The structural investigation of Ba4Bi3F17

Article abstract of DOI:10.1016/j.jssc.2003.08.022

The anion-excess ordered fluorite-related phase Ba₄Bi ₃F₁₇ has been synthesized by a solid state reaction of BaF₂ and BiF₃ at 873K. The crystal structure of Ba ₄Bi₃F₁₇ has been studied using electron diffraction and X-ray powder diffraction (a=11.2300(2)A, c=20.7766(5)A, S.G. R3, R_I=0.020, R_P=0.036). Interstitial fluorine atoms in the Ba₄Bi₃F₁₇ structure are considered to form isolated cuboctahedral 8:12:1 clusters. The structural relationship between Ba₄Bi₃F₁₇ and similar rare-earth-based phases is discussed.

Full text of DOI:10.1016/j.jssc.2003.08.022

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 312–318
The structural investigation of Ba Bi F
4
3 17
a
E.N. Dombrovski, T.V. Serov, A.M. Abakumov, E.I. Ardashnikova,
a
a
a,ꢀ
a
V.A. Dolgikh, and G. Van Tendeloo
b
a
Department of Chemistry, Moscow State University, Moscow 119992, Russia
EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium
b
Received 21 April 2003; received in revised form 4 August 2003; accepted 20 August 2003
Abstract
The anion-excess ordered fluorite-related phase Ba Bi F has been synthesized by a solid state reaction of BaF and BiF at
4
3
17
2
3
˚
8
73 K. The crystal structure of Ba
4
Bi
=0.020, R
isolated cuboctahedral 8 : 12 : 1 clusters. The structural relationship between Ba Bi F and similar rare-earth-based phases is
3
F
17 has been studied usingelectron diffraction and X-ray powder diffraction ( a=11.2300(2) A,
˚
%
c=20.7766(5) A, S.G. R3; R
I
P
=0.036). Interstitial fluorine atoms in the Ba Bi 17 structure are considered to form
4
3
F
4
3 17
discussed.
r 2003 Elsevier Inc. All rights reserved.
4 3
Keywords: Fluorides; Ba Bi F17; Crystal structure; Fluorite-like; Cuboctahedral clusters
1
. Introduction
An association of anionic defects (so-called clustering)
Similar clusteringmodels were proposed for
fluorite-like Bi-containingphases because of the
3
close size of Bi and RE cations (K1 Bi O F
+
3+
ꢁx
x
z
2+2xꢁz
0.5ꢁx
is frequently used in description of physico-chemical
properties of solid solutions. However, almost every
time these models serve only as logical explanations and
remain without direct proofs. As a rule, only the exact
determination of atomic positions in the crystal struc-
ture of ordered phases derived from the solid solutions
allows to reveal these clusters.
[7], Sr1 Bi F
[9], Ba Bi F
[10], Na
2+x
ꢁx
Bi0.5+x(O,F)
x
2+x
1ꢁx
[11,12]); however, there are quite few
x
2+d
structural studies of ordered compounds (BiZr F [13],
3
15
MBiF (M=K, Rb) [14], CsBi F [15], Bi(O,F)
4
2
7
2.43ꢁ2.50
[16]). Bi-containingphases are much less studied than
RE-containingones because of difficulties in their
preparation. BiF is not as thermally stable as LnF
3
3
Clusteringin anion-excess fluorite-like phases was
first proposed when studyingoptical properties of
and less stable to pyrohydrolysis compared to LnF3,
more volatile and subjected to reduction to metallic Bi
with a material of the reaction vessel.
Ca1 Ln F solid solutions [1] and was confirmed
x 2+x
ꢁx
by structural investigations and a cluster modelling [2,3].
Cuboctahedral clusters (in this case 8 : 12 : 0 or 8 : 12 : 1;
the different cluster notations are given in [4]), which are
often used for description of the structure and properties
of fluorite-derived phases were found for the first time
for the rare-carth (RE)-containingcompound in 1973
The ordered phase Ba1 Bi F
x 2+x
(0.45oxo0.50)
ꢁx
was previously found on the isothermal section
ꢂ
(700 C) of the BaF –BiF system [17]. The compound
2
3
with Ba0.55Bi0.45F2.45 composition was described as
˚
hexagonal with cell parameters a=11.22 A and
˚
c=20.76 A. The goal of the present work is to prepare
(
KY F [5]). These cuboctahedral clusters may interact
3
this compound in pure form and to perform a structural
investigation using X-ray powder diffraction and
electron diffraction.
10
in different ways giving rise to several types of ordered
phases [6] and can be present in disordered phases as
well [7]. The progress in understanding of clustering is
presented in [8].
2. Experimental
ꢀ
The startingmaterials were hi hg -purity ( 499.9%)
Correspondingauthor. Fax: +095-939-0998.
E-mail address: ard@inorg.chem.msu.ru (E.I. Ardashnikova).
barium and bismuth fluorides. BaF was treated with
2
0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.08.022

