Soft-chemistry synthesis and characterization of bismuth oxyfluorides and ammonium bismuth fluorides

Article abstract of DOI:10.1111/j.1551-2916.2005.00721.x

Two bismuth oxyfluorides, BiOF and BiO_0.5F₂, have been prepared by soft chemistry. These two compounds are of interest for application as new type of positive electrode active materials in secondary lithium battery. BiOF was obtained directly from the fluorination of Bi ₂O₃ by a saturated aqueous solution of NH₄F. BiO_0.5F₂ was obtained by the hydrolysis of β-NH ₄BiF₄ at 70°C. The ammonium bismuth fluoride β-NH₄BiF₄, which is a new orthorhombic form of NH₄BiF₄, was obtained by reacting BiF₃ in a saturated aqueous solution of NH₄F. Two other ammonium bismuth fluorides, (NH₄)_0.5BiF_3.5 and NH ₄Bi₃F₁₀, were also prepared by annealing β-NH₄BiF₄ at a relatively low temperature under inert atmosphere. The existence of the fluorite (NH₄) _0.5BiF_3.5 is reported for the first time in this paper.

Full text of DOI:10.1111/j.1551-2916.2005.00721.x

J. Am. Ceram. Soc., 89 [2] 645–651 (2006)
DOI: 10.1111/j.1551-2916.2005.00721.x
r 2005 The American Ceramic Society
ournal
J
Soft-Chemistry Synthesis and Characterization of Bismuth Oxyfluorides
and Ammonium Bismuth Fluorides
M. Bervas,w,z,y B. Yakshinskiy,z L. C. Klein,y and G. G. Amatucciz,y
Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854
Two bismuth oxyfluorides, BiOF and BiO0.5F2, have been pre-
pared by soft chemistry. These two compounds are of interest
for application as new type of positive electrode active materials
in secondary lithium battery. BiOF was obtained directly from
the fluorination of Bi2O3 by a saturated aqueous solution of
NH4F. BiO0.5F2 was obtained by the hydrolysis of b-NH4BiF4
at 701C. The ammonium bismuth fluoride b-NH4BiF4, which is
a new orthorhombic form of NH4BiF4, was obtained by reacting
BiF3 in a saturated aqueous solution of NH4F. Two other
ammonium bismuth fluorides, (NH4)0.5BiF3.5 and NH4Bi3F10,
were also prepared by annealing b-NH4BiF4 at a relatively low
temperature under inert atmosphere. The existence of the fluo-
rite (NH4)0.5BiF3.5 is reported for the first time in this paper.
state synthesis using Bi2O3 and BiF3 as precursors. As demon-
strated by Livshits et al.,11 who have conducted an 19F NMR
study on the compound, BiO0.5F2 is an excellent fluorine ion
conductor. The fluorite structure of BiO0.5F2 is considerably
defective as fluorites have an MX2 stoichiometry, but a complete
structural analysis has never been performed.
We were interested in preparing BiOF and BiO0.5F2 via a so-
lution synthesis technique, at room temperature without the use
of HF. As described in this paper, this was made possible by the
fluorination of Bi2O3 or BiF3 by a saturated aqueous solution of
NH4F. Our preference for NH4F over HF is justified by the
facts that, although great caution is also necessary, NH4F is
much safer to use than HF (MSDS Health rating ‘‘High’’ for
NH4F and ‘‘Extreme’’ for HF12) and easier to handle as it is a
solid rather than a liquid. Moreover, the use of NH4F as pre-
cursor allowed us to isolate new compounds of the ammonium
bismuth fluoride and oxyfluoride families.13
During the preparation of BiOF and BiO0.5F2, we also iso-
lated a variety of ammonium bismuth fluorides compounds,
some of them previously unknown. These compounds are also
reported in this paper. These compounds were characterized by
means of combined X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FT-IR), thermal gravimetric analysis
(TGA), differential scanning calorimetry (DSC), and X-ray
photoelectron spectroscopy (XPS).
I. Introduction
S a continuation of the work carried out by our group
on iron fluoride/carbon nanocomposite, where it was
A
shown for the first time that reversible conversion reactions
with lithium were possible at high voltage for energy storage,1
we investigated conversion reactions in other metal fluorides.
