Studies on fluorination of Y2O3 by NH 4HF2

Article abstract of DOI:10.1016/j.tca.2011.03.032

Fluorination of Y₂O₃ by NH₄HF₂ to obtain pure YF₃ is studied in this work by observing the mass changes at room temperature and by employing TGA and DTA at higher temperatures, and observing the subsequent phase changes by X-ray diffraction. Fluorination begins at room temperature with the formation of (NH₄) ₃Y₂F₉ and NH₄F, and continues with the later further fluorinating any leftover Y₂O₃ to ammonium yttrium fluorides with the evolution of NH₃. Similarly, on heating a mixture of Y₂O₃ and NH₄HF ₂, the reaction proceeds sequentially through the formation of (NH₄)₃Y₂F₉, NH₄Y ₂F₇ and finally YF₃. Any Y₂O ₃ still remaining reacts with YF₃ to form yttrium oxyfluorides, which continue as impurity. Substantial evaporation of NH ₄HF₂ is possible even before it could participate in the reaction. This disturbs the stoichiometry of the charge causing oxygen to remain in final YF₃. The compound reported in literature as YF ₃·1.5NH₃ appears to be NH₄Y ₂F₇.

Full text of DOI:10.1016/j.tca.2011.03.032

Thermochimica Acta 520 (2011) 145–152
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Studies on fluorination of Y2O3 by NH4HF2
Abhishek Mukherjeea, Alok Awasthia,∗, Saurabh Mishrab, Nagaiyar Krishnamurthya
a Materials Processing Division, Bhabha Atomic Research Centre, Mumbai 400085, India
b Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorination of Y2O3 by NH4HF2 to obtain pure YF3 is studied in this work by observing the mass changes
at room temperature and by employing TGA and DTA at higher temperatures, and observing the subse-
quent phase changes by X-ray diffraction. Fluorination begins at room temperature with the formation of
(NH4)3Y2F9 and NH4F, and continues with the later further fluorinating any leftover Y2O3 to ammonium
yttrium fluorides with the evolution of NH3. Similarly, on heating a mixture of Y2O3 and NH4HF2, the
reaction proceeds sequentially through the formation of (NH4)3Y2F9, NH4Y2F7 and finally YF3. Any Y2O3
still remaining reacts with YF3 to form yttrium oxyfluorides, which continue as impurity. Substantial
evaporation of NH4HF2 is possible even before it could participate in the reaction. This disturbs the sto-
ichiometry of the charge causing oxygen to remain in final YF3. The compound reported in literature as
YF3·1.5NH3 appears to be NH4Y2F7.
Received 16 October 2010
Received in revised form 30 March 2011
Accepted 31 March 2011
Available online 12 April 2011
Keywords:
Ammonium yttrium fluorides
Rare earth fluoride
Fluorination
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
NH4HF2 is studied in this work. There is no evidence to suggest
solubility of oxygen in YF3 and the absence of yttrium oxyfluo-
Metallothermic reduction of rare-earth oxides results in rare-
earths which contain substantial amount of oxygen [1]. Fluorides
are therefore used for the preparation of rare-earths with less oxy-
gen. Rare-earth fluorides are non-hygroscopic and are therefore
preferred over cheaper chlorides. Rare earths are highly reac-
tive and their preparation has to be done in an oxygen-free inert
atmosphere. If oxygen purity of the starting fluoride could be
ensured besides maintaining the inert atmosphere, then its reduc-
tion should result in a purer rare-earth product.
rides in the product has been considered sufficient to term YF3 as
oxygen-free.
It is known in the literature [5,6] that the fluorination of
rare earth oxides by NH4HF2 occurs as per the following over-
all reaction (1) at temperatures about 300 ◦C. To ensure complete
fluorination, 30% excess NH4HF2 is used than the stoichiometric
amount.
Y2O3 + 6NH4HF2 = 2YF3 + 6NH4F + 3H2O
(1)
Fluorination of rare earth oxides can be done using fluorine
gas (F2) [2,3], hydrogen fluoride gas (HF) [4], aqueous hydrofluoric
acid (HF) [5], ammonium fluoride (NH4F) or ammonium hydro-
gen fluoride (NH4HF2). Of these, fluorine and hydrogen fluoride
are corrosive and poisonous gases and thus are difficult to handle.
Moreover, there remains a possibility of unreacted oxide remaining
in the product [2,6], at least during the static-bed fluorination car-
ried out using these gases. Aqueous hydrofluoric acid is again highly
corrosive. NH4F is highly hygroscopic. Contact of water vapour and
rare-earth fluoride has to be avoided, as there remains a possibil-
ity of oxygen contamination due to pyrohydrolysis of the fluoride
[7,8]. Fluorination of the oxide by NH4HF2 therefore appears to be
the most convenient method for obtaining oxygen-free fluoride.
The preparation of oxygen-free YF3 by the reaction of Y2O3 with
NH4F, H2O and excess NH4HF2 evaporate at these temperatures
and thus only YF3 remains in the solid form.
Kalinnikov et al. [9] have recently studied the changes occur-
ring on heating a mixture of NH4HF2 and Y2O3 in 4.5:1 ratio by
thermal analysis techniques and have found that the fluorination
proceeds through two ammonium yttrium fluorides, (NH4)3Y2F9
and NH4Y2F7. In the present work, the effects of varying the NH4HF2
content of the charge have been studied. The occurrence of fluori-
nation at room temperature is also examined in detail. Attempts are
made to identify the factors, which could lead to oxygen remaining
in the product during scale-up.
