Soft chemical synthesis of NaYF4 nanopowders

Article abstract of DOI:10.1134/S0036023608110028

Hydrated NaYF₄ powders dominated by the metastable high-temperature cubic phase were precipitated with a fivefold NaF excess from acid solutions of yttrium nitrate. Transformation into the stable hexagonal phase (a = 5.969(2), c = 3.503(1) ?) occurs under heating with an exotherm at ?400°C.

Full text of DOI:10.1134/S0036023608110028

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 11, pp. 1681–1685. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © P.P. Fedorov, S.V. Kuznetsov, V.V. Voronov, I.V. Yarotskaya, V.V. Arbenina, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 11,
pp. 1802–1806.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Soft Chemical Synthesis of NaYF4 Nanopowders
P. P. Fedorova, S. V. Kuznetsova, V. V. Voronova,
I. V. Yarotskayab, and V. V. Arbeninab
a Prokhorov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia
b Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571 Russia
E-mail: ppf@lst.gpi.ru
Received December 24, 2007
Abstract—Hydrated NaYF4 powders dominated by the metastable high-temperature cubic phase were precip-
itated with a fivefold NaF excess from acid solutions of yttrium nitrate. Transformation into the stable hexago-
nal phase (a = 5.969(2), c = 3.503(1) Å) occurs under heating with an exotherm at ~400°C.
DOI: 10.1134/S0036023608110028
Phase formation in the NaF–YF3 system has been
studied repeatedly [1–9]. Phases formed in this system
are of interest as optical materials for laser matrices
[10–14], luminophors [15–17], solid electrolytes [7],
and other applications. A high-temperature fluorite
phase of variable composition Na0.5 – xY0.5 + xF2 + 2x exists
in this system (Fig. 1). The melting-point curve of this
phase has a maximum at 60.5 0.5 mol % YF3. With
temperature depression, this fluorite phase decomposes
by a eutectoid scheme into the low-temperature NaYF4
phase and an ordered fluorite-derived phase stable
EXPERIMENTAL
The starting chemicals used were NaF (pure for
analysis grade), yttrium oxide (99.99%), nitric acid
(pure for analysis grade), and distilled water.
Sodium fluoride and yttrium fluoride coprecipita-
tion was carried out as follows. An aqueous NaF solu-
tion with concentration 1.06 mol/L was dropped to an
acid solution of yttrium nitrate (c
= 0.53 mol/L,
Y(NO3)3
pH 1) prepared by dissolving Y2O3 in HNO3. A fivefold
excess of NaF was used. After settling, the precipitate
within 700–575°ë. The refined composition of this was decanted, washed with distilled water two times,
filtered, and then dried under a lamp.
phase, which was discovered by Thoma [4, 5], corre-
sponds to the formula unit Na7Y13F46. This phase is iso-
structural to orthorhombic phases with R = Tm–Lu. An
NaY2F7 phase having a complex low-symmetry X-ray
diffraction pattern (a structural analogue of the phases
with R = Er or Ho) was revealed [9].
Thermal analysis was carried out on a Q-1500 D
derivatograph in Alundum crucibles in air. The sample
size was 0.25 g. Heating curves were recorded at
10 K/min. X-ray powder diffraction analysis was car-
ried out on DRON 4M equipped with a pyrolytic graph-
ite monochromator using CuKα radiation. Scanning
electron microscopy was carried out on JEOL 5910.
Test samples were coated with gold before experi-
ments.
Thus, the compound NaYF4 described in [1, 2] can
be treated as dimorphic with the polymorphic transition
at 705°ë. Its high-temperature fluorite polymorph can
dissolve YF3 in the amount corresponding to the limit-
ing composition Na0.345Y0.655F2.31. The low-temperature
trigonal polymorph [3] is isostructural to the native
mineral gagarinite and NaRF4 compounds (where R is
a rare-earth element) [19].
RESULTS AND DISCUSSION
An electron micrograph of a dried precipitate is
shown in Fig. 2; its X-ray diffraction pattern, in Fig. 3a.
NaRF4 compounds are promising luminophores. The precipitate consists of spherical particles with sizes
of 100–300 nm. These particles mostly contain a fluo-
rite cubic phase (‡ = 5.4606 0.003 Å). There is also a
small amount of a hexagonal phase (‡ = 5.916(6),
Ò = 3.54(2) Å). Thus, the major phase of the precipitate
is the high-temperature fluorite phase (which is meta-
stable at ambient temperature and pressure) and the
minor phase is the stable low-temperature phase. The
unit cell parameter of the cubic phase according to the
Nanopowders are known to have improved luminescent
properties [20]. Low-temperature syntheses is of inter-
est in this context [16, 17, 20, 21]. Hydrous NaRF4
compounds were precipitated from aqueous chloride
solutions as early as in [22, 23].
This work studies the soft chemical synthesis of
NaYF4 powders.
1681