We have recently reported that a reversible conversion reaction
occurs at high voltage in the bismuth fluoride/carbon nano-
composite.2,3 To gain a better understanding of the system
and to show the influence of the ionicity of the metal fluoride
bond on the output voltage we also came to study bismuth
oxide and oxyfluorides. We have demonstrated that reversible
conversion was also possible in those compounds, and that
BiOF and BiO0.5F2 are very interesting active materials as
they combine the good electrochemical activity of the oxide
with the high voltage of the fluoride.4 This paper relates
the synthesis of these two bismuth oxyfluorides compounds
through soft chemistry and the investigation of new ammoni-
um bismuth fluorides intermediates formed during the fabrica-
tion process.
II. Experimental Procedure
(1) Physical Characterization
XRD was performed on a Scintag X2 (Cupertino, CA) with
CuKa radiation. All the patterns presented in this paper were
obtained under the identical conditions of 0.81/min in continu-
ous scan, between 101 and 651 in 2y.
FT-IR analyses were conducted on a Thermo Nicolet Avatar
360 (Waltham, MA). Transmittance spectra were obtained with
the KBr pellet technique with a dilution of about 1/100. Atten-
uated total reflectance (ATR) experiments were performed on
the Smart Endurance accessory of the Avatar 360. A back-
ground was collected before each sample spectrum. Fifty scans
were taken for the background and each spectrum in the ATR
mode and 100 in the transmittance mode.
TGA and DSC analyses were conducted on the Q50 and
Q100 instruments from TA (TA Instruments, New Castle, DE),
respectively. The experiments were conducted under argon
between room temperature and 4201C at a scanning rate of
101C/min. The sample pans were in aluminum in both cases.
XPS quantitative analyses were performed using an XSAM
800 KRATOS apparatus (Manchester, U.K.) with a 127 mm
radius concentric hemispherical analyzer (CHA). An AlKa
source with a photon energy of 1486.6 eV is used for exciting
X-ray photoemission, and photoelectrons are detected by the
CHA operated in the fixed retarding ratio mode FRR5 (survey
scans), and in the fixed analyzer transmission mode FAT40 with
a pass energy of 40 eV (detail scans). One scan was conducted to
record the survey spectra, and two to six scans to record the
detail spectra. The XPS quantification method, based on the
comparison of relative intensities of photoelectron peaks, ena-
BiOF is tetragonal, space group P4/nmm. Aurivillus was the
first to study this compound, which he prepared by annealing
BiF3 at 6701C in air, and provided a thorough description of
its structure.5 Other groups have worked on BiOF and usually
prepared it by solid-state synthesis from Bi2O3 and BiF3.6,7
Only a few solution techniques have been used to prepare
BiOF and they required either moderate temperatures or an
HF treatment.8
BiO0.5F2 has a cubic fluorite structure, space group Fm3m. It
has been studied by several groups over the years7,9,10 but, from
what we have found, has been prepared exclusively from solid-
V. A. Hackley—contributing editor
Manuscript No. 20659. Received June 9, 2005; approved August 3, 2005.
Supported by U.S. Government.
wAuthor to whom correspondence should be addressed. e-mail: bervas_mathieu@
hotmail.com
zEnergy Storage Research Group.
yDepartment of Ceramic and Materials Engineering.
zLaboratory for Surface Modification.
645

646
Journal of the American Ceramic Society—Bervas et al.
Vol. 89, No. 2
bles us to calculate the atomic fraction for each component, as-
suming their total intensities to be 100% and using the cor-
responding sensitivity factors. The measurements have been
performed under UHV conditions with a residual pressure of
about 10ꢀ9 Torr.
III. Characterization
(1) BiOF and a-NH4BiF4
The results of a preliminary study on the reactivity of bismuth
oxide with ammonium fluoride in solution as a function of the
molar ratio R 5 [NH4F]/[Bi2O3] are shown in Fig. 1. At low mo-
lar ratio, BiOF precipitates according to the chemical reaction:
(2) Synthesis Approach
Bi2O3 þ 2NH4F ! 2BiOF þ 2NH3 þ H2O
(1)
A preliminary study on the reactivity of Bi2O3 with NH4F in
solution was first conducted. For that purpose 15 mL of an
aqueous solution of ammonium fluoride was poured into a glass
vial containing 0.5 g of bismuth oxide (Aldrich, Milwaukee,
WI). The NH4F concentration was chosen so that the molar
ratio R 5 [NH4F]/[Bi2O3] varied between 1 and 180. Bi2O3 was
stirred in the NH4F solution for 3 h at room temperature after
which the reaction product was filtered, dried under vacuum
at 701C for 1 day, and characterized. The reaction of Bi2O3
with NH4F occurred within a few minutes and goes to
completion, as attested by the total change of color of the
solid from yellow to white. Stirring periods of 1 day instead of
3 h were tried; however, XRD performed on the reaction prod-
ucts did not reveal any noticeable difference. A molar ratio
R 5 [NH4F]/[Bi2O3] of 180 corresponds to the maximum possi-
ble ratio for a fixed oxide concentration as the NH4F concen-
tration is in that case at, or very close to, the solubility limit of
ammonium fluoride in water.
