Hydrofluoride synthesis of fluorides of some rare-earth elements

Article abstract of DOI:10.1023/A:1022237514902

Thermal gravimetric, X-ray phase, IR spectroscopic, and chemical analyses were applied to study the reaction of yttrium and neodymium oxides with NH ₄HF₂.

Full text of DOI:10.1023/A:1022237514902

Russian Journal of Applied Chemistry, Vol. 75, No. 11, 2002, pp. 1760 1764. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 11,
2002, pp. 1796 1800.
Original Russian Text Copyright
2002 by Kalinnikov, Makarov, Tikhomirova, Elizarova, Kuznetsov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Hydrofluoride Synthesis of Fluorides
of Some Rare-Earth Elements
V. T. Kalinnikov, D. V. Makarov, E. L. Tikhomirova,
I. R. Elizarova, and V. Ya. Kuznetsov
Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Resources,
Kola Scientific Center, Russian Academy of Sciences, Apatity, Murmansk oblast, Russia
Received April 15, 2002
Abstract Thermal gravimetric, X-ray phase, IR spectroscopic, and chemical analyses were applied to study
the reaction of yttrium and neodymium oxides with NH4HF2.
Fluorides of rare-earth elements (REE) are used in
electrochemical recovery of these metals by electroly-
sis of molten salts and in production of glasses for
active and passive waveguides; they are of interest for
production of laser media based on complex REE
fluorides [1 4].
for obtaining yttrium, neodymium, and gadolinium
fluorides by (a) hydrofluoride synthesis and (b) pre-
cipitation from solutions were compared with respect
to the concentration of oxygen-containing anionic im-
purities in the final products.
As starting substances we used REE oxides of
analytically pure grade and twice recrystallized
NH HF of the same grade. Thermogravimetric
A study of the reaction of REE oxides with am-
monium hydrofluoride, including identification of the
intermediates, is of interest not only for controlling
the process, but also for developing and optimizing
the hydrofluoride synthesis of complex fluorides of
4
2
studies were carried out on a Q-1500D derivatograph
in platinum crucibles, with weighed portions of 350
400 mg. As reference served calcined Al O . Meas-
2
3
the composition M Ln F
, where M is an alkali
urements were done in dynamic and quasi-isothermal
m
n m+3n
1
metal and Ln is an REE, to be used as stock for
growth of single crystals.
modes at a heating rate of 5 deg min . X-ray phase
analysis was carried out on a DRON-3 diffractometer
with CuK radiation. IR absorption spectra were
recorded on a UR-20 spectrophotometer. Chemical
analysis for fluoride ions was performed by pyrohy-
drolysis followed by potentiometric determination
(with F -selective electrode). Analysis for metals was
done by flame emission spectrometry. The content of
ammonia was determined by the Kjeldahl method.
The concentrations of oxygen-containing anionic im-
purities in the final synthesis products was found
using linear voltammetry in halide melts [8]. The
measurements were done under isothermal conditions
in an argon atmosphere. The fluorides synthesized
were introduced into a KCl KF melt of eutectic com-
position. A glassy carbon (SU-2000) crucible with
the melt was placed in a hermetically sealed stainless
steel retort. The crucible simultaneously served as
a counterelectrode. Molybdenum was used as materi-
al of the quasi-reference electrode. Voltammograms
were measured on a glassy carbon (SU-2000) elec-
trode using an Autolab computerized electrochemical
A disadvantage of the conventional method for ob-
taining REE fluorides by precipitation from solution
is the relatively high content of oxygen-containing
anionic impurities.
Use of ammonium hydrofluoride as a fluorinating
agent has a number of advantages, since NH HF is
4
2
close in reactivity to anhydrous hydrogen fluoride, is
inexpensive, and can be readily dehydrated and regen-
erated from gaseous components, and reactions in-
volving this agent can be carried out without complex
equipment [5 7].
Despite extensive published data on hydrofluoride
synthesis of REE fluorides, there is no common opin-
ion concerning the scheme of the reactions in the
literature. Therefore, we performed in this work a
detailed study of reactions of yttrium and neodymium
oxides with ammonium hydrofluoride and examined
the composition and order of thermal decomposition
of the intermediates formed. In addition, the methods
1070-4272/02/7511-1760$27.00 2002 MAIK Nauka/Interperiodica