Besides being used for fluorinating rare earth oxides, NH4HF2
has also been employed for fluorination of ZrO2 [10–12], BeO [13],
Sr2CuO3 [14], UO2 [15], ThO2 [16], V2O3 [17] etc., and the method-
ologies adopted in the present work may be useful for optimizing
the process parameters for these oxides as well, at least during the
dry processing.
∗
Corresponding author. Tel.: +91 22 25595111; fax: +91 22 25505151.
E-mail address: aawasthi@barc.gov.in (A. Awasthi).
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.03.032

146
A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
2. Experimental
14
12
Yttrium oxide (Y2O3) powder used in this work was of 99.9%
purity. Commercial grade (97% pure) crystals of ammonium bifluo-
ride (NH4HF2) and ammonium fluoride (NH4F) were used. Isolated
compounds (NH4)3Y2F9 and NH4Y2F7 used further in the experi-
ments were made by heating the mixtures of NH4HF2 and Y2O3 in
at least 7.5:1 mole ratio at temperatures 275–375 ◦C for sufficient
duration in argon flow and confirming the products by mass change
and X-ray diffraction.
Samples of NH4HF2 and Y2O3 were prepared by mixing the com-
pounds together in the required ratio until the mixture visually
appeared uniform. The mixtures have been designated according
to their stoichiometry. For example, a mixture containing NH4HF2
and Y2O3 in 3:1 mole ratio is termed 3NH4HF2 + Y2O3 mixture. A
few mixtures were kept at room temperature for varying dura-
tions in plastic bags. Most of the mixtures were treated at high
temperatures.
10
8
6
4
2
NH HF +Y O
2 3
4
2
3NH HF +Y O
4
2
2 3
4.5NH HF +Y O
4
2
2 3
0
6.5NH HF +Y O
4
2
2
3
-2
-4
-6
-8
-10
-12
-14
0
50
100
150
Time in hrs
200
250
300
350
Fig. 1. Mass changes observed in various samples at room temperature with time.
Simultaneous thermogravimetric analysis (TGA) – differential
thermal analysis (DTA) curves for NH4HF2 and a few mixtures of
Y2O3 and NH4HF2 were generated in a commercial unit (model SDT
Q800 of TA Instruments make) with about 10 mg charge heated at
10 ◦C/min rate in a platinum cup under nitrogen flow (100 ml/min
for NH4HF2 and NH4F, and 50 ml/min for the mixtures) and with
alumina as the reference material. The simultaneous TGA–DTA
unit was available only for some experiments. Further experiments
were carried out in a home-made DTA unit with about 0.5 g charge
in static air atmosphere in a vitreous silica cup with alumina as the
reference. The rate of heating was nominally kept 10 ◦C/min in the
home-made unit, although the unit had an initial thermal inertia,
due to which the increase in temperature upto 70 ◦C was slower
and non-linear. Some of the runs carried out in the home-made
DTA unit were stopped after specific DTA events and the phases
present in such products were analysed.
Y O [83-0927]
(NH ) Y F [43-0840]
4 3
NH Y F [43-0847]
4 2 7
2
3
2
9
6.5NH HF +Y O
2 3
4
2
4.5NH HF +Y O
2 3
4
2
3NH HF +Y O
2 3
4
2
NH HF +Y O
4
2
2
3
Phase identification was carried out by X-ray diffraction (XRD)
in an Inel-make unit (model MPD) with Cu-K␣1 radiation at 30 mA
40 kV using a curved position-sensitive detector, thereby observing
the diffraction data in complete 2ꢀ range of interest simultaneously.
The XRD unit was equipped with parabolic mirrors to enhance the
intensity of the X-ray beam. The samples were spinned during the
XRD analysis around the vertical axis, thereby eliminating diffrac-
tion effects due to orientation in r and ꢀ directions. A satisfactory
XRD pattern could be recorded in this unit in 3–4 min.
10
20
30
40
50
60
2θ in degrees
Fig. 2. XRD of samples of various compositions after being kept at room temperature
for about 360 h (exact duration is indicated by the final points in Fig. 1).
in Fig. 1. Table 1 shows the phases identified by XRD in these
old samples and the corresponding changes in the mass. Presence
of ammonium yttrium fluorides in these samples confirmed the
occurrence of a chemical reaction at the room temperature.
However, the relative intensities of some of the peaks of
(NH4)3Y2F9 were not constant in these samples (Fig. 2). Exam-
ination of the isolated compounds using scanning electron
microscope indicated that, unlike NH4Y2F7 and Y2O3, the particles
of (NH4)3Y2F9 apparently had a long needle-like structure. In the
presence of H2O, they probably oriented themselves in a preferred
way in the z-direction while placing the sample on the XRD holder.
The effects of preferred orientation in z-direction could not be elim-
inated by spinning and these apparently caused the variation in the
peak heights of (NH4)3Y2F9.
3. Results and discussion
3.1. Reactions at room temperature
All the samples became reasonably warm during mixing of the
powders, alluding to the occurrence of an exothermic chemical
reaction even at room temperature. Another interesting obser-
vation was made in a few sealed bags containing the samples.
After placing the samples, these bags were completely deflated to
remove the air inside and the seals were closed. Some of the bags
became inflated after some time, thereby indicating that the reac-
tion had a gaseous product. Smell of ammonia could be observed
from some of the bags.
Table 1
The mass changes and the phases observed in the mixtures of NH4HF2 and Y2O3
when kept at room temperature for about 360 h after mixing.
Appreciable mass changes were also observed with time even
at room temperature (Fig. 1), which are possible only if a gas or
vapour is absorbed or desorbed from the samples. The masses of the
samples with NH4HF2:Y2O3 mole ratio of less than 4.5 initially fast
decrease and then become constant with time. However, sample
6.5NH4HF2 + Y2O3 shows increase in mass.
