Fluorite-like phases in the BaF2-BiF3-Bi 2O3 system - Synthesis, conductivity and defect clustering

Article abstract of DOI:10.1016/j.materresbull.2005.02.007

A fluorite-like solid solution Ba_{1 - x}Bi_xO _zF_{2 + x - 2z} on the basis of cubic BaF₂ was synthesised in the BaF₂-Bi₂O₃-BiF₃ system and the homogeneity range at 873 K was determined. The samples were studied by X-ray powder diffraction and electron diffraction, and their transport properties were measured by the complex impedance method at 300-623 K. Tendencies of variation of lattice parameters and transport properties were determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at 573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like phase with lattice parameters a = 9.5355(4) ?, c = 18.151(1) ?.

Full text of DOI:10.1016/j.materresbull.2005.02.007

Materials Research Bulletin 40 (2005) 821–830
www.elsevier.com/locate/matresbu
Fluorite-like phases in the BaF2–BiF3–Bi2O3 system—synthesis,
conductivity and defect clustering
a
T.V. Serov a, E.N. Dombrovski a, E.I. Ardashnikova a, , V.A. Dolgikh ,
*
Malika El Omari b, Mohamed El Omari b, A. Abaouz b, J. Senegas c,
N.G. Chaban d, A.M. Abakumova, G. Van Tendeloo e
a Department of Chemistry, Moscow State University, 119992 Moscow, Russia
¨
`
Department of Chemistry, Faculty of Science, Moulay Ismaıl University, 50000 Meknes, Morocco
b
c
`
´
Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB), 87 Av. A. Schweitzer, 33608 Pessac Cedex, France
d Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow 117571, Russia
e EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received 9 November 2004; received in revised form 10 December 2004; accepted 2 February 2005
Abstract
A fluorite-like solid solution Ba1 ꢀ xBixOzF2 + x ꢀ 2z on the basis of cubic BaF2 was synthesised in the BaF2–
Bi2O3–BiF3 system and the homogeneity range at 873 K was determined. The samples were studied by X-ray
powder diffraction and electron diffraction, and their transport properties were measured by the complex
impedance method at 300–623 K. Tendencies of variation of lattice parameters and transport properties were
determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at
573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like
˚
˚
phase with lattice parameters a = 9.5355(4) A, c = 18.151(1) A.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds; C. X-ray diffraction; C. Impedance spectroscopy; C. Electron diffraction
* Corresponding author. Tel.: +7 95 939 55 02; fax: +7 95 939 09 98.
E-mail address: ard@inorg.chem.msu.ru (E.I. Ardashnikova).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.02.007