A larger volume of an aqueous solution of NH4F at the sol-
ubility limit was then prepared by dissolving 73.8 g of crystalline
NH4F into 100 mL of deionized water in a Teflon container. The
dissolution was complete after about 3 hours of vigorous stirring.
BiOF and b-NH4BiF4 were prepared at room temperature from
the reaction of Bi2O3 or BiF3 with this saturated NH4F aqueous
solution. (NH4)0.5BiF3.5 and NH4Bi3F10 were obtained by an-
nealing b-NH4BiF4 under Argon. Finally BiO0.5F2 was prepared
by hydrolyzing b-NH4BiF4 in deionized water heated to 701C.
All the compounds mentioned in this paper were filtered and
dried under vacuum at 701C after precipitation. When the sat-
urated NH4F aqueous solution was utilized the synthesis was
carried out in Teflon vials, otherwise it was conducted in glass
vials. Once the synthesis was completed, the different samples
were then redried overnight at 1201C under vacuum and then
stored in a glove box filled with Helium.
Traces of BiOF can already be seen on the pattern of the sample
made with a ratio as low as 1. A concentration ratio equal to 5 is
enough to fully react Bi2O3. Nonetheless a ratio of 20 is neces-
sary to obtain the pure BiOF. A few unidentified peaks at
2y ꢁ 121, 271, and 29.51 (marked by a star symbol in Fig. 1)
are present on the patterns of powder prepared from a lower
ratio. The obtained BiOF is identical to the one first described
by Aurivillus, which he prepared by solid-state oxidation.5
It crystallizes in the tetragonal system (space group P4/nmm).
˚
The values of the lattice parameters are a 5 3.7417.002 A
˚
and c 5 6.2107.004 A, in good agreement with the values of
˚
a 5 3.7469 A and c 5 6.226 A given by Aurivillus.
˚
A large batch of BiOF was prepared by stirring 6 g of Bi2O3
into 3 mL of a saturated aqueous NH4F solution and 8 mL of
deionized water at room temperature for 1 day. The indexed
XRD pattern of the resulting BiOF is shown in Fig. 2. It reveals
a successful formation of the phase pure BiOF. FT-IR was per-
formed on BiOF and the results, not shown here, exhibited very
weak bands characteristic of ammonium ions. However, from
the intensity ratio of those bands with the inorganic band it was
clear that the residual NH14 ions were only present as traces.
The XRD patterns of Fig. 1 indicate that at higher NH4F
concentration a compound different than BiOF is produced.
The XRD pattern of this new compound matches almost per-
fectly the reported diffraction pattern of the monoclinic form of
NH4BiF4.14 The chemical reactions involved can be either one,
or both, of the following, depending on whether or not BiOF is
formed as a reaction intermediate:
Bi2O3 þ 8NH4F ! 2NH4BiF4 þ 6NH3 þ 3H2O
BiOF þ 3NH4F ! NH4BiF4 þ 2NH3 þ H2O
(2)
(3)
#
# α-NH BiF
4
4
#
#
#
#
#
#
#
#
R=180
# #
#º
#
#
##
º
# # º#
#º
#
#
# ##º#
##
R=100
R=75
R=50
R=45
R=40
º
º
º
º
º
BiOF
º
º
R=20 º
R=10
R=5
R=1
º
º
º
º
º
º
º
º
*
unid.
*
*
*
*
Bi O
2
3
10.0
16.0
22.0
28.0
34.0
40.0
46.0
52.0
58.0
64.0
Degrees 2θ (°)
Fig. 1. Formation of BiOF and NH4BiF4 from the reaction of Bi2O3 with aqueous NH4F solutions.