Sample
% Mass change when
XRD was carried out
Phases identified by
XRD (Fig. 2)
NH4HF2 + Y2O3
−9.6
−13.9
−5.4
14.4
NH4Y2F7, Y2O3
(NH4)3Y2F9, Y2O3
(NH4)3Y2F9
3 NH4HF2 + Y2O3
4.5 NH4HF2 + Y2O3
6.5 NH4HF2 + Y2O3
The samples were analysed by XRD (Fig. 2) after being kept at
room temperature for about 360 h as indicated by the final points
(NH4)3Y2F9

A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
147
If material balance for the following reaction (2) is considered,
7NH4HF2 + 2Y2O3 = 2NH4Y2F7 + 6H2O(vapour) + 5NH3 (2)
the mass loss on complete consumption of NH4HF2 present in the
sample of NH4HF2 + Y2O3 should be 9.7%. It is reasonable to con-
sider H2O in vapour form, as the duration of the experiment is long
enough to indeed allow its evaporation. Similarly, the mass loss for
the reaction (3)
9NH4HF2 + 2Y2O3 = 2(NH4)3Y2F9 + 6H2O(vapour) + 3NH3
(3)
in samples 3NH4HF2 + Y2O3 and 4.5NH4HF2 + Y2O3 should be 13.4
and 34.5% respectively. The observed mass losses and the XRD data
(Figs. 1 and 2, Table 1) are consistent with these calculations, if the
occurrence of reaction (2) for sample NH4HF2 + Y2O3 and of reaction
(3) for sample 3NH4HF2 + Y2O3 is considered. However, the mass
loss for sample 4.5NH4HF2 + Y2O3 was substantially lower, whereas
sample 6.5NH4HF2 + Y2O3 showed mass gain.
NH4F is known to be highly hygroscopic and the reason of the
mass gain may lie in this fact. The moisture pickup by NH4F and
NH4HF2 exposed to air was observed; those had gained 76.3% and
3.3% mass respectively in a similar duration. If the following reac-
tion (4) occurs, then not only the product H2O should remain in the
sample, product NH4F should also pickup moisture from air.
Fig. 3. Change in the XRD pattern of NH4HF2 + Y2O3 sample with time at room tem-
perature indicating the formation of (NH4)3Y2F9 first and its conversion to NH4Y2F7
with time.
present work indicate its presence. Room temperature reac-
tions with NH4HF2 resulting in ammonium metallo-fluorides
have recently been reported for uranium oxide [15] and tho-
rium oxide [16]. The XRD patterns of Silva et al. [16] do not
show NH4HF2 and NH4F in the old thorium oxide samples. How-
ever, Yeamans et al. [15] could identify unreacted NH4HF2 in
4.3NH4HF2 + UO2 sample after the complete reaction, which should
result in 0.3NH4HF2 + (NH4)4UF8 + 2H2O at the time of XRD.
XRD patterns of NH4HF2 + Y2O3 sample recorded at different
times since mixing are shown in Fig. 3 and these indicate the devel-
opment of phases with time in the sample. Peak of (NH4)3Y2F9
at 27.10◦ appeared within 30 min of sample preparation. The XRD
pattern recorded after three days had all the peaks of (NH4)3Y2F9
and also of NH4Y2F7 (Note: The occurrence of twin peaks around
15◦ and 27.5◦ in the XRD pattern of NH4Y2F7 is discussed in Sec-
tion 3.6). The peaks of (NH4)3Y2F9 completely disappeared in the
pattern recorded after fifteen days and the peaks of NH4Y2F7 fully
developed. This indicates that (NH4)3Y2F9 forms first and gets sub-
sequently converted to NH4Y2F7 by reacting with remaining Y2O3
according to reaction (6). The overall reaction can still be repre-
sented by reaction (2), which in fact is the combination of reactions
(3) and (6).
6NH4HF2 + Y2O3 = (NH4)3Y2F9 + 3H2O + 3NH4F
(4)
If the moisture pickup data by the product NH4F and also by the
remaining NH4HF2 (in case of sample 6.5NH4HF2 + Y2O3) is con-
sidered, samples 4.5NH4HF2 + Y2O3 and 6.5NH4HF2 + Y2O3 should
gain 13.6% and 14.4% mass respectively on completion of reaction
(4). The observed mass change in 4.5NH4HF2 + Y2O3 sample was in
between the calculated mass changes for reactions (3) and (4), so
probably both of these reactions occurred in this sample. Similarly,
the mass gain data for sample 6.5NH4HF2 + Y2O3 matches with the
calculated data for reaction (4), thereby indicating the occurrence
of this reaction. Also, the XRD data is noisier for this sample (Fig. 2),
apparently due to Compton scattering because of the presence of
H2O [18].
However, neither the excess reactant NH4HF2 nor the product
NH4F was detected in the presence of yttrium compounds by XRD
(Table 1, Fig. 2). Yttrium atom due to its substantially higher atomic
number (Z) scatters X-rays very strongly as compared to hydrogen,
nitrogen or fluorine atoms [18], and therefore the peaks of yttrium
containing compounds in XRD are substantially stronger than those
of the compounds of low-Z atoms like NH4HF2 and NH4F. XRD of a
mixture of NH4HF2 and (NH4)3Y2F9 in as high a mole ratio as 9:5
was recorded; no peaks of NH4HF2 were observed.
7(NH4)3Y2F9 + 2Y2O3 = 9NH4Y2F7 + 6H2O(v) + 12NH3
(6)
It could thus be concluded that NH4HF2 reacts with Y2O3 even
at room temperature to form ammonium yttrium fluorides with
NH4F and H2O as the products. NH4F too is not stable in the pres-
ence of Y2O3. If Y2O3 unreacted with NH4HF2 remains and NH4F
is also present in a sample, then following reaction (5) also occurs.