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T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
1. Introduction
The investigation of anion-excess fluoride and oxyfluoride fluorite-like solid solutions and the problem
of anion defect clustering attract a lot of attention during the last decades [1–6]. The clusters formed in
such phases are defined by four integer numbers n1:n2:n3:n4 where n1 is the number of vacant anion
positions, n2 is the number of interstitial anions of F0 type (1/2,u,u; u ꢁ 0.35–0.40), n3—the number of
interstitial anions of F00 type (v,v,v; v ꢁ 0.41), n4—the number of interstitial anions of F000 type (w,w,w;
w ꢁ 0.28–0.30) [7]. It is stated that defects forming large cubooctahedral clusters 8:12:n:0 (where n = 0
or 1) [8], are not involved in long-range anion motion. But cubooctahedral clusters may enhance ionic
conductivity because they distort the surrounding fluorite matrix and lower the energy barriers of anion
migration. If the concentration of the cubooctahedra is too high, they reduce the conductivity [9]. Smaller
clusters, like 1:0:n:0 (n = 2, 3 or 4) [4], or 4:4:3:0 [10] enhance the electrical properties. Formation of
different clusters usually depends on the size of the cations and the concentration of the interstitial anions.
Cubooctahedral clusters were reported for the KBi3F10 and Ba4Bi3F17 [11] ordered phases, and for the
K1 ꢀ xBixF1 + 2x [1] and K1 ꢀ xBixOzF1 + 2x ꢀ 2z solid solutions [2]; the 1:0:3:0 clusters were found in the
ordered solid solution Bi(O,F)2.43–2.50 [4].
The investigation of the Ba1 ꢀ xBixF2 + x system [12,13] at 973 K showed a progressive transformation
of the 4:4:3:0 clusters to the 8:12:1:0 cubooctahedral clusters with increasing x. It is therefore interesting
to explore the clustering when oxygen is introduced into the structure. A first investigation of the BaF2–
Bi2O3–BiF3 system was performed in [14], where the composition–transport property dependence was
analysed for two series of samples: BaF2-Bi2O3 (Ba1 ꢀ 2z/3Bi2z/3F2 ꢀ 4z/3Oz, 0.05 < z < 0.20) and
Ba1 ꢀ xBixF2 + x ꢀ 0.30O0.15 (0.10 < x < 0.35). The aim of the present work is the synthesis and
investigation of the Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution in order to establish a correlation between
composition, unit cell parameters and conductivity.
2. Experimental
The starting materials were high-purity (>99.9%) barium and bismuth fluorides and bismuth oxide.
BaF2 was dried under dynamic vacuum (10ꢀ1 Torr) at 473 K for 2 h prior to the synthesis. The oxide
Bi2O3 was annealed at 1073 K for 12 h to remove traces of water, hydroxides and carbonates. BiF3 was
prepared from Bi(OH)3 by reaction with aqueous HF (40%) and the obtained precipitate was heated at
623 K for 3 h under a flow of gaseous HF. The ready to use starting materials were kept in a dessicator.
Samples of the fluorite-like oxyfluoride solid solution Ba1 ꢀ xBixOzF2 + xꢀ2z (i.e. Ba1 ꢀ xBix(O,F)2 + d
)
were obtained through solid state synthesis. The mixed powders of BaF2, BiF3 and Bi2O3, taken in
appropriate ratios, were ground in agate mortars and introduced into copper tubes. After degassing in
dynamic vacuum at 473 K for 2 h and filling with dry argon, the tubes were sealed, annealed at
873 ꢂ 10 K for 12 h and finally quenched in icy water. The compositions of the samples are presented on
Fig. 1. Compositions were taken along three types of lines:
- with a constant oxygen concentration z = constant,
- with a constant bismuth concentration x = constant,
- with a constant defects concentration d = x ꢀ z = constant.

T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
823
Fig. 1. Composition of the studied samples in the BaF2–Bi2O3–BiF3 system at 873 K: (a) single-phase samples; (b) multi-phase
samples; (c) anion-deficient part of the region.
X-ray powder diffraction analysis was performed with a Guinier camera FR-552 (Cu Ka1-radiation)
using germanium as an internal standard. This method was used for checking the phase composition both
after the synthesis and after the thermal treatment during the conductivity measurements. Several sa-
mples were also studied with a STADI-P diffractometer (Cu Ka1-radiation, curved Ge monochromator,
transmission mode, linear PSD). The indexation and cell parameters refinement were performed with the
program packages POWDER and STOE WinXpow.
Samples for electron microscopy were crushed in ethanol and the fragments were deposited on a holey
carbon grid. Electron diffraction (ED) experiments were carried out using a Philips CM20 microscope.
The conductivity measurements were carried out on powder samples pressed to form pellets with a
thickness ꢁ 1 mm and a diameter ꢁ 8 mm sintered under the same operating conditions as for synthesis.
Vacuum-evaporated gold was used as electrode material. The Au/pellet/Au devices were placed in a
quartz measurement cell, degassed at 473 K for 2 h and filled with dry argon to avoid eventual hydrolysis.
Conductivity was determined by the complex impedance method [15], using a frequency response
analyser Solartron 1260. The frequency range was 10ꢀ1 to 106 Hz over the thermal interval 300–623 K in
several temperature cycles. For each measurement temperature, the sample was kept at that temperature
for 30 min and after the measurement the temperature was automatically increased with a step 10 K.
3. Results and discussion
The homogeneity range of the solid solution Ba1 ꢀ xBixOzF2 + xꢀ2z at 873 K spreads up to x = 0.35 and
z = 0.20 (Fig. 1). The main part of the solid solution has an excess of anions compared to the

