ss)
Ca1 – xRxF2 + x (R = Er, Yb), Sr1 – xNdxF2 + x, and Ba1 – xCexF2 + x have been manufactured by coprecipitation
from aqueous solutions. The powders consist of rounded particles with sizes from 50 to 150 nm. The particles
with sizes of about 150 nm are aggregates of adhered nanoparticles. The nanoparticles are highly reactive and
sinter at low temperatures (T < 0.35Tm).
DOI: 10.1134/S0036023607030035
Trend analysis in photonics shows that fluoride-base
The goal of this work was to obtain nanopowders of
materials will play an important role in the nearest fluoride solid solutions M1 − xRxF2 + x (M = Ca, Sr, Ba;
future. The rationale for this claim is provided by the
combination of the following properties of fluorides:
the wide transmission window from 0.2 to 6 µm; “low”
phonon spectra, which prevent from bypass of the radi-
ative transitions of impurity ions by multiphonon relax-
ation; the ease of achieving significant doping levels of
active rare-earth ions (to 1021 cm–3); and the high heat
conductivity. Higher mechanical properties and higher
water resistance also distinguish fluorides from other
classes of compounds with wide transmission windows
(such as chlorides and chalcogenides).
R = Ce, Nd, Er, Yb) from aqueous solutions. Nanopar-
ticles are meant to have sizes less than 100 nm.
This is a nontrivial problem: the water solubilities of
alkaline-metal and lanthanide fluorides differ by sev-
eral orders of magnitude. In accordance, Ba1 – xNdxF2 + x
nanoparticles synthesized in work [12] had dramati-
cally inhomogeneous compositions, with their cores
being enriched in neodymium.
EXPERIMENTAL
It is attractive to create fluoride laser nanoceramics
[1–4], analogues of recently developed oxide laser
ceramics [5–7]: their transmission, spectral, and lasing
parameters can almost compete with single crystals,
and they are substitutes for single-crystal and glassy
laser materials. The breakthrough in this field was pro-
vided by the use of nanoparticle self-organization.
The chemicals used are listed in the table. Neody-
mium oxide was fully hydrolyzed and converted to
Nd(OH)3. It was calcined at 750°C for 2.5 h to convert
the reagent back to neodymium oxide.
The fluoride solid solution (ss) particles were syn-
thesized in air in the following way. Nitrate solutions of
alkaline-earth and lanthanide metals were prepared
from the metal nitrates and/or oxides. When lanthanide
oxides were used, excess nitric acid was added to the
solution until the oxides completely dissolved. Alka-
line-earth nitrate solutions were combined with the lan-
thanide nitrate solutions. The resulting solution was
dropped to aqueous hydrofluoric acid, which was in a
Teflon container and was magnetically stirred. The
solution clouded during dropping, and a suspension
appeared. The suspension was allowed to stand for set-
tling, and the clear supernatant was decanted. In view
of a possible coprecipitation of impurity nitrates, the
The strengths of laser ceramics over single crystals
are the feasibility of manufacturing large slabs with
desired shapes, improved mechanical properties,
homogeneous distribution and high concentrations of
activator ions, and optical transparency in cases where
it is difficult to obtain single crystals.
We chose to study solid solutions å1 – xRxF2 + x
,
where M stands for an alkaline-earth metal and R for a
lanthanide, for which physical properties can be varied
widely [8–10]. In preliminary experiments intended to
manufacturing of ceramics by hot pressing [11], mix-
tures of MF2 and RF3 were found to yield chemically
inhomogeneous ceramics with unsatisfactory optical precipitates were doubly washed with distilled water.
parameters. The precipitate obtained thus was dried under a lamp at
315

316
KUZNETSOV et al.
Quality of the chemicals used (Russia)
Compound
Several hydration schedules were chosen to study
the effect of adsorbate water on the particle size and
morphology: ethanol washing, long-term drying (for
~15 h) at 100–150°ë, and calcining for 4.5 h at 300°C.