Reactions (4) and (5) combine to give overall reaction (3).
Similarly, only (NH4)3Y2F9 (and not NH4Y2F7) and Y2O3 were
observed even in freshly-prepared and quickly-analysed samples
of 3NH4HF2 + Y2O3, 4.5NH4HF2 + Y2O3 and 6.5NH4HF2 + Y2O3; no
NH4Y2F7 was observed in such samples. This further confirms that
(NH4)3Y2F9 forms earlier than NH4Y2F7, even when the other prod-
uct is NH4F and not NH3.
9NH4F + Y2O3 = (NH4)3Y2F9 + 3H2O + 6NH3
(5)
3.2. Home-made DTA unit and observing the room temperature
reaction
To further confirm that NH4F too reacts with Y2O3 even at room
temperature, a mixture of NH4F and Y2O3 in 6:1 mole ratio was
made and analysed by XRD after being kept at room temperature
for some time. The XRD showed (NH4)3Y2F9 peaks apart from Y2O3
peaks.
The occurrence of a reaction between Y2O3 and NH4HF2 at room
temperature was reported by Patwe et al. [19]. However, they had
identified the product as (NH4)3YF6·1.5H2O, having the same XRD
pattern, due to being isostructural, as that of (NH4)3LaF6·1.5H2O.
However, neither this yttrium compound was confirmed by the
subsequent researchers, nor the XRD patterns recorded in the
Before discussing the results further, it may be appropriate to
mention here that the sensitivity of the home-made DTA unit was
inferior as compared to that of the professionally made simul-
taneous TGA–DTA unit. As mentioned earlier, the increase in
temperature upto 70 ◦C was slower and non-linear in the home-
made unit, whereas the temperature rise was linear right from the
room temperature in the combined TGA–DTA unit. When the flu-
orine content of the charge was more (for example, in samples
with NH4HF2 to Y2O3 mole ratio of 3:1 or more), the interac-

148
A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
tion of fluorine with the quartz cups slightly influenced the DTA
curves recorded in the home-made unit, as indicated by the kinks
in the baselines in Fig. 4. However, the XRD patterns of the prod-
ucts remaining in the DTA cup were not affected, as the product of
fluorination of quartz is gaseous at these temperatures. The pow-
der collected from the cooler portions of the unit was identified as
(NH4)2SiF6, whereas the product remaining in the DTA cup did not
have XRD peaks of any silicon compound.
According to thermodynamic data, YF3 is more stable than Y2O3
[20]. Tischer and Burnet [2] have described the preparation of YF3
by the reaction of Y2O3 with fluorine gas. This reaction should pro-
ceed through YOF and YF3·6–4YOF compounds sequentially with
oxygen evolution. Oxidation of yttrium fluoride or the oxyfluorides
in air would therefore not be possible and thus the absence of the
inert atmosphere in the home-made DTA unit should not make a
difference. However, as the runs were carried out in static air in this
unit, the beginning of a reaction having a gaseous product would
be recorded at a higher temperature.
NH HF
(a)
(b)
4
2
20%
NH F
4
NH HF
4
2
NH F
4
5%/min
o
145 C
Fig. 4(a and b) shows the DTA curves of the freshly pre-
pared (charged within 5 min of mixing) and the one-day old
3NH4HF2 + Y2O3 charges obtained respectively in the home-made
DTA unit. An exothermic peak appeared in the DTA of the fresh
charge just above the room temperature, corresponding to the for-
mation of (NH4)3Y2F9 as discussed in Section 3.1. This peak did not
appear for the one-day old charge, as the formation of (NH4)3Y2F9
was over at room temperature by the time the DTA was recorded.
Fig. 4 also indicates that a slight increase in temperature makes the
kinetics of the room temperature reaction faster. A similar exother-
mic peak was observed by Kalinnikov et al. [9].
NH HF
o
4
2
125 C
o
NH F
4
110 C
Exo
(c)
o
165 C
10μV
Endo
100
200
300
400
500
600
Temperature in oC
3.3. Investigation of high temperature events
Fig. 5. Curves obtained by simultaneous TGA–DTA of NH4HF2 and NH4F; (a) TGA
curves (b) DTG curves and (c) DTA curves.
The curves obtained by simultaneous TGA–DTA of NH4HF2 and
NH4F are shown in Fig. 5 and those obtained with freshly-prepared
mixtures of NH4HF2 to Y2O3 mole ratios of 1–6.5 are given in Fig. 6.
Differential thermogravimetry (DTG) data obtained by differentiat-
ing the TGA data has also been included in Figs. 5 and 6. A few events
are seen in all the samples. However, boiling of H2O at 100 ◦C is not
clearly recorded in these curves (Fig. 6) as it evaporates continu-
ously and very less amount of it is available by the time its boiling
point is reached.
ever, as is apparent from the TGA and the DTG curves (Fig. 5a and
b), appreciable evaporation of NH4HF2 began even before its melt-
ing point was reached, thereby indicating that it has a high vapour
pressure. The next DTA event began around 145 ◦C for NH4HF2,
and led to its complete evaporation as indicated by TGA, appar-
ently due to boiling. Similarly, the simultaneous TGA–DTA curves
of NH4F (Fig. 5) indicated its melting and boiling points to be 110
and 165 ◦C respectively, and its appreciable evaporation right from
its melting point onwards.