HYDROFLUORIDE SYNTHESIS OF FLUORIDES
complex. The residual current densities found from
1761
T, C
(a)
m, %
voltammograms of the supporting melt characterized
its purity; for oxygen, these current densities were
2
about 5 mA cm .
In [9], complex yttrium fluorides were studied
thermogravimetrically, and it was demonstrated that
the reaction of Y O with NH HF starts at 60 80 C
2
3
4
2
to give NH Y F NH F, which dissociates into
4
2 7
4
NH Y F and yttrium trifluoride upon heating. In
4
2 7
[10], no formation of NH Y F NH F in the reaction
4
2 7
4
of Y O with NH F was observed, but yttrium com-
2
3
4
pounds of the composition (NH ) Y F and NH Y F
4 3 2 9
4 2 7
were synthesized.
A thermogram of a mixture of Ln O with NH HF
2
2
3
4
in 1 : 6 molar ratio is shown in Fig. 1a. The reaction
is accompanied by a series of exo- and endothermic
effects. The endothermic effect at 145 C can apparent-
ly be attributed to melting of ammonium hydrofluo-
ride, which intensifies the reaction. At 236 C, excess
NH HF boils, giving an endothermic peak in the
T, C
(b)
m, %
4
2
DTA curve. Hydrofluorination is complete at a tem-
perature of about 350 C.
To elucidate the scheme of the reaction between
Y O and NH HF , we also performed measurements
2
3
4
2
in the quasi-isothermal mode. Figure 2a shows a typi-
cal thermogravigram of a mixture of Y O and
2
3
NH HF in 1 : 4.5 molar ratio, in which three plateaus
4
2
can be observed. An analysis of the experimental
m values corresponding to each of the plateaus
revealed the following scheme of the process:
Fig. 1. Thermograms of Ln O mixed with NH HF in
2
3
4
2
1 : 6 molar ratio: (T) temperature and ( m) weight loss;
the same for Fig. 2. (a) Yttrium oxide and (b) neodymium
oxide.
Y2O3 + 4.5NH4NF2
(NH4)3Y2F9
(NH4)3Y2F9 + 1.5NH3 + H2O, (1)
m, %
NH4Y2F7 + 2NH3 + 2HF,
2YF3 + NH3 + HF.
(2)
(3)
(a)
NH4Y2F7
The theoretical weight losses corresponding to each
stage of synthesis and amounting to, respectively,
16.5, 31.8, and 39.5 wt % are in good agreement with
the experimental values (Fig. 2a).
T, C
The products formed at 170, 250, and 350 C were
analyzed by the methods mentioned above; the results
obtained confirmed the reaction scheme represented
by Eqs. (1) (3).
m, %
(b)
Figure 3 shows X-ray diffraction patterns of
Y O : NH HF mixtures with the molar ratio of
2
3
4
2
1 : 4.5, measured at different temperatures. The results
of X-ray phase analysis of the products formed at
170 and 250 C (Figs. 3a, 3b) are in good agreement
with [10] and indicate formation of, respectively,
T, C
Fig. 2. Thermograms of Ln O mixed with NH HF :
(a) yttrium oxide, 1 : 4.5 molar ratio, and (b) neodymium
oxide, 1 : 4 molar ratio.
2
3
4
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002