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E.N. Dombrovski et al. / Journal of Solid State Chemistry 177 (2004) 312–318
313
gaseous HF on heating (623 K, 3 h) to remove traces of
water and degassed under dynamic vacuum (B10 Pa) at
Samples for electron microscopy were made by
crushingthe crystals in ethanol and depositingfra-g
ments on a holey carbon grid. Electron diffraction (ED)
and EDX analysis were carried out usinga Philips
CM20 microscope with a LINK-2000 attachment.
4
73 K for 2 h. BiF was prepared from Bi(OH) by a
3 3
reaction with aqueous HF (40%) and the precipitate
obtained was heated at 623 K for 3 h under flow of
gaseous HF. Ready to use starting materials were kept
in a desiccator. The mixed powders of BaF and BiF
2
3
were pressed to pellets and introduced into copper tubes.
Primary investigations showed absence of interactions
of copper with studied fluorides and oxyfluorides at
temperatures lower than 923 K when neither water
vapor nor oxygen is present in the reaction atmosphere.
After degassing in dynamic vacuum at 473 K for 2 h and
fillingwith dry ar go n, the tubes were sealed, heated at
3. Results and discussion
The X-ray diffraction pattern of the Ba Bi F sample
4
was indexed on a hexagonal lattice with the unit cell
˚
parameters a=11.224(1) A, c=20.782(4) A, which are in
3 17
˚
good agreement with those reported previously [17]. The
parameters of the hexagonal unit cell can be related to
pffiffiffiffiffiffiffi
a 2 3 (a —the parameter of fluorite subcell). The
8
(
73710 K for 24 h and quenched in icy water. The best
almost monophase) sample was obtained when BiF3
was taken with the ratio BaF :BiF =0.55:0.45.
the parameters of fluorite subcell as a ¼ a 7=2; c ¼
fl
pffiffiffi
fl
fl
2
3
reflections –h + k + l a 3n were systematically absent
which indicates a R-centered unit cell. Apart from this
extinction condition, no other conditions were observed.
Electron diffraction patterns (Fig. 1) reveal a clear
relationship between the basic fluorite structure and the
structure of Ba Bi F . The brighter spots on the ED
X-ray powder diffraction analysis for all samples was
performed with a focusingGuinier camera FR-552
(CuKa1 radiation) using egr manium as an internal
standard. X-ray powder diffraction data for structure
refinement were collected on a STADI-P diffractometer
CuKa -radiation, curved Ge monochromator, trans-
4
3 17
(
patterns can be indexed on a face-centered cubic lattice
˚
1
mission mode, linear PSD). The WINCSD program
package was used for the structure determination from
X-ray powder data [18]. The final refinement was
performed with RIETAN-2000 program [19].
with cell parameter aE6.0 A; these spots belongto the
fluorite sublattice. Weaker spots are superstructure
reflections, they were successfully indexed in a hexago-
˚
˚
nal unit cell with aE11.2 A and cE20.8 A, in g ood
4 3
Fig. 1. Electron diffraction patterns of Ba Bi F17. Indexes belonging to the fluorite subcell and the hexagonal supercell are marked with the
subscripts f and h, respectively.