February 2006
Bismuth Oxyfluorides and Ammonium Bismuth Fluorides
647
under vacuum and this routine is repeated several consecutive
times. The compound formed has an orthorhombic symmetry
˚
˚
with lattice parameters a 5 7.7577.022 A, b 5 13.2127.040 A,
˚
and c 5 14.8297.062 A. As will be shown shortly, this
compound is a new ammonium bismuth fluoride, b-NH4BiF4.
BiOF remains as a minor second phase in the ammonium
bismuth fluoride even after the eighth stirring in solution A.
Its amount has been estimated at 5% from the intensity ratio of
the XRD pattern.
As will be described below, b-NH4BiF4 was utilized as a pre-
cursor for the low-temperature synthesis of a variety of other
bismuth fluorides and we were interested in preparing with a
high purity. This turned out to be possible when BiF3, instead
of Bi2O3, was used as bismuth precursor. The XRD pattern of
b-NH4BiF4 prepared by stirring 2 g of BiF3 two successive times
in 10 mL of solution A, each one followed by a filtering and
drying at 701C in vacuum sequences, is presented in Fig. 4. This
XRD pattern is devoid of BiOF, or any other second phase,
reflections. A quantitative XPS analysis has been conducted on
this sample. The results, presented in Table I, clearly confirm
our proposed stoichiometry.
10.0 16.0 22.0 28.0 34.0 40.0 46.0 52.0 58.0
64.0
Degrees 2θ (°)
Fig. 2. X-ray diffraction pattern of BiOF prepared by stirring Bi2O3 in
a saturated aqueous solution on NH4F.
NH4BiF4 crystallizes in the monoclinic system (space group
P21/c). As will be discussed in the next paragraphs, NH4BiF4 is
actually a polymorphic compound, just like KBiF4, RbBiF4,
and TlBiF4.15–17 For this reason we will now call the monoclinic
phase a-NH4BiF4. The unit cell dimensions, calculated from 33
Several reports on the synthesis of NH4BiF4 exist in the lit-
erature,8,18,19 most of which use solution techniques, but we
could not find any account of this orthorhombic form. This
material is isomorphic to the orthorhombic a form of RbBiF4
prepared by Matar et al.17 by a 5 days annealing at 2701C of
RbF and BiF3 in a sealed gold tube. The published lattice pa-
˚
peaks between 101 and 651 in 2y, are a 5 8.2797.041 A,
˚
˚
b 5 7.6057.044 A, c 5 6.4157.034 A, and b 5 93.0251. Within
˚
error, they match the values of a 5 8.317 A, b 5 7.597 A,
c 5 6.486 A, and b 5 93.51 given by Aurivillus well, except
for the c lattice parameter that is slightly contracted.
˚
14
˚
˚
rameters for a-RbBiF4 were a 5 13.2667.030 A, b 5 7.7837
˚
˚
XRD reveals that a molar ratio R 5 [NH4F]/[Bi2O3] of 180 is
not sufficient to eliminate the residual BiOF altogether. From an
estimate of the XRD intensity ratio the amount of residual
BiOF is approximately 10%. The NH4F concentration utilized
for the synthesis of this sample is at, or very close to, the sol-
ubility limit of NH4F in water. This molar ratio is thus the
highest possible at room temperature for the fixed volume and
fixed amount of oxide of this study. Therefore, we have not been
able to prepare a batch of completely pure monoclinic a-
NH4BiF4 with this low-temperature synthesis technique. It is
known that a-NH4BiF4 readily decomposes into BiOF when
exposed to water.18 The BiOF second phase may partially come
from atmospheric H2O-induced decomposition of a-NH4BiF4
post synthesis. However, we did not notice any increase in the
amount of BiOF with the exposure time in air.
.030 A, and c 5 14.9427.030 A, very close to the one we
obtained for b-NH4BiF4. The intensities of the different Bragg
reflections are however much different in b-NH4BiF4 and
a-RbBiF4. For instance the most intense reflection on the
pattern of b-NH4BiF4 is (002), whereas in a-RbBiF4 the most
intense reflection is (040) and the intensity of (002) is only 30%
of I040. This is not surprising as this orthorhombic structure is
ordered and NH14 and Rb1 have almost the same ionic radius
but very different X-ray scattering factors.
For all the other results presented in the rest of this paper,
b-NH4BiF4 will refer to the compound prepared using BiF3
as precursor.