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T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
Table 1
˚
Fluorite cell parameter (a, A) of single-phase samples of the Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution
z
x
0.00
6.200(2)
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00
0.05
0.10
0.15
0.20
6.146(2)
6.159(3)
6.123(2)
6.128(2)
6.123(3)
6.125(4)
6.102(2)
6.107(3)
6.102(2)
6.109(2)
6.107(2)
6.072(2)
6.084(2)
6.082(2)
6.088(1)
MPS
6.025(1)
6.039(2)
6.069(1)
6.073(2)
MPS
MPS
MPS
MPS
6.026(2)
6.029(2)
6.032(1)
MPS
MPS
6.023(1)
MPS
MPS: multi-phase samples at 873 K.
stoichiometric fluorite MX2. Some samples of the solid solution fall into the anion deficient part of the
system (they are marked with ‘‘*’’ on Fig. 1). Both anion excess and anion deficient samples form a
homogeneity region of solid solutions at 873 K along the d coordinate (ꢀ0.05 < d < 0.27). The cell
parameters of the single-phase samples of the fluorite-like solid solution Ba1 ꢀ xBixOzF2 + x ꢀ 2z are
gathered in Table 1.
The temperature dependence of the ionic conductivity is shown in Fig. 2 for samples of the
Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution along the x = 0.05, 0.20, 0.30 and the z = 0.15 lines as a plot of
log(sT) versus inverse absolute temperature. The Arrhenius-type behavior is obvious. The activation
energy is calculated from the formula sT = s0 exp(DEs/kT). The ionic conductivity and activation energy
Fig. 2. Temperature dependence of conductivity for the samples of the Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution for different
compositions: x = 0.05: sample 1—Ba0.95Bi0.05O0.00F2.05 (d = 0.05), sample 2—Ba0.95Bi0.05O0.05F1.95 (d = 0). x = 0.20: sample
3—Ba0.80Bi0.20O0.10F2.00 (d = 0.10), sample 4—Ba0.80Bi0.20O0.15F1.90 (d = 0.05). x = 0.30: sample 5—Ba0.70Bi0.30O0.05F2.20
(d = 0.25), sample 6—Ba0.70Bi0.30O0.15F2.00 (d = 0.15). z = 0.15: sample 7—Ba0.65Bi0.35O0.15F2.05 (d = 0.20), sample 6—
Ba0.70Bi0.30O0.15F2.00 (d = 0.15), sample 4—Ba0.80Bi0.20O0.15F1.90 (d = 0.05), sample 8—Ba0.85Bi0.15O0.15F1.85 (d = 0).

T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
825
Table 2
log(s423 K), Vꢀ1 cmꢀ1, of single-phase samples of the Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution
z
x
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00
0.05
0.10
0.15
0.20
ꢀ8.27 [12]
ꢀ6.22
ꢀ7.63
–
ꢀ5.69
ꢀ5.00
ꢀ5.32
ꢀ5.25
ꢀ5.06
–
–
MPS
MPS
MPS
MPS
ꢀ6.63
MPS
ꢀ5.43
ꢀ6.10
ꢀ5.49
–
ꢀ5.52
–
ꢀ4.91
ꢀ7.02
ꢀ4.91
MPS
ꢀ5.50
ꢀ4.90
MPS
ꢀ5.51
MPS
MPS: multi-phase samples at 873 K.
data are summarized in Tables 2 and 3. An unified dependence of either log(s) or DEs on concentration
was not observed for any line with x or z = constant. It is clear from Fig. 2 that the composition
dependence of conductivity is different for different lines: for x = 0.05—s decreases with d decreasing
(samples 1 and 2); for x = 0.20—s increases with d decreasing (samples 3 and 4); for x = 0.30—s is
practically independent on d (samples 5 and 6); for z = 0.15—the dependence s on d has a maximum
(samples 7, 6, 4 and 8).
Probably there exist different regions in the homogeneity range of the solid solution. They differ in the
character of the concentration dependence of the conductivity (Table 2): the conductivity increases
(region I), the conductivity is maximal and almost does not change (region II), the conductivity decreases
(region III). These differences are likely to stem from differences in the defect structure within different
areas of the whole solid solution. These three regions are less obvious on the concentration dependence of
cell parameters (Table 1).
In the first region (I) closest to BaF2, the properties rapidly change upon composition—ionic
conductivity increases on increasing bismuth concentration (x) from log s423 K (Vꢀ1 cmꢀ1) = ꢀ8.27
˚
˚
to ꢀ4.90. The cell parameter decreases (from 6.200(2) A for BaF2 to 6.088(2) A for
Ba0.80Bi0.20O0.15F1.90) according to the substitution of the large barium cation by the smaller bismuth
cation. The dependence of the cell parameter on composition can generally be approximated by a plane
a = 6.19(4) ꢀ 0.571(8)x ꢀ 0.0283(1)d, but the cell parameter is more sensitive to the Bi content (x) than
to the anion excess (d). The first concentration region can be roughly bordered by the low content of
‘‘extra’’ anions (d ꢁ 0.10).
Moving the composition further away from BaF2 (increasing x and d), the ionic conductivity changes
very slightly and the decrease of the cell parameter is slower than in the first region. In this region (II) the
Table 3
Activation energy (eV) of single-phase samples of the Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution
z
x
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00
0.05
0.10
0.15
0.20
0.92 [12]
0.69
0.71
–
0.52
0.63
0.68
0.67
0.65
–
–
MPS
0.60
0.60
0.59
MPS
MPS
MPS
MPS
0.79
0.66
0.71
0.68
–
0.58
–
0.67
0.64
MPS
0.69
MPS
MPS
MPS: multi-phase samples at 873 K.