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314
2
˚
agreement with the cell parameters determined from
powder X-ray diffraction data. Indexes belonging to the
fluorite subcell and the hexagonal supercell are marked
with the subscript letters f and h, respectively. The
reciprocal lattice vectors of the fluorite subcell and the
hexagonal supercell are related by the matrix:
(ADP) (B19 A ). The refinement of the occupancy
factor of this position gave the value of 0.71(3).
However, in the Ba Ln F structure the F8 atom is
4
3 17
shifted from the ideal (0,0,1/2) position into a general
18f position with an occupancy factor of 1/6. The
refinement of the F8 atoms in 18f position resulted in
even slightly negative ADP. Some fluorine atoms also
had negative ADPs since the correct refinement of the
atomic displacement parameters of such light scatterers
as fluorine is hampered by the presence of Ba and Bi in
the structure and by strongcorrelations between the
ADPs of the anion positions, which were symmetrically
dependent in the fluorite subcell. This circumstance was
overcome by the refinement of all fluorine atoms with
the common ADP. ADPs for Ba and Bi cations were
refined independently in an isotropic approximation.
The final refinement led to good values of reliability
factors R =0.020 and R =0.036. The crystallographic
0
1
C
3
=2 ꢁ1=2 ꢁ1
B
@
ꢁ1=2
2
ꢁ1
3=2 A
2
2
Only –h + k + l=3n reflections were observed, which
%
%
corresponds to possible R3m; R3m; R3 and R3 space
groups. The sample was found to be very sensitive to the
intense electron beam. Under beam irradiation the
superstructure reflections gradually become weaker
and finally disappear, then the sample decomposes. To
avoid decomposition, ED patterns were taken with
widely spread weak electron beam. However, high-
resolution electron microscopy investigation of
Ba Bi F was not possible since the sample decomposes
I
P
parameters, reliability factors, atomic coordinates and
the most relevant interatomic distances for Ba Bi F
3 17
4
are listed in Tables 1–3. The experimental, calculated
and difference X-ray patterns are shown in Fig. 2.
Further details of the crystal structure investigation can
be obtained from the Fachinformationszentrum Karls-
ruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax:
(49) 7247-808-666; E-mail: crysdata@fiz.karlsruhe.de),
on quotingthe depository number CSD-391210, the
names of the authors and the chemical formula.
The Rietveld refinement from X-ray powder diffrac-
tion data does not allow one to differentiate between
two possible alternatives of arrangement of the F8
atoms in the structure: either these atoms are located at
3b position with an occupancy of 0.71 or at 18f position
with an occupancy of 1/6. The former results in an
overall amount of anions in the structure less than 17
and requires a partial replacement of fluorine by oxygen
to keep the electroneutrality. Oxygen may appear in the
substance because of partial pyrohydrolysis that cannot
be totally excluded. The resultingcomposition of the
phase then corresponds to Ba Bi F16.72O0.14. The latter
4
3 17
under a focused electron beam.
The Ba:Bi ratio was determined by EDX analysis
performed with BaL and BiM lines at 12 points on four
different crystallites. The determined Ba:Bi=
4
.02(5):2.98(5) ratio confirms the proposed Ba Bi F
4 3 17
composition. Thus, the almost monophase stochio-
metric sample of Ba Bi F was obtained when starting
BiF was taken with a small excess in relation to this
4
3 17
3
composition. Probably the excessive BiF was removed
3
from the reaction due to its volatility and possible
reduction.
The crystal structure of Ba Bi F was refined from
4
3 17
X-ray powder diffraction data. Careful analysis of the
raw X-ray powder profile revealed the presence of three
reflections of admixture phases with intensities of 0.4–
1
.2%. The admixture phases were identified as BaF2
˚
d=3.57 A, I/I =0.8%) and metallic Bi (d=3.279 A,
˚
(
I/I =1.2%; d=2.273 A, I/I =0.4%). Their appearance
o
˚
o
o
can be explained by a partial reduction of BiF3 to
metallic Bi, which often leads to a difference between the
initial bulk composition and the final stoichiometry.
Since the admixture reflections have very low intensities
and do not overlap with the reflections of the main
phase, their impact to the diffraction profile was
subtracted usingprofile decomposition prior to the
refinement.
4
3
was proved by means of monocrystals Ba Ln F
3 17
(Ln=Y, Yb) studies [20].
4
Table 1
Selected parameters from Rietveld refinement of X-ray powder data
for Ba Bi
4
3 17
F
The cation and anion positions for the initial structure
model were taken from the Ba Ln F (Ln=Y, Yb) [20]
structure, which has similar cell parameters: Ba Y F —
Space group
˚
a (A)
˚
c (A)
R3%
11.2300(2)
20.7766(5)
4
3 17
4
3 17
Z
Cell volume (A )
3
Calculated density (g/cm )
6
2269.16(8)
˚
˚
a=11.075(1) A,
c=20.372(2) A;
Ba Yb F —a=
4
3
3
17
˚
˚
1.000(1) A, c=20.262(1) A. The R3 space group was
˚
%
1
chosen by analogy with the Ba Ln F compounds. At a
6.583
9p2yp102; 0.01
2y range, step (deg)
Number of reflections
Refinable parameters
4
3 17
281
31
first step the F8 atom was placed at (0,0,1/2) position
3b) with full occupancy, but this resulted in an
abnormally high atomic displacement parameter
(
R
I
, R
P
, RwP
0.020; 0.036; 0.047