(3) (NH4)0.5BiF3.5 and NH4Bi3F10
The TGA and DSC plots of b-NH4BiF4 collected under Argon
at a scanning rate of 101C/min for temperatures up to 4201C are
presented in Fig. 5. The important weight loss occurring be-
tween 1501 and 3201C is attributed to the decomposition of
NH4F in the form of ammonia and HF. The corresponding
chemical reaction is:
(2) b-NH4BiF4
Instead of stirring Bi2O3 in an increasingly concentrated NH4F
solution as we did in the study reported in the previous section
of this paper, we stirred Bi2O3 several consecutive times in a
saturated aqueous NH4F solution. As illustrated in the XRD
patterns of Fig. 3, a phase transformation occurs when Bi2O3 is
stirred for 1 day in 10 or 15 mL of an aqueous solution saturated
in NH4F, filtered and dried at 701C for approximately 1 day
DT
b-NH4BiF4 ꢀ! NH3 þ HF þ BiF3
(4)
The total experimental weight loss in that range of temper-
ature is 12.5%, very close to the theoretical value of 12.2%
for reaction 4. XRD was conducted on the powder after the
DSC scan and the pattern obtained, shown in Fig. 6 (pattern c),
could as expected clearly be identified as the pure orthorhombic
BiF3 phase. This result also supports our previous experimental
data regarding the composition of the new phase, b-NH4BiF4.
As we have not been able to prepare a-NH4BiF4 with a good
purity, we could not verify whether or not its thermal decom-
position was similar.
As can be observed on the TGA plot in Fig. 5, the weight loss
in b-NH4BiF4 happens in three separate steps. These steps are
coupled with three endothermic peaks on the DSC plot. These
results show that NH4F does not evolve from b-NH4BiF4 in one
single step but rather in three distinct ones. To assess the nature
of the intermediate compounds formed after the different steps,
b-NH4BiF4 was annealed under Ar. The DSC and TGA plots of
the powders obtained after 2 h annealing at 250 and 2901C are
presented in Figs. 7 and 8, respectively, and the corresponding
XRD patterns are shown in Fig. 6.
°
°
°
°
β-NH BiF
4
4
×8
°
° ° °°
°
°
°
°
°
° °° ° °° ° °°° °°° °° °° °° °° ° °° ° °
×6
×5
×4
×3
×2
×1
BiOF
10.0 16.0 22.0 28.0 34.0 40.0 46.0 52.0 58.0 64.0
Degrees 2θ (°)
Fig. 3. Preparation of b-NH4BiF4 from Bi2O3 and a saturated aqueous
solution of NH4F. BiOF remains as a second phase.

648
Journal of the American Ceramic Society—Bervas et al.
Vol. 89, No. 2
3342
3000
2657
2314
β-NH BiF
4
4
obtained by stirring BiF
3
in a saturated NH F aqueous solution
4
1971
1628
1285
942
600
257
10.0
16.0
22.0
28.0
34.0
Degrees 2θ (°)
Fig. 4. Preparation of pure b-NH4BiF4 from BiF3 and a saturated aqueous solution of NH4F.
40.0
46.0
52.0
58.0
64.0
cal weight losses necessary to transform (NH4)0.5BiF3.5 into
BiF3 and (NH4)0.33BiF3.33 into BiF3, respectively. Hence, the
thermal decomposition of b-NH4BiF4 under Argon can be sum-
marized as follows:
Table I. Quantitative X-ray photoelectron spectroscopy
(XPS) Results for b-NH4BiF4. The C Peak comes From the
Substrate and the O Peak is from Surface Contamination
Position Raw Sensitivity
(eV) area factor
Atomic
Normalized atomic
concentration
Peak
concentration (%)
250ꢂC
b-NH4BiF4 ꢀ! ðNH4=FÞ0:5BiF3:5
ꢀð1=2Þ
F 1s 684.01 4437 1.000
O 1s 531.90 1098 0.660
N 1s 402.00 470 0.420
C 1s 285.00 1354 0.250
Bi 4f 160.80 8226 7.400
32.3
12.1
8.1
39.4
8.1
4
1
1
(5)
290ꢂC
330ꢂC
ꢀ! ðNH4=FÞ0:33BiF3:33 ꢀ! NH4F
ꢀð1=6Þ
ꢀð1=3Þ
If a (NH4)1ꢀxBixF1ꢀ2x type of notation is adopted, as pro-
posed by Laborde and Re
au20 for the solid solution M1ꢀx
´
BixF1ꢀ2x (where M 5 K, Rb, and Tl), NH4BiF4, (NH4)0.5BiF3.5
,
The experimental total weight losses on the TGA plots
of the two powders obtained after annealing b-NH4BiF4
at 2501 and at 2901C are 6.4% and 4.3%, respectively.