826
T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
conductivity and the cell parameter vary in the range ꢀ5.69 < log s423 K (Vꢀ1 cmꢀ1) < ꢀ4.91 and
˚
˚
6.025(1) A < a < 6.107(3) A. The x ꢁ 0.30 line can be considered as an approximate boundary of this
region II.
The sample with the largest x value (x = 0.35, z = 0.15) is characterized by a sharp drop of the ionic
ꢀ1
conductivity down to ꢀ6.63 Vꢀ1 cm ; its cell parameter is a = 6.023(1) A. Such conductivity decrease
was also observed for the Ba1 ꢀ xBixF2 + x solid solution at compositions with a high anion excess
(x > 0.35) [12].
˚
By analogy with results published earlier for the Ba1 ꢀ xBixF2 + x solid solution [13] the variation of the
properties of the oxyfluoride solid solution with composition can be correlated with changing defect
types. The composition–property dependence can be explained by the coexistence of three entities (point
defects, small clusters and large cubooctahedra) in the solid solution, with each of them prevailing in one
of the three composition parts. A continuous transition from point defects to cubooctahedral clusters
8:12:n:0 (where n = 0 or 1) via intermediate 4:4:3:0 clusters was considered in [13] for Ba1 ꢀ xBixF2 + x
;
the defects constituting the cubooctaherdral clusters were found not to contribute to the conductivity. It is
possible that in region III (x > 0.30, 0.10 < d < 0.27) with maximal Bi content and anion excess, the
concentration of the cubooctahedral clusters is high enough to suppress the conductivity by blocking
ion migration pathways and decreasing the number of free carriers. In region II (0.10 < x < 0.30,
0.10 < d < 0.27), the formation of few cubooctahedra on a background of dominating small clusters
enhances the conductivity by deforming the fluorite matrix and lowering the energy of migration barriers
[9]. In region I (0 < x < 0.30, d < 0.10) the conductivity seems to depend on the concentration of point
defects since a monotonic increase of the conductivity versus composition is observed. It should be
noted that monotonic changes are observed both in anion-deficient (marked with ‘‘*’’ in Fig. 1) and
anion-excess parts of this region that expands on both sides of the stoichiometric AX2 composition
(ꢀ0.05 < d < 0.10).
A distinctive feature of the fluorite-like Ba1 ꢀ xBixOzF2 + xꢀ2z (i.e. Ba1 ꢀ xBix(O,F)2 + d) solid solution
is a small change of the cell parameters with changing oxygen content (Table 1). Apparently this
phenomenon can be explained by influence of two competing trends. The substitution of Fꢀ by slightly
larger O2ꢀ (RF = 1.17 A, R0 = 1.24 A) leads to a decrease of the total number of anions in according
with the scheme: 2Fꢀ = O2ꢀ. Thus increasing z causes a decrease in d. Cell parameter is more sensitive to
Bi content (x) rather than to the anion excess (d) and oxygen content (z).
ꢀ
2ꢀ
˚
˚
The observed correlation between the conductivity properties, cell parameter and composition allows
to define the concentration region with the best conducting properties. It can be selected based on the data
presented in Tables 1 and 2. For Ba1 ꢀ xBixOzF2 + x ꢀ 2z this region corresponds to the region II where the
conductivity is maximal and almost does not change (log s423 K (Vꢀ1 cmꢀ1) ꢁ ꢀ4.9–ꢀ5.5) in the broad
composition region (x ꢁ 0.25–0.30, d > 0.10).
Beyond the compositional borderline of region II (x ꢁ 0.3) the conductivity decreases. This means that
at a Ba:Bi ratio of 2:1 apparently a novel ordered fluorite-related phase in the BaF2–Bi2O3–BiF3 system
appears. In accordance with the cluster formation model the structure of this novel phase can contain
ordered cubooctahedra similar to those discovered earlier in Ba4Bi3F17 [11].
In order to detect possible superstructures due to ordering of the defects, samples of the
Ba1 ꢀ xBixOzF2 + x ꢀ 2z solid solution were studied by electron diffraction. The typical ED patterns
for two samples Ba0.85Bi0.15F2.15 (i) and Ba0.70Bi0.30O0.10F2.10 (ii) are presented in Fig. 3. ED patterns of
sample (i) revealed no additional features which could indicate a deviation from the cubic fluorite
˚
disordered structure. They can be indexed on a face-centered cubic lattice with a ꢁ 6.1 A, which agrees