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315
Table 2
Positional and thermal parameters for Ba
4 3 17
Bi F
2
˚
Atom
Position
x/a
y/b
z/c
B
iso (A )
Ba1
Ba2
Bi
6c
18f
18f
18f
18f
18f
18f
18f
6c
0
0
0.2629(2)
0.0851(1)
0.08171(9)
0.0372(9)
0.2171(9)
0.6118(7)
0.8372(8)
0.2913(8)
0.853(1)
0
0.58(9)
0.91(5)
1.06(2)
0.23(9)
0.23(9)
0.23(9)
0.23(9)
0.23(9)
0.23(9)
0.23(9)
0.23(9)
0.2252(2)
0.0850(2)
0.483(1)
0.238(2)
0.145(1)
0.582(1)
0.451(1)
0
0.0275(2)
0.6202(1)
0.090(2)
0.061(1)
0.181(1)
0.017(1)
0.083(1)
0
F1
F2
F3
F4
F5
F6
F7
3a
18f
0
0.962(20)
0
0.925(5)
a
F8
0.491(6)
a
Occupancy factor g=1/6.
Table 3
Selected interatomic distances for Ba
positions are situated in octahedrally coordinated sites
and thus are ‘‘interstitial’’ in the parent fluorite structure
˚
3
Bi F17 (A)
4
0
Ba1–F1 2.562(15) ꢄ 3
Ba1–F2 2.586(15) ꢄ 3
Ba1–F3 3.202(14) ꢄ 3
Ba1–F6 2.41(2)
Ba2–F1 2.80(2)
Ba2–F2 2.630(13)
Ba2–F2 2.76(2)
Bi–F1 2.48(2)
Bi–F1 2.49(2)
Bi–F2 2.58(2)
(F type [2]). These atoms in Ba Bi F form cuboctahe-
4
3 17
dral entities and the F8 position is situated in the center
of the cuboctahedra. The F8 anions are also situated in
octahedral hollows of the cationic sublattice and are of
Ba2–F3 2.981(13)
Ba2–F4 2.936(14)
Ba2–F4 3.224(12)
Ba2–F5 2.644(11)
Ba2–F5 2.65(2)
Bi–F3 2.344(11)
Bi–F3 2.386(12)
Bi–F4 2.309(15)
Bi–F4 2.482(14)
Bi–F5 2.322(12)
Bi–F8 2.60(16)
Bi–F8 2.72(11)
Bi–F8 2.99(20)
0
0
F
type [2]. The centered cuboctahedron of fluorine
atoms, which is characteristic for the structure of
Ba Bi F , is equivalent to a cuboctahedral cluster
4
3 17
Ba2–F5 2.710(12)
Ba2–F6 2.714(10)
Ba2–F7 2.973(2)
(8:12:1) reported in a number of anion-excess fluorite-
type solid solutions [8, 10, 21, 22] and ordered phases [6].
The F7 position is situated in the center of a cube
formed by the F5 and F6 atoms and is also an
0
0
‘
‘interstitial’’ one of F type.
The entire anionic sublattice is presented in Fig. 5. It
The structure of the Ba Bi F phase can be described
4
3 17
as a derivative from the fluorite structure. All cation
positions deviate slightly from the ideal positions in the
fluorite sublattice. The cation sublattice is shown in
Fig. 3 where the hexagonal supercell (solid line) and the
cubic fluorite subcell (dotted line) are outlined. The
cation sublattice has six close-packed layers, twice more
in comparison with the parent fluorite structure. The
doublingoccurs due to orderingof the Bi and Ba
cations. The coordination polyhedra for the cations
are presented in Fig4 . The Ba1 atoms are situated in a
can also be described as an alternation of six layers of
‘‘buildingparts’’—centered cubes and cuboctahedra.
Cubes formed of the F5 and F6 atoms (shaded dark) are
centered by the F7 anions, cuboctahedra of the F3 and
F4 atoms (shaded) are centered by the F8 ones. The F1
and F2 atoms are not involved in formation of these
‘‘blocks’’ and are shown as separate circles.
The crystal structure of the Ba Bi F phase contains
4
3 17
centered cuboctahedral clusters in the anion sublattice
(Fig. 6). An insertion of excess fluorine atoms (com-
pared to the initial fluorite structure) and a decrease of
the volume of the fluorite subcell because of partial
substitution of barium by bismuth (for Ba Bi F
3 17
1
0-fold polyhedron (Fig. 4a), which can be described
as a cube with one apex substituted by a triangular face
formed by the F3 atoms (shaded). The Ba2 atoms have
CN=11 and their polyhedron (Fig. 4b) is also a cube
with a triangle of one F3 and two F4 atoms instead of
one apex (shaded) and with a cap above one of the
square faces (F7 atom, shaded dark). The polyhedron
around Bi is a one-capped square antiprism (Fig. 4c,
CN=8+1), the cap is the F8 atom (shaded dark).
Anion positions in Ba Bi F can be divided into
4
˚
˚
asub=6.000(1) A, for BaF a=6.200(2) A) do not lead
2
to abnormally short F–F distances and even the shortest
˚
F–F distance in these cuboctahedra (2.821 A) exceeds
ꢁ
˚
the doubled ionic radius of an F anion (1.29 A).
Probably, the association of several defects: eight
ꢃ
anionic vacancies V in tetrahedral positions and 13
0
F
interstitial fluorine atoms F , and their interaction with
4
3
17
i
ꢃ
three groups. The F1, F2, F5 and F6 positions, which
correspond to the tetrahedral hollows in the close
packing, are ‘‘regular’’ referred to the parent fluorite
structure (where all tetrahedral sites are occupied and all
octahedral ones stay vacant). Anions in the F3 and F4
six Bi atoms in the face centers (Fig. 6), provides
Ba
stability to the cuboctahedral cluster. Perhaps this is the
reason why cuboctahedral clusters in such compounds
are present over a large temperature and composition
range.