These values are very close to 6.11% and 4.29%, the theoreti-
and (NH4)0.33BiF3.33 can be rewritten as (NH4)1/2Bi1/2F2,
(NH4)1/3Bi2/3F7/3, and (NH4)1/4Bi3/4F10/4, respectively.
As can be seen in Fig. 6 (pattern a), (NH4)0.5BiF3.5 is a pure
fluorite compound, space group Fm3m, of lattice parameter
˚
a 5 5.9897.001 A. This is to the best of our knowledge the first
time that the existence of this compound is being reported.
(NH4)0.5BiF3.5 is isostructural with b-KBiF4 and g-RbBiF4,21–23
which remain to this day two of the best solid-state fluorine ions
conductors ever discovered.24
102
100
98
2
(NH ) BiF
4 0.5
3.5
10
to NH Bi F
4
3
The XRD pattern of (NH4)0.33BiF3.33 (or NH4Bi3F10), pre-
sented in Fig. 6 (pattern b), can also be indexed in the Fm3m
0
˚
space group with a lattice parameter a 5 11.9467.012 A. The
partial unit cell of NH4Bi3F10, which is isomorphic with
KBi3F10 and RbBi3F10,22,25 is presented in Fig. 9. It is com-
posed of an alternation of two subcells [NH4Bi3F8]21 and
[NH4Bi3F12]2ꢀ. The former has a fluorite structure in which
−2
−4
−6
−8
−10
96
the anions occupy all the tetrahedral sites of the (NH14 , Bi31
)
94
FCC and hence form a cube, while in the latter they occupy the
48i interstitial sites and are organized in a cubo-octahedron.
Ninzansky and Rehspringer25 were the first to synthesize
NH4Bi3F10 from a solid-state reaction between Bi2O3 and
92
90
NH4F. These authors reported
a
lattice parameter
˚
a 5 11.9747.001 A slightly larger than the one we have found,
possibly because of the presence of some residual oxygen anions
on the fluorine lattice of their compound.13
NH Bi F
3 10
4
β-NH BiF
4
4
to BiF
88
3
to (NH ) BiF
4 0.5
3.5
Contrary to the pure fluorite (A1,VA1)Bi31(Fꢀ, VFꢀ)4 com-
pounds that have a fully disordered cation sub-lattice,26 all the
cubic ABi3F10-type compounds reported in the literature have
an ordered structure in which the A1 cations occupy all the
corner positions of the subcells and the Bi31 ions occupy the
centers of each face.22,25 However, the ordering of the cations
seems to be only partial in our NH4Bi3F10 as the fluorite reflec-
86
0
200
300
400
500
600
Temperature (°C)
Fig. 5. Thermogravimetric analysis and differential scanning calorime-
try plot of b-NH4BiF4 up to 4201C. The experiment was conducted
under Argon at a scanning rate of 101C/min.

February 2006
Bismuth Oxyfluorides and Ammonium Bismuth Fluorides
649
(111)
(c) BiF
3
(222)
(b) NH Bi F
3 10
4
(440)
(611)
(400)
(200)
(220)
(640)
(444)
(420)
(422)
(600)
(620)
(642)
(800)
(111)
(220)
(a) (NH ) BiF
4 0.5
3.5
(200)
(311)
(222)
(400)
64.0
10.0
16.0
22.0
28.0
34.0
40.0
46.0
52.0
58.0
Degrees 2θ (°)
Fig. 6. X-ray diffraction (XRD) patterns of (a) (NH4)0.5BiF3.5 and (b) NH4Bi3F10 prepared by annealing b-NH4BiF4 2 h at 2501C and 2 h at 2901C in
Argon, respectively. The XRD pattern (c), characteristic of the BiF3, collected after a differential scanning calorimetry run in Argon at 4201C on b-
NH4BiF4 is also presented on this figure.