1682
FEDOROV et al.
t, °C
1
2
3
4
1000
F + β-YF
L
3
800
L + F
F
F + β-YF
3
L + NaF
L + NaYF
4
600
NaF + NaYF
4
NaYF + β-YF
4
3
NaF
20
40
60
80
YF
3
mol %
Fig. 1. NaF–YF phase diagram according to [9]. Notations: L, melt; F, Na
Y
F
fluorite phase; 1, DTA data; 2, single-
3
0.5 – x 0.5 + x 2 + 2x
phase samples; 3, two-phase samples as probed by X-ray powder diffraction; 4, data borrowed from [18].
concentration curve given in [24] corresponds to the
formula unit Na0.486Y0.514F2.028
The heat treatment was followed by thermogravim-
etry (Fig. 4). The endotherm at 80–102°ë with an atten-
dant weight loss was evidently due to the elimination of
adsorbate water. Further heating brought about a strong
exotherm at 388–404°ë. After cooling, X-ray powder
diffraction pattern (Fig. 3b) only showed the stable low-
temperature NaYF4 phase (‡ = 5.969(2), Ò = 3.503(1) Å).
.
The phase composition of the precipitate did not
change after storage for 7 months. To remove adsorbed
impurities (primarily nitrates), we heat-treated the pre-
cipitate at 250°ë in flowing argon for 9.5 h until pH in
the aqueous solution through which leaving argon was
bubbled stopped changing. This heat treatment likewise
did not change the phase composition.
To summarize, yttrium fluoride and sodium fluoride
coprecipitation with the fivefold excess of the latter
generates mixtures of two (trigonal + cubic) phases.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 11 2008

SOFT CHEMICAL SYNTHESIS OF NaYF4 NANOPOWDERS
1683
between holmium and erbium in the series of fluoride
compounds.
Notably, although the resulting phase composition
is not equilibrium, this state is stagnant. Transition into
the equilibrium trigonal phase does not occur during
long storage. This is surprising if we recall that the sam-
ple contains some percentage of the equilibrium phase;
that is, there is no need to overcome the potential bar-
rier to the generation of crystallization centers.
The formula unit of the as-synthesized compound
derived from weight loss is NaYF4 · 0.39ç2é. This for-
mula unit agrees with data in [22, 23], where the
amount of sorbate water in NaRF4 · nç2é compounds
was n = 0.2–0.5. Dehydration at 100°ë (in fact, boiling
out) signifies that water is retained in the precipitate by
physisorption.
1 µm
Fig. 2. Electron micrograph of a precipitate.
Proceeding from the particle sizes in the as-synthe-
sized precipitate and the fact that the minimal surface
area per adsorbate water molecule is ~0.1 nm2 [25], we
estimated the thickness of the adsorbate surface water
These results agree with the results by Batsanova et al.
[23], who demonstrated that NaRF4 compounds are pre-
cipitated from aqueous solutions as hexagonal phases
for R = Sm–Ho and as cubic phases for R = Er and Yb.
In its crystal-chemical characteristics, yttrium stands layer. The resulting value was 11 4 monolayers.
I/I , %
0
100
(a)
As-synthesized NaYF4 powder
90
20
10
0
100
(b)
After DTA
80
60
40
20
0
10
20
30
40
50
60
70
2θ, deg
Fig. 3. X-ray powder diffraction patterns for (a) an as-synthesized NaYF precipitate and (b) the same powder after thermogravimetry.
4
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 11 2008

1684
FEDOROV et al.
404
TG
DTG
426
388
80
119
DTA
102
T
Time
Fig. 4. Thermoanalytical curves for a dried sample; sample size: 0.25 g; heating rate: 10 K/min; temperature: in degrees Centigrade.
4. R. E. Thoma, G. M. Hebert, H. Insley, and C. F. Weaver,
Rare Earth Research II. Proceedings of the 3rd Confer-
ence on Rare Earth Research (Gordon & Breach, New
York/London, 1963), Ed. by K. S. Vorres, p. 297.
ACKNOWLEDGMENTS
The authors are grateful to S.V. Lavrishchev for
electron microscopy experiments.
This work was supported by the Russian Founda-
tion for Basic Research (project no. 08-03-12080-ofi).
S. V. Kuznetsov ackowledges support from the Regional
Public Foundation for Support of National Science.
5. R. E. Thoma, G. M. Hebert, H. Insley, and C. F. Weaver,
Inorg. Chem. 2 (5), 1005 (1963).
6. L. Pontonner, These (L’Universite Scientifique et Medi-
cale et l’Institut National Polytechnique de Grenoble,
1985).
7. L. Pontonner, Solid State Ionics 9/10, 549 (1983).
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