1762
KALINNIKOV et al.
(NH ) Y F and NH Y F . Figure 3c presents an
4 3 2 9
4 2 7
X-ray diffraction pattern of yttrium trifluoride formed
at 350 C.
The results of chemical analysis of intermediate
synthesis products also confirm the formation of
(NH ) Y F and NH Y F .
4 3 2 9
4 2 7
Data on the reaction of Nd O with ammonium tri-
2
3
(a)
fluoride were reported in [10], where a compound of
the composition NH NdF was obtained by keeping
4
4
a mixture of Nd O and NH F at 130 150 C for 1
2
3
4
3 days. Rakov presented in his review [6] data ob-
tained by Polish researchers who established that this
reaction proceeds in stages to give, in the first stage,
the compound (NH ) NdF , and then, at 350 400 C,
4 3
6
NH NdF , which decomposes at 500 C to yield
4
4
(b)
NdF [6].
3
A thermogram of a mixture of Nd O and NH HF
2
2
3
4
in 1 : 6 molar ratio is shown in Fig. 1b. The reaction,
accompanied by a series of endo- and exothermic ef-
fects, is complete at temperatures higher than 270 C.
The results obtained in studies in the quasi-iso-
thermal mode are presented in Fig. 2b. The thermo-
gravigram of a mixture of Nd O and NH HF in
(c)
2
3
4
2
1 : 4 molar ratio is characterized by the presence of
two plateaus. With account of the experimental values
of m corresponding to each plateau, the presumed
scheme of the process is as follows:
2 , deg
Fig. 3. X-ray diffraction patterns of reaction products
formed from Y O : NH HF 1 : 4.5 mixture.
(2 ) Bragg angle. (a) (NH ) Y F , synthesis temperature
a
=
2
3
4
2
Nd2O3 + 4NH4HF2
2NH4NdF4
2NH4NdF4 + 3H2O + 2NH3, (4)
2NdF3 + 2NH3 + 2HF. (5)
4 3 2 9
170 C; (b) NH Y F , 250 C; and (c) YF , 350 C.
4
2 7
3
The theoretical weight losses of 15.6 and 28.7 wt %,
corresponding to reactions (4) and (5), are in good
agreement with the experimental values (Fig. 2b).
The products of synthesis at temperatures corre-
sponding to the plateaus in the thermogravimetric
curve were subjected to X-ray phase analysis. X-ray
diffraction patterns of Nd O : NH HF mixtures with
(a)
2
3
4
2
the molar ratio of 1 : 4 at 150 and 250 C are shown in
Fig. 4. The X-ray diffraction pattern of the product
formed at 150 C is in agreement with the results of
[10] and indicates the formation of NH NdF .
4
4
As an example of the measurements performed, the
table presents the results obtained in determining oxy-
gen in gadolinium fluoride, together with summarized
data on the content of oxygen in yttrium and neo-
dymium fluorides prepared by different methods.
(b)
Fig. 4. X-ray diffraction patterns of reaction products
formed from an Nd O : NH HF 1 : 4.5 mixture.
(a) NH NdF , synthesis temperature 150 C, and (b) NdF ,
With increasing content of gadolinium, neodymi-
um, and yttrium oxides in the melt, the peak corre-
sponding to the oxidation wave of oxygen-containing
=
2
2
3
4
4
4
3
250 C.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002