1
101
100
99
4
101
10
99
98
97
96
95
94
93
2
0
0
−1
−2
−3
−4
−5
−6
−2
−4
−6
−8
−10
98
(NH ) BiF
4 0.5
3.5
10
97
to NH Bi F
4
3
NH Bi F
3 10
4
96
to BiF
3
NH Bi F
3 10
to BiF
4
3
95
0
100
200
300
400
Temperature (°C)
0
100
200
300
400
500
Temperature (°C)
Fig. 8. Thermogravimetric analysis and differential scanning calorime-
try plots of NH4Bi3F10 prepared by annealing b-NH4BiF4 2 h at 2901C
in Argon. The experiment was conducted under Argon at a scanning rate
of 101C/min.
Fig. 7. Thermogravimetric analysis and differential scanning calorime-
try plots of (NH4)0.5BiF3.5, prepared by annealing b-NH4BiF4 2 h at
2501C in Argon. The experiment was conducted under Argon at a
scanning rate of 101C/min.
tions are much more intense than the reflections brought about
by the formation of the superstructure. Hence, the corner posi-
31
1ꢀx
tions of the FCC lattice must be occupied by ‘‘(NH14 )xBi ’’
cations and the centers of the face by ‘‘(NH14 )1ꢀxBix31’’ cations,
where 0.5oxo1.
3+
Bi
The FT-IR spectra of b-NH4BiF4, (NH4)0.5BiF3.5, and
(NH4)0.33BiF3.33 are compared in Fig. 10. This figure is divid-
ed into two parts: (a) that shows the vibrations bands of the
ammonium ions on spectra collected in the ATR mode; and (b)
that shows the Bi–F vibration bands on spectra collected in
transmittance mode. The spectrum of the pure macro-ortho-
rhombic BiF3 has been added to serve as a reference. The po-
sitions of the maxima of the different peaks are given in Table
II. The strong intensity of u1, the totally symmetric mode, and
−
F
+
NH
4
Fig. 9. NH4Bi3F10 partial unit cell. The fluorite [NH4Bi3F8]21 subcell is
2ꢀ
shown on the left and the [NH4Bi3F12
]
subcell is shown on the right.

650
Journal of the American Ceramic Society—Bervas et al.
Vol. 89, No. 2
100
80
2
υ
4
υ
1
NH Bi F
3 10
4
2
υ
υ
3
4
60
υ 4
100
2
υ
4
90
υ
1
(NH ) BiF
3.5
υ
4 0.5
3
υ 4
80
100
80
60
υ 4’
υ 4
β-NH BiF
2
4
4
υ
4
υ
3
υ
(a) ATR
1
2
υ
4’
υ4’’
40
4000
3500
3000
2500
2000
1500
1000
500
−1
Wavenumber (cm
)
100
80
NH Bi F
3 10
4
Bi-F
60
100
90
(NH ) BiF
4 0.5
3.5
80
Bi-F
Bi-F
100
90
β-NH BiF
4
4
80
100
BiF Pnma
3
80
(b) Trans.
Bi-F
1000
500
900
800
700
600
−1
Wavenumber (cm
)
Fig. 10. Fourier transform infrared patterns of b-NH4BiF4, (NH4)0.5BiF3.5, and NH4Bi3F10. (NH4)0.5BiF3.5 and NH4Bi3F10 were prepared by
annealing b-NH4BiF4 2 h at 2501C and 2 h at 2901C in Argon, respectively; (a) attenuated total reflectance mode, showing the vibrations of the
(NH4)1 ions and (b) transmittance mode, showing the Bi–F vibrations. The spectrum of orthorhombic BiF3 has been added for reference.
Table II. Positions of the Maxima of the Vibrations Bands of Figure 10
NH14 u3 (cmꢀ1
)
NH14 u1 (cmꢀ1
)
NH14 2u04 (cmꢀ1
)
NH14 2u4 (cmꢀ1
)
NH14 u04 (cmꢀ1
)
NH14 u4 (cmꢀ1
)
NH14 u040 (cmꢀ1
)
Bi–F (cmꢀ1
)
b-NH4BiF4
(NH4)0.5BiF3.5
NH4Bi3F10
BiF3
3205
3221
3226
3061
3076
3090
2916
2848
2850
2850
1463 (shoulder) 1417 (shoulder)
1417
1420
1402
457
464
457
457
2918
the splitting of the bending vibration u4 in three components
indicate that the NH14 ions do not have a perfect Td symme-
try.27,28 The absence of inorganic vibration bands, other than
Bi–F, between 400 and 1000 cmꢀ1 suggests that these com-
pounds are indeed pure fluorides.
