Materials based on BIFEVOX and bismuth or iron simple oxides nanopowders


202

Krylov A.A., Emelyanova Yu.V., Buyanova E.S.,  
Morozova M.V., Vylkov A.I., Chuykin A.Yu.

Chimica Techno Acta. 2017. Vol. 4, No. 3. P. 202–208.
ISSN 2409–5613

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05 A.A. Krylov1, Yu.V. Emelyanova1, E.S. Buyanova1,  

M.V. Morozova1, A.I. Vylkov2, A.Yu. Chuykin2
1Ural Federal University

19 Mira St., 620000, Ekaterinburg, Russia
2Institute of High Temperature Electrochemistry UB RAS

20 Akademicheskaya St, 620137, Ekaterinburg, Russia

Materials based on BIFEVOX and bismuth  
or iron simple oxides nanopowders

Compositions of composite materials based on BIFEVOX and nanopowders of 
bismuth and iron oxides have been obtained. The absence of chemical interac-
tion between the components has been proved, the total electrical conductivity 
of materials in the average temperature region has been determined. It has 
been shown that under the selected formation conditions, it has not yet been 
possible to achieve significant improvement of the functional characteristics of 
heterogeneous compositions in comparison with individual phases. However 
positive results on chemical and structural stability give way to further inves-
tigations.

Keywords: BIMEVOX; Oxygen-ion conductors; Electrical conductivity; Impedance spectros-
copy.

Received: 22.09.2017; accepted: 17.10.2017; published: 20.10.2017.

© Krylov A.A., Emelyanova Yu.V., Buyanova E.S., Morozova M.V., Vylkov A.I.,  
Chuykin A.Yu., 2017

Introduction
The family of solid electrolytes 

with the general formula Bi4V2–xFeхO11-δ 
(BIFEVOX) is characterized by high oxy-
gen-ion conductivity at intermediate tem-
peratures 550–950 K [1–5]. The high-tem-
perature γ-modification of the BIFEVOX 
solid solutions with tetragonal structure 
(space group I4/mmm) is obtained at 
0.3  ≤ x ≤ 0.5 iron concentration range. 
For this modification, the electrical con-
ductivity versus temperature dependence 
is linear, and the activation energy at high 
temperatures has a value of 0.2–0.4 eV, 
which is characteristic of BIFEVOX. The 
transition to an ordered γʹ modification 
with decreasing temperature is accompa-

nied by a small change of slope of the lg σ 
– 103/T dependence and, accordingly, an 
increase in the activation energy to 0.5–
0.7 eV. The tetragonal γ-modification of 
the Bi4V2–xFexO11–δ is sufficiently stable in 
a wide range of thermodynamic parame- 
ters (T = 298 – 1073 K, lg pO2 (atm) = 
–0.68 to –18.0) [4, 5]. The change of the 
structure into the orthorhombic one oc-
curs in atmosphere with low oxygen con-
tent (lg pO2 (atm) <–14.0) at temperatures 
above 773 K. However, decomposition 
of the sample does not occur. Evaluation 
of the structural and thermal stability of 
BIFEVOX in air at long time exposures (at 
least two weeks at the same temperature) 



203

in the temperature range 723–1083 K re-
vealed no changes in the structure or ap-
pearance of any additional phases [6].

In the last decade composite electro-
lytes are actively studied as alternative 
electrolyte materials. It has been shown 
that in this way it is possible to improve 
the quality of the material and remove 

some disadvantages of individual electro-
lytes [7]. There are examples of creating 
composite materials with BIMEVOX as 
their components [8–11]. For the modifi-
cation of BIFEVOX based electrolyte ma-
terials, the approach using simple oxides 
nanopowders is used in this paper.