Purity grade
Yb2O3
Nd2O3
Chemically pure
Chemically pure
The samples were studied by powder X-ray diffrac-
tion on a DRON 4M diffractometer using CuKα radia-
tion with a pyrographite monochromator on the dif-
fracted beam. The apparent crystallite size (ACS) D and
strain  were determined as described in work [13].
Certain difficulties arose in choosing a standard for D
and  determinations. Attempts to use a disintegrated
CaF2 single crystal were unsuccessful: powder X-ray
diffraction reflections were broaden because of micros-
trains. The standard used was powdered high-purity
Ca(NO3)2 · 4H2O
Er(NO3)3 · 5H2O
Ce(NO3)3 · 6H2O
Sr(NO3)2
Ba(NO3)2
HF (sln)
Chemically pure
Pure
Pure
Pure for analysis
Chemically pure
Chemically pure
High purity (osch 5–4)
Pure for analysis
NH4F
HNO3 (sln)
1
grade calcium fluoride (Fig. 1) .
T ≤100°ë. To prepare Ba1 – xCexF2 + x ss, an ammonium
fluoride solution was used instead of hydrofluoric acid.
Scanning electron microscopy (SEM) was carried
out on a JEOL 5910 instrument. Test powders were
coated with gold before investigation.
RESULTS AND DISCUSSION
Solid solution Ca0.9Er0.1F2.1 was the model for work-
ing out technology. A representative micrograph of the
precipitate prepared as described above (Fig. 2) shows
particle sizes from 50 to 150 nm; large particles are
aggregates of adhered fine particles.
For a precipitate of the same composition (Fig. 3a),
the ACS and microstrains were measured as described
above: 67 nm and 5.3 × 10–3, respectively. The X-ray
diffraction pattern of the precipitate corresponds to that
of a fluorite ss (Fig. 4). This precipitate was washed
with ethanol. The result is demonstrated by the micro-
graph on Fig. 3b. SEM showed a decrease in the parti-
cle size, which was also confirmed by ACS calculations
(D = 40 nm, Â = 3.9 × 10–3).
28.9 ÏÍÏ
50 µm
×500
Fig. 1. Micrograph of high-purity CaF powder.
2
Long-term drying at 100−150°C yielded particles
displayed on Fig. 3c. One can see cubic particles with
sizes from 100 nm to several micrometers in the micro-
graph. Calcining at 300°C for 4.5 h yielded cubic parti-
cles with sizes of several micrometers (Fig. 3d). From
this, we infer that heat treatment increased the particle
size with simultaneous faceting, while ethanol washing
did not affect the particle morphology.
Once the preparation of erbium-doped calcium flu-
oride ss was worked out, we proceeded with the synthe-
sis of Ca1 – xYbxF2 + x ss. The results of the synthesis
were analogous (Figs. 5, 6).
Experiments were carried out to manufacture
Sr1 - xNdxF2 + x. These experiments showed that a sin-
gle-phase ss was synthesized when hydrofluoric acid
was taken in a more than 20-fold excess over the nitric
0.1 µm
10 × 103
1
Prepared by A.A. Fakeev at the Institute of Reagents and High-
Purity Compounds.
Fig. 2. Micrograph of Ca Er
F
particles.
0.9 0.1 2.1
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PREPARATION OF NANOPOWDERED M1 – xRxF2 + x
317
10 × 103
10 × 103
1 µm
1 µm
(a)
(c)
30 × 103
10 × 103
0.5 µm
1 µm
(b)
(d)
Fig. 3. Micrographs of Ca Er
F
particles (a) after precipitation by dropping the mixed nitrate solution to hydrofluoric acid,
0.9 0.1 2.1
(b) after ethanol washing, (c) after drying at 150°ë, and (d) after calcining at 300°ë.
Ι, %
100
80
60
40
20
0
30
40
50
60
70
80
2θ, deg
Fig. 4. X-ray diffraction pattern of Ca Er
F
ss (see Fig. 3a).