(4) BiO0.5F2
BiO0.5F2 is formed when 1 g of b-NH4BiF4 is stirred for 1 h in
20 mL of deionized water heated for 1 h at 701C, followed by
two times of 15 min each in 20 mL of water at room temper-

February 2006
Bismuth Oxyfluorides and Ammonium Bismuth Fluorides
651
first time that the existence of the fluorite (NH4)0.5BiF3.5 is re-
ported. It is isostructural with b-KBiF4 and g-RbBiF4,22,23
which are among the best solid-state fluorine ions conductors
ever discovered.24
(111)
Finally, an additional bismuth oxyfluoride, BiO0.5F2, has
been also prepared from the hydrolysis of b-NH4BiF4 at 701C.
(220)
(200)
(311)
References
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(400)
10.0 16.0 22.0 28.0 34.0 40.0 46.0 52.0 58.0
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Degrees 2θ (°)
Fig. 11. X-ray diffraction pattern of BiO0.5F2, prepared by stirring 1 g
of b-NH4BiF4 for 1 h in 20 mL of deionized H2O at 701C followed by
two times of 15min each in 20mL of water at room temperature.
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12HF and NH4F Material Safety Data Sheets.
97
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´
au, and P. Hagenmuller, ‘‘Synthese et
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Etude de la Conductivite
´
95
0
100
200
300
400
500
´
Temperature (°C)
´
Fig. 12. Thermogravimetric analysis plot of BiO0.5F2. The experiment
was conducted under Argon at a scanning rate of 101C/min.
´
Inorganic Fluorides, Edited by J. H. Simmons. Academic Press, New York, 1950.
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´
20
P. Laborde and J.-M. Reau, ‘‘Influence de la Substitution Oxygen-Fluor sur
70ꢂ
b-NH4BiF4 þ 0:5H2O ꢀ! BiO0:5F2 þ NH3 þ 2HF (6)
The XRD pattern of BiO0.5F2 is presented in Fig. 11. It has a
les Proprie
´ ´
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au, P. Laborde, and A. Rhandour, ‘‘Static Modelling of
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285–8 (1988).
˚
fluorite structure with a lattice parameter a 5 5.82870.001 A, in
˚
good agreement with the value of a 5 5.848 A reported in the
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563–70 (1983).
literature.7 Contrary to Kalinchenko et al.10 we did not find any
orthorhombic distortion in this oxyfluoride.
Like in the case of BiOF, only traces of residual ammonium
ions have been observed in BiO0.5F2 by FT-IR. The good
purity of the compound has been confirmed by TGA, as shown
in Fig. 12.
23R. D. Weir and E. F. Westrum Jr, ‘‘The Thermodynamics of the Divalent
Metal Fluorides. V. Heat Capacity of the Fast-Ion Conductor g-RbBiF4 from 7 to
346 K,’’ High Temperatures-High Pressures, 23, 73–8 (1991).
24J.-M. Re
´
au and J. Grannec, ‘‘Chapter 12: Fast Fluorine Ion Conductor,’’
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Hagenmuller. Academic Press, New York, 1985.
25D. Niznansky and J. L. Rehspringer, ‘‘Preparation and Characterization of
NH4Bi3F10,’’ J. Mater. Chem. Res., 7 [9] 2511–13 (1992).
IV. Conclusion
26J. L. Subeyroux, J.-M. Re
´
au, S. Matar, G. Villeneuve, and P. Hagenmuller,
BiOF and an orthorhombic form of NH4BiF4 have been synthe-
sized at room temperature from the fluorination of Bi2O3 or BiF3
by a saturated aqueous solution of NH4F. This new orthorhom-
bic form of NH4BiF4, named b-NH4BiF4 to differentiate it from
the monoclinic a form, is isostructural with a-RbBiF4.17
(NH4)0.5BiF3.5 and NH4Bi3F10 were prepared by annealing
b-NH4BiF4 at a relatively low temperature under Ar. It is the
‘‘Etude par Diffraction Neutronique des Solutions Solides K1ꢀxBixF112x et
Rb1ꢀxBixF112x,’’ Solid State Ionics, 6 [1] 103–11 (1982).
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Compounds Part A: Theory and Applications in Inorganic Chemistry, 5th edition,
J. Wiley & Sons, New York, 1997.
28L. F. Bovey, ‘‘The Infrared Absorption and Reflection Spectra of the
Ammonium Halides,’’ J. Opt. Soc. Am., 41 [11] 836–48 (1951).
&