T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
827
Fig. 3. Electron diffraction patterns of two different Ba1 ꢀ xBixOzF2 + x ꢀ 2z samples. (i) Ba0.85Bi0.15F2.15 (left column), (ii)
Ba0.70Bi0.30O0.10F2.10 (right column).
with X-ray diffraction results. This observation confirms the absence of short- or long-range interactions
of defects in this sample and can be considered as representative for the concentration region II.
Sample (ii) is located at the border between the regions II and III. ED studies were conducted after
conductivity measurements and hence after additional thermal treatment at 623 K. ED patterns from this
sample exhibit a pronounced superstructure, which can be indexed on a body-centered tetragonal lattice
˚
˚
with cell parameters a ꢁ 9.53 A, c ꢁ 18.10 A (Fig. 3). Those cell parameters are related to the initial
pffiffiffiffiffiffiffi
fluorite parameter afl by the equations a = afl* 5=2, c = 3*afl. The reciprocal lattice vectors of the fluorite
subcell and the tetragonal supercell are related by the matrix:
0
@
1
A
3=2 1=2
ꢀ1=2 3=2
0
0
3
0
0
Except for the h + k + l = 2n reflection condition no other conditions were found; this corresponds to the
¯
following set of space groups: I4/mmm, I4/m, I4mm, I422, I4m2, I42m, I4 and I4.
¯
¯
The phases M2RF7 (M = Ca, Sr; R = rare-earth) [6] have a similar relation of the cell parameters to
the fluorite subcell and can be proposed as a structural prototype for the ordered phase in the sample (ii).
The I4/m space group can be chosen by analogy with the M2RF7 compounds. The lower ionic