3.3.1. TGA–DTA curves of NH4HF2 and NH4F
Melting point of NH4HF2 is reported as 126.5 ◦C [21]. The first
peak in the DTA curve (Fig. 5c) of NH4HF2 was endothermic; it
appeared at 125 ◦C and thus could be ascribed to melting. How-
3.3.2. Simultaneous TGA–DTA curves of mixtures–till ammonium
fluorides are present
The first DTA peaks of all the samples (Fig. 6c) corresponded
more or less with the melting of NH4HF2, considering the difference
in the flowrate of the cover gas for the runs of NH4HF2 and the
mixtures.
Exo
d
c
Endo
The second DTA peaks were exothermic. These began at the tips
of the first peaks and overlapped with the third peaks as well in all
the samples and thus were not very apparent. However, as observed
from the DTG curves (Fig. 6b), there was an increase in the rate of
mass loss when the second peaks began, and this independently
confirmed the occurrence of a DTA event. Like the exothermic event
observed in the DTA curve (Fig. 4) recorded in the home-made unit,
the event corresponding to these exothermic peaks also appears to
be the reaction of NH4HF2 with Y2O3. However, the peaks appeared
at a higher temperature in the combined TGA–DTA unit as the effec-
tive rate of heating here was higher, considering the initial thermal
inertia of the home-made DTA unit. Although the reaction must
have begun right at the room temperature, its rate became observ-
able only after the melting of NH4HF2 as the contact surface for the
reaction to occur then increased. Increase in the rate of a reaction
b
a
100
200
300
400
500
Temperature in oC
Fig. 4. DTA generated in the home-made DTA unit, (a) fresh 3NH4HF2 + Y2O3 sample,
(b) one-day old 3NH4HF2 + Y2O3 sample, (c) (NH4)3Y2F9 and (d) NH4Y2F7. The kinks
in the baseline are due to interaction of quartz with fluorides of the charge.

A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
149
YF [74-0911]
3
(NH
)
Y F
NH Y F
4 2 7
4 3
2
9
(a)
S6_5
S4_5
S3
S1
a)
o
450 C
o
282 C
S1
S3
6
o
235 C
S4_5
S6_5
room temperature
40
10
20
NH
30
50
2θ
2
YF
(
)
Y F
NH Y F
Y O
YF .4-6YOF [80-1124]
3
5%/min
(b)
3
4 3
2
9
4
2
7
2
3
3
3
b)
5
5
S1
S3
1
4
6
2
S4_5
S6_5
o
500 C
6
1
4
o
270 C
2
Exo
3
10 V
c)
room temperature
40
Endo
600
100
200
300
400
500
10
20
30
50
2θ
Temperature in oC
Fig. 7. XRD patterns obtained after heating (a) 7.8NH4HF2 + Y2O3 and (b)
3NH4HF2 + Y2O3 samples to various temperatures in home-made DTA unit. (Section
3.6 may be referred regarding the peaks of NH4Y2F7.)
Fig. 6. Curves obtained by simultaneous TGA–DTA of freshly prepared mixtures
with NH4HF2 to Y2O3 mole ratios of 1–6.5; (a) TGA curves (b) DTG curves and (c)
DTA curves. Samples are indicated as per the following, S1: NH4HF2 + Y2O3, S3:
3NH4HF2 + Y2O3, S4 5: 4.5NH4HF2 + Y2O3, S6 5: 6.5NH4HF2 + Y2O3. The beginning
of the DTA events is marked by an arrow and the peak number for S1 and S6 5 as
representative indication and the events for other samples can be correspondingly
identified. Most of the events are identifiable on the DTG curves and not-so-apparent
ones are marked there as well.
3.3.3. Simultaneous TGA–DTA curves of mixtures–after complete
evaporation of ammonium fluorides
To help in assessing the events occurring beyond the third peaks
obtained in the simultaneous TGA–DTA unit (Fig. 6), a few sam-
ples of 7.8NH4HF2 + Y2O3 and 3NH4HF2 + Y2O3 compositions were
heated in the home-made DTA unit to temperatures until a DTA
event was crossed for each sample. The XRD pattern obtained after
this heating and the maximum temperature to which the sample
was heated are given in Fig. 7(a) for 7.8NH4HF2 + Y2O3 samples and
in Fig. 7(b) for 3NH4HF2 + Y2O3 charges.
Fig. 6c indicates that the fourth DTA peaks for all the composi-
tions were over around 260 ◦C. The product obtained after heating
7.8NH4HF2 + Y2O3 sample upto 282 ◦C was NH4Y2F7 (Fig. 7a),
whereas a mixture of NH4Y2F7 and excess Y2O3 was obtained on
heating 3NH4HF2 + Y2O3 sample upto 270 ◦C (Fig. 7b). Thus, the
event corresponding to the fourth peak could be the complete
decomposition of (NH4)3Y2F9 to NH4Y2F7 according to the follow-
ing reaction (7).
on melting of one of the reactants has been observed in other sys-
tems as well [22]. The conversion to (NH4)3Y2F9 is complete in the
second peaks either due to the complete consumption of Y2O3 or
of NH4HF2/NH4F according to reactions (3–5).
The third DTA peaks (Fig. 6c) were endothermic and appeared
around 140–155 ◦C to indicate the boiling of excess NH4HF2, if still
present, and boiling of NH4F product of reaction (4). The third peaks
became appreciably bigger with NH4HF2 content of the starting
mixture, as the amount of NH4F forming or NH4HF2 remaining
unreacted in the charge till this temperature was higher. This fur-
ther alludes that the third peaks are not associated with any change
in the yttrium compounds. As evaporation is associated with mass
loss, the DTG peaks corresponding to the third DTA peaks also
increased in size with NH4HF2 content of the charge (Fig. 6c).