Experimental
Samples of Bi4V2–xFexO11–δ (x = 0.3, 

0.5) solid solutions were synthesized ac-
cording to the standard ceramic technolo- 
gy [2]. The preparation of nanopowders 
of bismuth and iron oxides was carried 
out by laser evaporation of a target and 
condensation of vapors in a working gas 
stream at the Institute of Electrophysics of 
the Ural Branch of the Russian Academy of 
Sciences. In this method a fiber ytterbium 
laser LS-1 with diode pumping was used. 
The average radiation power was 1000 W 
with a smooth adjustment from 20 to 
100  %, the wavelength 1070 nm, and the 
radiation regime continuous or modulat-
ed. The evaporation targets were prepared 
by pressing from a coarse-grained oxide 
powder followed by annealing at a temper-
ature providing a partial sintering of the 
powder to provide mechanical strength 
of the compact. Composites were pre-
pared by mechanically mixing of the cor-
responding powders with a simple oxide  
content of 10 to 50 wt.%. The powders 

were pressed into pellets with a diameter 
of 10 mm on a hydraulic press in the form 
of pellets and annealed at 1073 K.

The phase composition of the final 
solid oxide products was checked by 
X-ray powder diffraction (DRON-3 dif-
fractometer, CuKα radiation, pyrolytic 
carbon monochromator, reflected beam). 
The particle size of the powders was de-
termined using a laser dispersion analyzer 
SALD-7101 Shimadzu. The morpholo- 
gy of the obtained powders and their 
chemical composition were studied using 
a JEOL JSM6390 LA scanning electron 
microscope equipped with a JED-2300 
energy dispersive X-ray detector. Thermal 
dilatometric analysis was performed on a 
DIL 402 C Netzsch dilatometer equipped 
with a vacuum furnace. Electrical con-
ductivity measurements of the ceramic 
samples were performed on Elins Z-3000 
impedance spectrometer in the tempera-
ture range 1073–473 K.

Results and discussion
1.  Synthesis and characterization of 

the materials
Bi4V2–xFexO11–δ (x = 0.3, 0.5) samples, 

obtained by the standard ceramic tech-
nology, are single-phase and have the 
structure of high-temperature tetragonal 
γ-modification (space group I4/mmm). 
The average particle size of Bi4V2–xFexO11–δ 

is in the range of 0.5–10 μm. The bismuth 
oxide nanopowder is single-phase, and 
is β-Bi2O3 with tetragonal structure. Iron 
oxide nanopowder contains three crys-
talline phases where iron is in different 
oxidation states, so its composition is de-
noted as FeOx. This is composed of Fe3O4 
(magnetite) with its content 69 %, Fe2O3 



204

(hematite, 10 %) and ε-Fe2O3 (21 %). The 
average particle size of nanopowders is in 
the range of 50–100 nm.

In accordance with the results of the 
XRD, the calculation of the unit cell pa-
rameters was carried out for β-Bi2O3 in 
the tetragonal structure (space group 
I4/mmm), for Fe3O4 in cubic (Fd-3m), 
Fe2O3 for rhombohedral (space group 
R-3c), ε-Fe2O3 for orthorhombic (space 
group Pna21) structure. The results are 
shown in Table 1.

Taking into account that the 
BIMEVOX materials are non-stable 
in a reducing atmosphere as well as 
Bi4V2–xFexO11–δ undergoes transition from 
tetragonal structure to orthorhombic one 
in air at ca. 773 K without decomposition 
[4, 5] the sample of Bi4V1.5Fe0.5O11–δ pre-
pared accordingly was investigated in the 
reducing atmosphere with log(pO2/atm) 
 < –14.0 by means of dilatometry equipped 
with special chamber in order to reveal 
a  possible structure transition. The study 
was carried out with sequential change 
of gas atmosphere from air via argon to 
mixture of argon and hydrogen, and back 
to air in the heating and cooling cycles. 
When the sample was found to be heated 
in air a slight change in the slope of curve 
1 at 890 K (see Fig. 1) corresponding to an 
order-disorder type γ ↔ γʹ phase transi-
tion [12] is observed. However, there is no 

such transition upon heating in the reduc-
ing atmosphere while a γ ↔ β phase tran-
sition is observed at ca. 850 K. It is worth 
noting that final cooling curve recorded 
in air after reduction-oxidation cycling 
did not show indication of any transition. 
The unit cell parameters a = 3.919 and c = 
15.509 Å of the cooled sample were found 
by XRD to remain practically unchanged 
as compared to those of Bi4V1.5Fe0.5O11–δ 
before the reduction-oxidation treatment. 
Aforementioned results indicate obvious-