828
T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
Table 4
X-ray powder pattern of the ordered tetragonal phase in the Ba0.70Bi0.30O0.10F2.10 sample
˚
dobs (A)
Relative intensity
h
k
l
2uobs (8)
2ucalc (8)
9.09
8.46
1.8
0.8
2.8
1.7
0.5
0.7
1.1
0.2
100.0
1.9
1.7
0.6
0.5
0.2
9.9
25.6
0.8
0.3
0.3
0.1
0.2
0.3
0.6
0.6
1.2
0.5
1.2
1.6
0.5
2.0
0.8
41.5
0.7
0.3
0.4
0.4
0.3
0.6
0.9
0
1
1
1
0
2
2
1
2
1
2
2
2
3
0
3
3
3
2
3
2
3
3
3
4
0
3
2
3
4
1
3
2
3
4
4
5
2
4
5
3
2
5
3
4
4
5
5
0
0
1
0
0
0
1
1
1
0
2
0
2
0
0
1
1
0
1
2
0
1
2
0
1
0
3
1
3
1
1
1
0
3
1
2
0
2
0
1
2
1
0
3
2
1
2
0
2
1
2
3
4
2
1
4
3
5
0
4
2
1
6
0
2
3
5
1
6
4
3
5
1
8
0
7
2
3
8
6
8
4
5
4
1
8
6
0
7
9
3
6
6
7
3
5
9.722
10.453
16.356
17.335
19.533
21.029
21.386
23.622
25.538
26.255
26.419
27.106
28.204
28.498
29.491
29.603
31.236
31.772
32.353
34.247
35.097
35.716
37.067
37.598
39.234
39.673
40.062
40.688
41.380
41.780
41.988
42.315
44.164
44.989
46.520
47.055
47.936
48.326
48.625
9.738
10.471
16.365
17.345
19.547
21.031
21.387
23.614
25.535
26.247
26.416
27.107
28.215
28.486
29.504
29.601
31.232
31.776
32.361
34.236
35.103
35.723
37.069
37.582
39.238
39.694
40.087
40.691
41.352
41.780
41.981
42.285
44.171
44.973
46.522
47.052
47.925
48.314
48.586
48.650
49.169
49.986
50.112
50.551
52.462
53.014
53.908
54.292
5.415
5.112
4.541
4.221
4.152
3.763
3.485
3.392
3.371
3.287
3.162
3.130
3.026
3.0152
2.8612
2.814
2.765
2.616
2.555
2.512
2.423
2.390
2.294
2.270
2.249
2.216
2.180
2.160
2.150
2.134
2.049
2.013
1.9506
1.9297
1.8962
1.8819
1.8709
1.8524
1.8237
1.8188
1.8044
1.7426
1.7271
1.6991
1.6876
0.6
6.8
22.9
0.5
5.4
0.3
1.4
0.5
49.145
49.970
50.113
50.542
52.469
52.976
53.920
54.317

T.V. Serov et al. / Materials Research Bulletin 40 (2005) 821–830
829
Table 4 (Continued )
˚
dobs (A)
Relative intensity
h
k
l
2uobs (8)
2ucalc (8)
1.6439
1.6351
1.6090
1.5973
1.5899
0.2
0.3
0.2
0.2
0.5
4
5
5
2
5
6
3
0
3
3
2
1
0
1
8
0
2
55.886
56.213
57.208
57.666
57.960
55.900
56.204
57.191
57.631
57.928
57.985
59.385
10
6
0
10
1.5547
0.4
59.400
˚
˚
Tetragonal I-centered, a = 9.5355(4) A, c = 18.151(1) A, F(30) = 86.4 (0.009, 39).
conductivity of the sample with x = 0.30, z = 0.10 as compared to the neighboring ones (Table 2) suggests
some structural changes during the impedance spectroscopy measurements. New examination of the
Ba0.70Bi0.30O0.10F2.10 sample after the impedance spectroscopy (slow heating to 623 K with slow
cooling) was performed with X-ray powder diffractometer and showed the presence of superstructure
reflections on the X-ray powder patterns. The unit cell determined from the ED data was used to index the
X-ray powder diffraction pattern. The indexation was performed on a tetragonal lattice with cell
˚
˚
parameters a = 9.5355(4) A, c = 18.151(1) A, (F(30) = 86.4 (0.009, 39)) (Table 4). Except the tetragonal
main phase, Ba4Bi3F17 and BaF2 phases were found as admixtures in the Ba0.70Bi0.30O0.10F2.10 sample.
Apparently, the new tetragonal phase crystallizes on cooling the fluorite-like solid solution, belonging
to the region III, at temperatures below 873 K. Evidently, such crystallization also occurred at an
annealing at T < 623 K during the conductivity measurement. To confirm this, a low temperature
annealing (573 K/6 h) of x = 0.3 solid solution was performed and indeed resulted in the formation of the
tetragonal ordered phase. The determination of the exact composition and the crystal structure of the new
tetragonal phase in the BaF2–Bi2O3–BiF3 system is now in progress and will be reported in a forthcoming
publication. The detected ordering is most probably the reason for the drastic decrease of conductivity for
the samples in region III.
Acknowledgments
The authors are grateful to the Russian Foundation for Basic Research (grant no. 02-03-32847) and
Russian Ministry of Education (grant no. T02-094-1158) for support. This work has been partially
performed within the framework of IAP V-1.
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