When the DTA of isolated (NH4)3Y2F9 was recorded in the home-
made DTA unit, only two peaks, occurring beyond about 200 ◦C,
were observed (Fig. 4(c)). This is in agreement with the explanation
that it is only (NH4)3Y2F9 which remains after the event of the third
peaks.
(NH4)3Y2F9 = NH4Y2F7 + 2(NH3 + HF)
(7)
It is not clear whether the evaporating species is NH4F or a mix-
ture of NH3 and HF gases at reaction temperatures; it is therefore
indicated as (NH3 + HF) in reaction (7) and further discussions. A
comment on its nature is made in Section 3.3.4. The peak-begin

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A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
temperature of the fourth peak varied between 200 and 215 ◦C
and increased with the NH4HF2 content of the starting charge.
Apparently, the onset of the reaction was delayed till the vapours
generated due to the evaporation of excess NH4HF2 left the reaction
zone.
Table 2
Products obtained on heating the mixtures of NH4HF2 and Y2O3 upto 500 ◦C.
Mixture composition
Products obtained
NH4HF2 + Y2O3
1.4NH4HF2 + Y2O3
YF3.4-6YOF, YOF, Y2O3
YF3.4-6YOF
2NH4HF2 + Y2O3
3NH4HF2 + Y2O3
4.5NH4HF2 + Y2O3 (and more NH4HF2)
YF3.4-6YOF
YF3.4-6YOF, YF3
YF3
As the temperature was increased further, two events appeared
to be occurring, one endothermic (fifth peaks) and the other
exothermic (sixth peaks) (Fig. 6c). The endothermic DTA peaks
were prominent when the charge had more NH4HF2. The peak was
not very apparent in the case of NH4HF2 + Y2O3 charge. The exother-
mic events followed the endothermic peaks. The exothermic event
was distinct in the case of NH4HF2 + Y2O3 charge, and appeared as
a broad peak of less height for the other charges.
When the DTA of isolated NH4Y2F7 recorded in the home-made
DTA unit (Fig. 4d), only one endothermic peak appeared around
350 ◦C and the product was analysed as YF3. The fifth DTA peaks
were endothermic and appeared around 305 ◦C (Fig. 6c) and, there-
fore, could be assigned to the decomposition of NH4Y2F7 of the
charge (Fig. 7a and b) to YF3 according to reaction (8).
peaks, although the DTG curves became horizontal, they did not
reach the zero value. This indicates that the equilibrium pressures
of (NH3 + HF) in reactions (7) and (8) were not negligible even
before the beginning of these peaks. In accordance with Le Chate-
lier’s principle, the reactions could slowly proceed rightwards even
at temperatures lower than that indicated by the DTA. Holding
(NH4)3Y2F9 at 185 ◦C for 4 h in the home-made DTA unit could
indeed yield a mixture of (NH4)3Y2F9 and NH4Y2F7. It is known
in the literature [5,6] that reaction (1) is carried out at 300 ◦C and
takes about 12 h to complete. However, for achieving faster reaction
rates, it is better to have a temperature higher than the peak-begin
temperature, so that the pressure of (NH3 + HF) is more than one
atmosphere.
Wang et al. [24] have reported the formation of nano-sized
YF3 of various shapes at room temperature by treating yttrium
nitrate with NH4F or NH4HF2 and cleaning the product by repeated
centrifuge filtering and ultrasonic cleaning. Ammonium yttrium
fluorides have been prepared by the reaction of yttrium nitrate
with NH4F [25]. Apparently, ultrasonic cleaning with water and
centrifuging caused effective removal of (NH3 + HF) leading to com-
pletion of reactions (7) and (8) at room temperature in Wang et al.’s
[24] experiments.
NH4Y2F7 = 2YF3 + (NH3 + HF)
(8)
The sixth and exothermic peaks appear to be occurring due to
the reaction between YF3 and the remaining Y2O3 (Fig. 7b) to
form yttrium oxyfluorides YF3·xY2O3 (corresponding to YOF or
YF3·4–6YOF) according to reaction (9).
YF3 + xY2O3 = YF3·xY2O3
(9)
Since reaction (9) is a solid-state reaction, the formation of yttrium
oxyfluorides was slow, as indicated by the broad DTA peaks. This
reaction (9) does not involve a mass change and therefore no
corresponding change on the DTG curves was observed for all
the samples but NH4HF2 + Y2O3. In case of NH4HF2 + Y2O3 charge,
reaction (9) began even before the decomposition of NH4Y2F7
was complete apparently due to a substantially higher amount
of Y2O3 being present in the sample. It may be noted here that,
thermodynamically, NH4Y2F7 and Y2O3 should react to directly
form YF3·xY2O3 at a temperature lower than the temperature
required for NH4Y2F7 decomposition to YF3. However, the kinet-
ics of YF3 formation appears to be much faster in comparison
to that of the direct formation of oxyfluorides, as is apparent
from the occurrence of the two identifiable DTA peaks (fifth and
sixth). It may be repeated here that Y2O3 can remain even in sto-
ichiometric samples due to the evaporation of NH4HF2 and this
apparently caused the small sixth peaks even in samples with
higher NH4HF2 content (Fig. 6). In their work, Kowalczyk et al.
[23] have assigned the final exothermic peak to the conversion of
YF3·1.5NH3 (actually NH4Y2F7 as discussed in Section 3.6) to YF3.
They apparently have not considered the hyperstoichiometry of
yttrium oxide resulting in the charge due to the higher evapora-
tion of the fluorinating agent as a result of different rates of heating
employed.