Table 1
Crystal structure parameters of the materials

Composition a ± 0.001, Å b ± 0.001, Å c ± 0.004, Å
Bi4V1.7Fe0.3O11–δ 3.919 3.919 15.468

Bi4V1.5Fe0.5O11–δ 3.918 3.918 15.524

β-Bi2O3 7.729 7.729 5.648

Fe3O4 8.356 8.356 8.356

Fe2O3 5.034 5.034 13.727

ε-Fe2O3 5.091 8.804 9.446

Fig. 1. Thermal strain of the Bi4V1.5Fe0.5O11–δ 
sample: 1 – heating up to 1023 K in air;  
2 – 30 minutes holding in air at 1023 K;  
3 – 1 hour holding in argon at 1023 K;  
4 – 2 hours holding in argon-hydrogen 

mixture at 1023 K (50 % H2 and 50 % Ar 
for this and next three steps); 5 – cooling in 
argon-hydrogen mixture, 6 – second heating 
in air-argon mixture; 7 – 30 minutes holding 

in argon-hydrogen mixture at 1023 K;  
8 - 1 hour holding in argon at 1023 K;  
9 – 3.5 hours holding in air at 1023 K,  

10 – cooling in air



205

ly in favor of high resistance of the BIFE-
VOX structure to alteration under reduc-
ing conditions. The value of the linear 
thermal expansion coefficient (LTEC) of 
the BIFEVOX before and after the oxida-
tion-reduction cycle also did not change 
significantly and remained in the range 
17–19∙10–6 K–1 (Fig. 2). Annealing in hy-
drogen atmosphere of the Bi4V1.7Fe0.3O11–δ 
sample at 1073 K for 8 hours was carried 
out to estimate the possibility of decom-
position of the Bi4V2–xFexO11–δ series at 
lgPO2 (atm) <–14.0. In addition to the 
Bi4V2–xFexO11–δ lines, peaks correspond-
ing to BiVO4 and Bi2O3 (or solid solutions 

based on them) as well as to metallic iron 
were found on the X-ray diffraction pat-
tern of the sample. These results show that 
samples of the BIFEVOX system, being 
annealed in air after the reduction, return 
to their original state with the same crys-
tal structure.

2. Preparation and characterization of 
composite materials.

X-ray phase analysis was used to test 
the possible interactions in the compo- 
site by annealing pellets of Bi4V1.7(1.5)
Fe0.3(0.5)O11–δ/x wt.%. Bi2O3 (FeOx) com-
posites at 1073 K. All X-ray diffraction 
patterns contain only composite compo-
nents lines, without extra reflexes.

As an additional method for deter-
mining the phase and element composi-
tion of composites, the scanning electron 
microscopy (SEM) method with the en-
ergy-dispersive microanalysis was used. 
For the sintered samples, the surface and 
cross-section of the composite pellets 
were examined. It was established that the 
surface of the samples is porous, consists 
of grains of various shapes and sizes, the 
visual contrast is determined by the to-
pography of the sample surface (Fig. 3).

Large grains of BIFEVOX and fine 
grains of nanopowder particles are clearly 

Fig. 2. LTEC change versus temperature: 
1 – first heating in air, 2 – cooling in air 

after reduction-oxidation cycle, 3 – heating 
in argon-hydrogen mixture, 4 – cooling in 

argon-hydrogen mixture

Fig. 3. The images of the surface of the composite samples:  
a – Bi4V1.5Fe0.5O11–δ/40 wt.%. Bi2O3; b – Bi4V1.7Fe0.3O11-δ/10 wt.%. FeOx

a b



206

visible, the iron oxide particles being ag-
gregated to a lesser extent and covering 
the coarse grains of the BIMEVOX com-
plex oxide. The particles of bismuth ox-
ide are combined into aggregates and fill 
the space between the coarse BIMEVOX 
particles. The chemical composition of 
the particles was estimated by energy-
dispersive X-ray spectroscopy (EDX), 
and results correspond to the nominal 
ratio of elements in simple and complex 
oxides, which additionally indicates the 
absence of interaction in the composi- 
tes under the selected processing condi-
tions. An example of the X-ray disper-
sion energy spectrum of a surface of the 
Bi4V1.7Fe0.3O11–δ/10 wt.%. FeOx composite 
is shown in Fig. 4.