0.9 0.1 2.1
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 3 2007

318
KUZNETSOV et al.
acid used for preparing the mixed nitrate solution.
Figure 7a shows the micrograph of a single-phase
neodymium-doped strontium fluoride ss sample, which
was not washed with distilled water before drying, after
long-term drying at 100–150°C. The particle-size cal-
culations from X-ray diffraction data gave D = 95 nm
and  = 4.4 × 10–3. Cubic particles with sizes ranging
from about 100 nm to several micrometers are seen in
the micrograph. Loose inclusions, which are apparently
amorphous strontium nitrate, are observed.
When the requirement to add excess hydrofluoric
acid is not fulfilled, the resulting sample consists of two
phases: a strontium fluoride ss and strontium nitrate.
We employed the standard precipitation procedure that
was worked out for calcium fluoride ss, but the precip-
itate was additionally washed two times. As a result,
there were no strontium nitrate peaks in the X-ray dif-
fraction patterns. Figure 8 displays the micrograph of
Sr1 – xNdxF2 + x ss after washing; one can see that loose
inclusions are absent.
10 × 103
0.1 µm
(‡)
For Ba1 – xCexF2 + x ss, analogous results were
obtained with the only difference that the mixed nitrate
solution was dropped to an ammonium fluoride solu-
tion. Dropping to aqueous hydrofluoric acid produced a
phase mixture, and we did not use HF as a fluorinating
agent. The precipitates showed background barium
nitrate peaks in their X-ray diffraction patterns, and we
had to wash them twice. As a result, we obtained a sin-
gle-phase Ba1 – xCexF2 + x precipitate whose X-ray dif-
fraction pattern is displayed on Fig. 9.
10 × 103
1 µm
(b)
Fig. 5. Micrographs of Ca
Yb F particles (a) after
x 2 + x
1 – x
precipitation by dropping the mixed nitrate solution to
hydrofluoric acid and (b) after calcining at 300°ë.
Ι, %
100
80
60
40
20
0
20
30
40
50
60
70
80
2θ, deg
Fig. 6. X-ray diffraction pattern of Ca
Yb F
ss particles after precipitation by dropping the mixed nitrate solution to hydrof-
1 – x
x
2 + x
luoric acid (see Fig. 5a).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 3 2007

PREPARATION OF NANOPOWDERED M1 – xRxF2 + x
319
Ι, %
100
80
60
40
20
BaF –CeF
2
3
10 × 103
1 µm
(‡)
0
20
30
40
50
60
70
2θ, deg
I, %
100
Fig. 9. X-ray diffraction pattern of Ba
pared by dropping the mixed nitrate solution to ammonium
fluoride after double washing.
Ce F
ss pre-
1 – x
x
2 + x
80
60
40
20
As a result of our investigations, we found that sin-
gle-phase calcium and strontium-based ss are manufac-
tured by dropping the relating nitrate solutions to
hydrofluoric acid followed by double washing of the
precipitates. The characteristic particle sizes are
50−150 nm. Particles with sizes larger than 100 nm are
agglomerates of fine particles (Figs. 2, 3b, 5a).
0
10
(b)
20
30
40
50
60
70
Long-term drying at 100–150°C induces particle
faceting (Figs. 3c, 7a). Faceting is also observed upon
dehydration at 300°C for 4.5 h (Figs. 3d, 5b).
2θ, deg
Fig. 7. Panel (a): a micrograph of Sr
Nd F
particles
1 – x
x
2 + x
Grain coarsening upon heating to 300°C proves the
high reactivity of the nanoparticles, namely, their sin-
terability at low temperatures (T < 0.35 Tm).
after precipitation by dropping the mixed nitrate solution to
hydrofluoric acid. Panel (b): the relating X-ray diffraction
pattern.
In summary, we have worked out the synthesis of
nanopowders for use as precursors for manufacturing
optical ceramics.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educa-
tion and Sciences (State Contract no. 02.435.11.2011,
July 15, 2005).
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