3.4. Products obtained with varying amounts of NH4HF2 in
charge
A few mixtures having different amounts of NH4HF2 were
heated upto 500 ◦C and the products thus obtained were identi-
fied. The results are presented in Table 2. A part of the equilibrium
ternary diagram of NH4F-YF3–Y2O3 system is constructed (Fig. 8)
based on these results, which is valid upto the temperature of sta-
bility of the constituents in condensed forms. Mann and Bevan [7]
could identify three compounds in with YF3·4YOF, YF3·5YOF and
YF3·6YOF compositions having very similar XRD patterns. As these
compounds have similar crystal lattices with differences only in
ordering, these have been indicated as one compound with the
composition range of YF3·4–6YOF. No indication of the existence
Y O
2 3
3.3.4. Identity of (NH3 + HF) and its removal at lower temperature
After the runs of heating 7.5–9NH4HF2 + Y2O3 charges at scores
of gram level at 275–375 ◦C were over, substantial amount of con-
densate was found at cooler portions of the setup. Product YF3
remained in the hot zone of the setup, whereas excess NH4HF2 and
product (NH3 + HF) evaporated away and condensed in the cooler
parts. This condensate was analysed to be NH4HF2 + NH4F mixture
by XRD. This still does not prove that the evaporating species of
reactions (7) and (8) are indeed NH3 and HF gases, but even if they
are, it is certain that they convert and condense to NH4F on cooling.
When there is no mass change in a sample, the value of DTG
should be observed as zero. Horizontal lines have been drawn in
the DTG curves in Fig. 6b corresponding to the zero values. It is
observed that prior to the beginning of the fourth and the fifth
YOF
YF .4-6YOF
3
NH Y F
2 7
YF
4
(NH ) Y F
4 3
3
2
9
Fig. 8. A portion of the equilibrium ternary diagram of NH4F-YF3–Y2O3 system
proposed from experimental observations of the present work.

A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
151
YF .1.5NH [28-1449]
3
3
NH Y F [43-0847]
4 2 7
10
15
20
25
30
35
2 in degrees
Fig. 9. XRD of the isolated NH4Y2F7 alongwith JCPDS data on YF3·1.5NH3 and NH4Y2F7.
of ammonium metallo-oxyfluorides compounds like NH4YxOyFz
(actually NH4F·xYOF or NH4F·x(YF3·4–6YOF)), which do not appear
in the JCPDS data, was found for yttrium metal (Fig. 7, Table 2).
The heating of NH4HF2 + Y2O3 charge yielded a mixture of
YF3·4–6YOF, YOF and Y2O3 (Table 2), although this charge is stoi-
chiometric for YOF formation. This further indicates that the solid
state reactions between yttrium oxide, oxyfluorides and fluoride
are slow as compared to the formation of yttrium fluoride from
ammonium yttrium fluorides (Section 3.3.3).
Similarly, 3NH4HF2 + Y2O3 charge is stoichiometric for the for-
mation of YF3, if NH3 (and not NH4F) is the product of the
fluorination reaction. However, YF3·4–6YOF was also obtained
apart from YF3 upon heating this mixture, as some amount of
NH4HF2 and probably also of NH4F intermediate evaporates during
heating without participating in fluorination (Section 3.3.2).
It may now be appropriate to compare NH4F and NH4HF2 as
the fluorinating agents. Fluorination by NH4F would also occur
sequentially, beginning from reaction (5) and proceeding through
reactions (6) if necessary, (7) and (8). Even if the formation of
NH4OH, a condensed phase, is considered by the reaction between
the products NH3 and H2O, reaction (5) remains a solid–gas reac-
tion, as three moles of NH3 still remain in the gaseous form. The
removal of ammonia is necessary for reaction (5) to proceed and
large surface would be favourable. On the other hand, reaction (4)
does not involve a gas and thus does not depend on the availabil-
ity of an open surface. The dissolution of the reactant NH4HF2 in
the product H2O helps in its better spreading to untouched por-
tions of Y2O3 due to wetting, thereby reducing the effects of the
possible pockets of excess Y2O3 caused due to imperfect mixing.
Therefore, fluorination with NH4HF2 should be inherently faster
and more complete. No NH3 is discharged to the atmosphere in
reaction (4) and NH4F product could be regenerated to NH4HF2 by
reacting it with HF.
3.5. Conditions for obtaining pure yttrium fluoride
It may be appropriate to mention here that, as fluorination of
yttrium oxide using NH4F and that using NH4HF2, both follow the
same paths reaction (5) onwards, and as the evaporation of NH4F is
substantial even before its boiling temperature, many researchers
have mistakenly assigned the first endothermic peak to the decom-
position of NH4F to NH4HF2 and NH3 [26–28].
It is clear from the earlier discussions that the formation of
YF3 by fluorination of Y2O3 using NH4HF2 proceeds sequentially
through reactions (4)–(8). If NH4HF2 content of the starting charge
were sufficient to allow the complete consumption of Y2O3 in reac-
tion (4) and also sufficient time were given for reaction (4) to
complete, reactions (5) and (6) are omitted from this sequence.
If the temperature of the charge is substantially increased before
the completion of reaction (4), some NH4HF2 would evaporate due
to its high vapour pressure leaving some Y2O3 unreacted, which
would ultimately contaminate the final product, YF3. Therefore,
for obtaining oxygen-free YF3, it is best to ensure the full conver-
sion to (NH4)3Y2F9 and complete disappearance of Y2O3 at either
room temperature or slightly higher temperature when the evap-
oration of NH4HF2/NH4F is negligible. The use of excess NH4HF2 in
the starting mixture may not be fully necessary. Once Y2O3 of the
charge gets completely converted to (NH4)3Y2F9, it could then be
fast heated to temperatures above 400 ◦C along with the use of a
sweeping gas for quicker formation of YF3. The bigger level runs
for preparation of YF3 in the present work were typically over in
about 45 min at 400 ◦C. The exact time required for complete fluo-
rination may substantially vary from reactor to reactor, as it would
depend on the gas sweeping rate, area of the gas–solid interface,
whether condensation is possible in cooler regions etc. apart from
the temperature in the reaction zone.