Determination of the electrochemical 
characteristics of the composite materials 
was carried out by the impedance spec-
troscopy method. Complex plane plots 
of the BIFEVOX solid solutions consist 
of two joint half-circles, showing behavi- 
or typical for the BIMEVOX family ionic 
conductors [2].

Fitting of the Cole-Cole plots was per-
formed using the equivalent electrical 
circuits method [2]. Typical temperature 
dependences of the total conductivity are 

shown in Fig. 5. The dependences of the 
total conductivity on temperature for the 
composites studied have the form cha- 
racteristic for the γ-modification of BIFE-
VOX. The change of slope is observed 
in the dependencies at the temperature 
range 750–850 K. It corresponds to the 
transition of BIFEVOX to an ordered γʹ-
modification with decreasing temperature 
and is accompanied by the increase of the 
activation energy from 0.4 to 0.7–0.8 eV. 
The behavior of all lg σ – 103/T depend-
encies, corresponding to the composites 
with different content and nature of the 
simple oxide added is similar.

As the concentration of the simple  
oxide increases, the conductivity decreas-
es. This situation is typical for the entire 
temperature range. This is probably due 
to an increase in the concentration of 
the less conducting phase, which are the 
simple oxides used in comparison with 
pure BIFEVOX. For example, according 
to [13, 14], for the β-Bi2O3 at 873  K the 
value of the total electrical conductivity is  

Fig. 4. EDX spectrum of the 
Bi4V1.7Fe0.3O11–δ/10 wt.%. FeOx  

composite surface

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200

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400

500

600

700

800

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L
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1.00 2.00

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Fig. 5. Total electrical conductivity versus 
temperature: 1 – Bi4V1.7Fe0.3O11–δ;  

2 – Bi4V1.5Fe0.5O11–δ; 3 – Bi4V1.5Fe0.5O11–δ/wt.% 
FeOx; 4 – Bi4V1.5Fe0.5O11–δ/wt.% FeOx;  

5 – Bi4V1.5Fe0.5O11–δ/40 wt.% Bi2O3;  
6 – Bi4V1.7Fe0.3O11–δ/10 wt.% Bi2O3



207

~10–3 Ohm–1 cm–1, for Bi4V1.7Fe0.3O11–δ in 
our work it is 1.5 × 10–2 Ohm–1 cm–1. The 
effect of an increase of the overall elec-
trical conductivity values of composite 

samples, which could be associated with 
a possible increase in the sintering quality 
of the bars, is not observed.

Conclusions
Thus, compositions of composite 

materials based on BIFEVOX and nano-
powders of bismuth and iron oxides have 
been obtained, the absence of interaction 
between them has been proved, the total 
electrical conductivity of materials in the 
region of average temperatures has been 
determined. It has been shown that, un-

der the selected formation conditions, it 
has not yet been possible to achieve sig-
nificant improvement of the functional 
characteristics of heterogeneous com-
positions in comparison with individual 
phases. However, the results obtained can 
serve as a basis for further searching for 
optimal solutions.

Acknowledgements
The work was partially supported by the Scholarship of the President (SP-3376.2016.1) 

and Russian Foundation for Basic Research (project No 17-53-04098).

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Cite this article as:
Krylov AA, Emelyanova YuV, Buyanova ES, Morozova MV, Vylkov AI, Chuykin AYu. 
Materials based on BIFEVOX and bismuth or iron simple oxides nanopowders. Chimica 
Techno Acta. 2017;4(3):202–8. DOI:10.15826/chimtech/2017.4.3.05.