New complex-oxide materials Bi25.6Ba0.4Mo10–2yMe″2yO69–d: Synthesis and characteristics

Article abstract of DOI:10.1134/S1070427217060040

An integrated study was carried out of complex-oxide materials based substituted bismuth molybdate Bi₂₆Mo₁₀O₆₉, which exhibit such potentially valuable characteristics as oxygen-ion conductivity at medium temperatures and catalytic activity. Compounds with formula Bi_25.6Ba_0.4Mo_10–2yMe'' _2yO_69–d were synthesized by the standard ceramic technology and characterized by the methods of X-ray phase analysis, scanning electron microscopy, and laser light scattering. The morphological characteristics of the ceramic based on Bi_25.6Ba_0.4Mo_10–2yMe'' _2yO_69–d were examined; the catalytic characteristics of granulated powders were studied for the example of ethylene oxidation; the method of impedance spectroscopy was employed to examine the temperature dependence of the electrical conductivity of the samples; it was shown that the conductivity of Bi_25.6Ba_0.4Mo_10–2yMe'' _2yO_69–d exceeds that of the matrix composition.

Full text of DOI:10.1134/S1070427217060040

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 6, pp. 853−861. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © Z.A. Mikhaylovskaya, E.S. Buyanova, S.A. Petrova, O.V. Russkikh, I.V. Nikolaenko, 2017, published in Zhurnal Prikladnoi Khimii, 2017,
Vol. 90, No. 6, pp. 696−704.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
New Complex-Oxide Materials Bi Ba Mo
Me'' O :
2y 69–d
25.6
0.4
10–2y
Synthesis and Characteristics
a
a
b
Z. A. Mikhaylovskaya *, E. S. Buyanova , S. A. Petrova ,
a
a,c
O. V. Russkikh , and I. V. Nikolaenko
a
Ural Federal University named after the first President of Russia B.N. Yeltsin,
ul. Bolʼshakova 77, Yekaterinburg, 620144 Russia
*
e-mail: zozoikina@mail.ru
b
Institute of Metallurgy, Ural Branch, Russian Academy of Sciences, ul. Amundsena 101, Yekaterinburg, 620016 Russia
c
Institute of Solid-State Chemistry, Ural Branch, Russian Academy of Sciences,
ul. Pervomaiskaya 91, Yekaterinburg, 620990 Russia
Received July 18, 2017
Abstract—An integrated study was carried out of complex-oxide materials based substituted bismuth molybdate
Bi Mo O , which exhibit such potentially valuable characteristics as oxygen-ion conductivity at medium tempera-
26
10 69
tures and catalytic activity. Compounds with formula Bi25.6Ba Mo
Me'' O
were synthesized by the standard
0.4
10–2y
2y 69–d
ceramic technology and characterized by the methods of X-ray phase analysis, scanning electron microscopy, and
laser light scattering. The morphological characteristics of the ceramic based on Bi25.6Ba Mo Me'' O were
0.4
10–2y
2y 69–d
examined; the catalytic characteristics of granulated powders were studied for the example of ethylene oxidation;
the method of impedance spectroscopy was employed to examine the temperature dependence of the electrical
conductivity of the samples; it was shown that the conductivity of Bi25.6Ba Mo
Me'' O
exceeds that of the
0.4
10–2y
2y 69–d
matrix composition.
DOI: 10.1134/S1070427217060040
Materials based on bismuth oxide are the object of
extensive studies because of possessing valuable applied
properties: ionic conductivity [1, 2] and ferroelectric
[11] and are chemically inert toward electrode materials
and, consequently, are a promising material for sensors
and electrochemical gas pumps.Aphotocatalytic activity
of this class of compounds has been reported [12].
[3] and catalytic properties [4]. Compounds of the Bi–
Me–O systems are for the most part described in terms
of fluorite-like structures; however, new structural types
have been found for complex bismuth–molybdenum
oxides at high bismuth concentration [5–9]. Of particular
interest as regards its structure and properties is
bismuth molybdate of composition Bi Mo O . It
There have been a rather large number of studies
concerned with the structure and properties of compounds
based on Bi Mo O . The crystal structure of
2
6
10 69
Bi Mo O has the form of infinite [Bi O ] columns
2
6
10 69
12 14 n
oriented along the y axis and surrounded with MoOn
polyhedra and isolated bismuth ions [6, 7]. It has been
shown that isostructural compounds or solid solutions can
be formed in different ways. First, this can be done by
substitution of molybdenum with the following atoms: Li,
Mg, Al, Ge, V [13], P, W [10], Co [14], Zr, Nb [15], Fe
2
6
10 69
is homogeneous at metal ratios in the range 2.6 ≤ Bi/
Mo ≤ 2.8 and exhibits properties of an oxygen-ion
−
3
–1
conductor with conductivity of ~10 S cm [5]. The
charge transfer in the bismuth molybdate structure occurs
predominantly in the 1D way along the crystallographic
axis y [10]. Further studies have shown that compounds
isostructural with bismuth molybdate can exhibit higher
[11]. Second, this can be done by substitution of isolated
bismuth ions with Pb [16], Ca, Sr, Ba [10, 17], Mg [17],
and Mn [18] and rare-earth elements (REEs) [19]. The
–2
–1
conductivities (10 S cm ) [10]. It has been reported that
compounds based on Bi Mo O have medium values
substitution in [Bi O ] has been only described for
2
6
10 69
12 14 n
–
6
–1
of the thermal expansion coefficient (14–16) × 10 K
tellurium.
8
53