3.6. Observations on the stoichiometry of ammonium yttrium
fluorides
Russo and Haendler [25] have reported (NH4)3Y2F9·H2O com-
pound, which they prepared by reacting aqueous solutions
of yttrium nitrate with NH4F. This compound converted to
(NH4)3Y2F9 at 92 ◦C. They have given the intensities of its XRD peaks
qualitatively [25] [JCPDS 28-0098]. As H2O also forms alongwith
(NH4)3Y2F9 in reactions (3) and (4), we have considered the pos-
sibility of the formation of this hydrated compound. However, the
XRD data of (NH4)3Y2F9·H2O is similar to that of (NH4)3Y2F9 [JCPDS
43-0840]. It is quite probable that the variation in relative intensi-
ties of the two data was due to preferred orientation as discussed
in Section 3.1 and the slight difference in d-spacings was due to the
use of a different equipment. Reactions (3) and (4) were over by the
time 92 ◦C was reached in DTA runs carried out in the home-made-
unit (Fig. 4(a and b)); no event at this temperature was observed.

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A. Mukherjee et al. / Thermochimica Acta 520 (2011) 145–152
It is possible that Russo and Haendler [25] assigned the DTA event
corresponding to the evaporation of free water to the dehydration
of (NH4)3Y2F9·H2O.
7. No indication of the existence of ammonium yttrio-oxyfluorides
(NH YxOyFz) was found.
8. YF3·41.5NH3 is actually NH4Y2F7. Also, (NH4)3Y2F9·H2O is possi-
bly (NH4)3Y2F9.
According to JCPDS database, the XRD patterns of NH4Y2F7
[JCPDS 43-0847] and YF3·1.5NH3 [JCPDS 28-1449] are quite sim-
ilar in d-spacings. Apart from the occurrence of some additional
peaks of low intensity in YF3·1.5NH3, its two highest peaks are actu-
ally twin peaks, at 0.325 and 0.322 nm (corresponding to 27.44 and
27.70◦ two-theta values for CuK␣1) and at 0.585 and 0.597 (corre-
sponding to 14.84 and 15.15◦ two-theta). The two highest peaks
of NH4Y2F7 have been reported to occur at 0.326 and 0.597 nm
(corresponding to 27.36 and 14.84◦) in this database. Clear twin
peaks around these angles are seen in the XRD of NH4Y2F7 in
Figs. 3 and 7(a and b). XRD of the isolated NH4Y2F7 had clear twin
peaks at 14.94◦ and 15.25◦ and at 27.36◦ and 27.61◦ (Fig. 9). Heating
about ten grams of this compound at 450 ◦C till there was no further
mass change resulted in YF3 with 12.5% mass loss, against the sto-
ichiometric mass loss of 11.2 and 14.9% on conversion of NH4Y2F7
and YF3·1.5NH3 to YF3, respectively. As there is no possibility of
mass gain due to contamination or so in a carefully carried-out
experiment with handling losses being still possible, this hints that
the compound was indeed NH4Y2F7 and not YF3·1.5NH3. JCPDS data
on YF3·1.5NH3 has been quoted from Markovskii et al. [26], who
obtained this compound by heating 3NH4F + YOF mixture at 210 ◦C
for 1 h and also 6NH4F + Y2O3 mixture at 220 ◦C for 0.5 h. JCPDS
data on NH4Y2F7 is taken from the work of Rajeshwar and Secco
[29], who prepared the compound by reacting Y2O3 with NH4F and
decomposing the (NH4)3Y2F9 thus obtained. Apart from assess-
ing its stoichiometry by material balance calculations, Rajeshwar
and Secco [29] also obtained the infrared spectrum of the com-
pound, which had characteristics of NH4+. It is quite possible that
Markovskii et al. [26] and Rajeshwar and Secco [29] both obtained
the same compound and the later missed the close adjacent peaks.
In the present work, therefore, the compound is designated as
NH4Y2F7 despite the occurrence of the twin peaks, which were
observable apparently due to good peak-to-noise ratio of the XRD
unit resulting from the use of parabolic mirror.
9. For obtaining oxygen-free YF3, it is better to first ensure the com-
plete conversion of the oxide to (NH4)3Y2F9. The use of excess
NH4HF2 in the starting mixture may not be fully necessary.
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4. Conclusions
1. Fluorination of Y2O3 with NH4HF2 proceeds sequentially
through the formation of (NH4)3Y2F9, NH4Y2F7 and finally YF3.
2. Reaction of NH4HF2 with Y2O3 begins at room temperature with
appreciable rate. (NH4)3Y2F9 and NH4F form first. Like NH4HF2,
NH4F also is highly unstable in the presence of Y2O3. It further
participates in the reaction right from the room temperature
with the release of NH3.
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4. NH4HF2 has a high vapour pressure. Its substantial evaporation
before it could react with Y2O3 is possible during fast heating.
5. The decomposition temperatures of (NH4)3Y2F9 to NH4Y2F7 and
of the later to YF3 are indicated as 200–215 ◦C and 305 ◦C by
DTA. However, due to the high vapour pressure of the evap-
orating species, it is possible to obtain observable amounts of
the decomposition products at lower temperatures after suffi-
cient time. Completion of these reactions noticeably depends on
removal of the vapours from the reaction zone.
6. If Y2O3 is present even after the formation of YF3, then it finally
reacts with it to form yttrium oxyfluoride (YOF or YF3·4–6YOF).
Oxygen remains in this form as an impurity in YF3.