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316
4 3 17
Fig. 2. Experimental, calculated and difference X-ray diffraction profiles for Ba Bi F .
4 3
Fig. 4. Cation polyhedra in Ba Bi F17: (a) Ba1 polyhedron—a cube
with one apex substituted by a triangle (shaded); (b) Ba2 polyhedron—
a cube with a triangle instead of one apex (shaded) and with a cap
above one of the square faces (shaded dark); (c) Bi polyhedron—a one-
capped square antiprism, the cap shaded dark.
The Ba Bi F , Ba Ln F (Ln=Y, Yb [20], Er [23])
3 17
4
3
17
4
and Pb Y F O [24] phases can be described with the
8
6 32
same structural model. They are based on a cationic
sublattice with a fixed 4:3 ratio of two- and three-
charged cations, and ‘‘regular’’ anions in the F1–F6
positions. Variable occupation of cubic (F7) and
4 3
Fig. 3. Cationic sublattice of Ba Bi F17. The cubic fluorite subcell
(dotted line) and hexagonal supercell (solid line) are shown.

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317
oxyfluoride Pb Y F O [24] and probably our phase are
8
6 32
inside of the predicted homogeneity range of the solid
II III
solution based on the phase M M F .
3
4
17
4. Conclusions
In the present work the synthesis conditions of the
ordered fluorite-related phase Ba Bi F are described
4
3 17
and a structural information is obtained, which proves
the existence of the 8:12:1 anionic cuboctahedral clusters
in this compound.
Similar rare-earth-based phases are stable at 800–
000 K; however, they can only be obtained through
1
crystallization from the melt only for small RE cations
(
(
RE=Gd–Lu, Y). Compounds with large rare earths
Ce–Sm) are stable only at lower temperatures (673 K)
3
+
[
25]. For the largest La cation the ordered Ba La F
4 3 17
compound is not observed. The Bi-containingphase
perfectly gets in this row of isostructural phases
accordingto its size factor because the crystallo gr aphic
3
+
3+
˚
radius of Bi (CN=8, R(Bi ) 1.25 A [26]) is close to
3
+
the larger RE radii (CN=8, R(Nd )=1.26 A [26]).
˚
Acknowledgments
Fig. 5. Anionic sublattice of Ba
(shaded) are centered by F8, cubes of F5 and F6 (shaded dark) are
centered by F7, separate circles—F1 and F2.
4 3
Bi F17. Cuboctahedra of F3 and F4
The authors are grateful to the Russian Foundation
for Basic Research (Grant 02-03-32847) and Russian
Ministry of Higher Education (Grant T02-094-1158) for
support. Acknowledgment is made to the donors of the
American Chemical Society Petroleum Research Fund
for partial support of this research (project 38459 -AC5).
References
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ꢁ
achieved through a partial substitution of F ions by
. Then the anion homogeneity range of such phases
2ꢁ
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II III
3
II III
4
4
II III
4
16
3
15
(
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II III
M M X (i.e. M M F ) compositions. It means
17
4
3
3
17
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