HYDROFLUORIDE SYNTHESIS OF FLUORIDES
Content of oxygen in REE fluorides
1763
(KCl KF)eut LnF3 melt
Content in LnF3
O2
Method of LnF3 synthesis
LnF3,
wt %
jap,
mA cm
Ln2O3,
wt %
Ln2O3,
c 102, wt %
2
wt %
c
104, mol %
Gadolinium fluoride
Hydrofluoride:
in air
0.867
0.331
0.579
0.708
2.042
0.341
0.214
1.203
0.851
0.981
0.350
0.126
0.578
0.129
0.046
0.017
0.076
0.964
0.343
0.124
0.568
in air with repeated fluorination
in inert atmosphere
Precipitation from solution with sub-
sequent calcination in a vacuum
LnF3 of ultrapure grade
0.116
0.073
0.409
3.376
5.728
1.948
0.577
0.076
0.568
Yttrium fluoride
Hydrofluoride:
in air
0.487
0.699
0.848
1.057
0.528
0.264
2.736
1.368
0.684
5.623
1.956
0.807
1.195
0.416
0.172
13.124
4.565
1.884
in air with repeated fluorination
in inert atmosphere
Neodymium fluoride
Hydrofluoride:
in air
0.878
1.239
1.960
27.590
4.109
0.839
38.680
5.761
1.176
43.959
4.642
0.600
6.273
0.663
0.0856
52.942
5.596
0.723
in air with repeated fluorination
in inert atmosphere
halide complex of a rare-earth metal in voltammo-
grams measured at polarizing voltage sweep rate of
0.1 V s becomes more pronounced. Figure 5a shows
I, A
(a)
1
a typical family of curves for the above process,
measured with gadolinium oxide added to the melt.
On reaching the limiting solubility of REE oxide
(gadolinium oxide in the given case), the anode cur-
rent at the peak corresponding to oxidation of the
oxygen-containing halide complex levels off (Fig. 5a,
a
E, V
curves 5 7). The dependences j = f([Ln O ]) show
p
2 3
a
a portion corresponding to constant j (Fig. 5b). The
p
2
(b)
j, mA cm
initial linear portion (before leveling-off of the anode
current) is described by the equations
j = 294.020c for gadolinium oxide,
j = 38.617c for yttrium oxide,
j = 71.323c for neodymium oxide,
(6)
(7)
(8)
c, wt %
Fig. 5. (a) Voltammograms of oxidation of an oxygen-
containing complex of gadolinium in a KCl KF Gd O
2
3
melt at 735 C on a glassy carbon electrode relative to a
molybdenum quasi-reference electrode and (b) oxidation
current j of oxygen-containing gadolinium complex vs.
Gd O content c in a melt of the KCl KF eutectic at 735 C.
where j is the anode current density at the peak corre-
sponding to oxidation of the oxygen-containing ion
2
(mA cm ), and c is the oxide content of the melt
2
3
1
(a) Polarization rate 0.1 V s . (I) Current and (E) potential.
(wt %).
2
Electrode area (cm ): (1 3) 0.23 and (4 7) 0.27. Gd O
2
3
Addition of an REE oxide to a melt under study in
amounts a fortiori exceeding the maximum soluble
content (wt %): (1) 0.21, (2) 0.425, (3) 0.638, (4) 0.896,
(5) 1.012, (6) 1.32, and (7) 1.617.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002

1764
KALINNIKOV et al.
amount (about 5 wt % in the case in question) and
sampling for determining the content of Ln O made
lowered to a minimum the contaminating effect of
the apparatus.
2
3
it possible to plot the temperature dependences of the
limiting solubility of the REE oxides. The functions
obtained are described by the equations
ACKNOWLEDGMENTS
The study was carried out in accordance with the
project High-Temperature Chemical and Electro-
chemical Syntheses of New Compounds Based on
Less-Common and Rare-Earth Metals, within the
framework of the integrated program of the Russian
Academy of Sciences.
clim = 0.0015T
clim = 0.0136T
clim = 0.0139T
0.0794 for gadolinium oxide, (9)
8.4549 for yttrium oxide, (10)
0.2602 for neodymium oxide, (11)
where c
is the limiting content of Ln O in the
2 3
REFERENCES
lim
melt (wt %), and T is the temperature of the melt ( C).
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tioned REEs made it possible to give preference to
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1998, p. 50.
the hydrofluoride method of LnF preparation in an
3
inert atmosphere [oxygen content of the REE fluoride
4
was (0.12 1.88) 10 mol %].
CONCLUSIONS
(1) Reactions of Y O and Nd O with ammoni-
2
3
2 3
4. Zamoryanskaya, M.V., Petrova, M.A., and Semeno-
va, T.S., Izv. Ross. Akad. Nauk, Neorg. Mater., 1998,
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found to be determined by their content in the starting
oxides; the content of anionic impurities (oxygen-
containing in the first place) can be lowered by creat-
ing an inert or fluorinating atmosphere in synthesis.
A comparison of various methods for the synthesis of
yttrium, gadolinium, and neodymium fluorides as
regards the content of oxygen-containing anionic
impurities in the final products demonstrated that
the best results are obtained with the hydrofluoride
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10
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002