854
MIKHAYLOVSKAYA et al.
The most promising area of research for improving
keeping at a point for 1 s), with the use of the software
packages Fullprof suite [22] and TOPAS [23]. Powder
samples were studied by IR Fourier spectroscopy with
a Nicolet 6700 spectrometer by the FTIR method in the
the conducting characteristics of materials of this kind,
compared with monosubstituted materials, consists
in obtaining compositions with double substitution
and studying their properties. Upon introduction of
substituents into the molybdenum and bismuth sublattice,
the formula of the solid solutions takes the general form
–
1
range 400–1600 cm . The particle size of the powders
was determined with a Shimadzu SALD-7101 dispersity
analyzer. The morphology of the resulting powder and
ceramic samples was examined by scanning electron
microscopy (SEM, JEOLJSM 6390LAscanning electron
microscope with a JEIOL JED 2300 energy-dispersive
attachment andAnalysis Station, JED series, Version 3.7
software package [24]). To study their catalytic properties,
powder samples were granulated, with the s = 1.4–1.6 mm
fraction selected. The ethylene oxidation reaction was
performed in a flow-through reactor. The content of
hydrocarbons and carbon oxides in the starting and final
reaction mixtures was determined with a Testo-350 XL
gas analyzer. Ceramic samples for electrical conductivity
studies were prepared by compaction of powders of the
final product, followed by annealing at the final synthesis
temperature (24 h). The edge surfaces of the sintered
ceramic pellets were covered with finely dispersed
platinum. The measurements by impedance spectroscopy
were made in a two-contact cell with platinum electrodes
at 573–1073 K in the cooling mode with an Elins Z-3000
impedance meter at frequency varied within the range
from 3 MHz to 10 Hz. The impedance hodographs were
analyzed by the method of equivalent circuits (Zview
software, Version 2.6b, Scribner Associates, Inc.).
Bi26–2xMe' Mo
Me'' O69–d. For example, the following
2x
10–2y
2y
compositions have been studied: Bi Me' Mo Me''O
2
4
2
8
2
69–d
(
Me' = Ca, Sr, Ba, and Me'' = Cr, W) [20] and
Bi13−2xTe Mo8−2xV2+2xO68 [21]. These compositions
2x
demonstrated positive changes in electrical conductivity,
compared with the matrix compound.
Our study is devoted to synthesis and analysis
of the properties of bismuth molybdate with double
substitution: Bi25.6Ba Mo
Me'' O69–d, where Me'' are
0.4
10–2y
2y
d-elements (Co, Fe, V). The fixed barium concentration
corresponds to the best conducting composition of
the monosubstituted bismuth molybdate of the series
Bi26–2xBa Mo O
[17]. The type of the second
2x
10 69–d
substituting cation was chosen with consideration for
the results of our preceding studies, which enabled an
effective comparison of the structural characteristics
and target properties of mono- and disubstituted
compositions Bi25.6Ba Mo
Me'' O69–d.
0.4
10–2y
2y
EXPERIMENTAL
Powders of the complex oxides Bi25.6Ba Mo
0.4
10–
Me'' O (0.0 ≤ y ≤ 1) were synthesized by the standard
2y
2y 69–d
RESULTS AND DISCUSSION
ceramic technology from the oxides Bi O (99.9%), MoO
2
3
3
(
99.5%), Fe O (99.9%), V O (99.8%), Co O (99.5%)
2 3 2 5 2 3
The method of solid-phase synthesis was
used to obtain solid solutions of compositions
and barium carbonate BaCO (99.8%). On being mixed
3
and homogenized, the stoichiometric mixture of the
starting compounds was pelletized and then annealed. The
pelletization was performed by the isostatic compaction
method. To minimize the contact between a pellet and
the crucible material in the course of annealing, we used
a powder bed of substituted bismuth molybdate of the
corresponding composition. The synthesis was performed
in the following temperature modes: the first stage at
Bi25.6Ba Mo
Co O69–d, Bi Ba Mo
Fe O69–d,
0.4
10–2y
2y
25.6
0.4
10–2y 2y
Bi25.6Ba Mo
V O69–d. The structural characterization
0.4
10–2y 2y
was performed by X-ray phase analysis (XPA), with a
typical X-ray diffraction pattern shown in Fig. 1. The
limits of the existence region of the solid solutions are
presented in Table 1. A narrowing of the homogeneity
ranges and stabilization of the monoclinic modification
(space group P2/C) of bismuth molybdate are observed
for the solid solutions under study, compared with similar
monosubstituted composition. This is in all probability due
to the structural deformation caused by the introduction
of barium. On the whole, the change in the width of the
homogeneity range for the solid solutions under study
is correlated with the sizes of cations substituting each
other in the series with both mono- and disubstitution
6
23 K (48 h, with quenching to room temperature), and
the second, at 1123 K (24 h, slow cooling). The boundaries
of the homogeneity regions, regions in which structural
modifications exist, and structural parameters of the solid
solutions Bi25.6Ba Mo
were determined by X-ray powder diffraction (Bruker
Me'' O
(Me'' = Co, Fe, V)
0.4
10–2y
2y 69–d
Advance D8, Cu radiation, Ni-filter, θ/θ configuration
at 2θ in the range 5–105° with a step of 0.021° and
K
α
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

NEW COMPLEX-OXIDE MATERIALS
855
I, rel. units
1
2000
1
2
I, rel. units
3
0
2
5
30
35
2θ, deg
1
2000
1
2
3
0
2
0
40
60
80 2θ, deg
Fig. 1. Typical X-ray diffraction patterns of substituted Bi Mo O : (1) Bi25.6Ba0.4Mo9.9Co0.1O69–d, (2) Bi25.6Ba0.4Mo9.6Fe0.4O69–d
,
26
10 69
(3) Bi25.6Ba0.4Mo8.8V1.2O69–d. (I) Intensity and (2θ) Bragg angle.
(
rMo6+ = 0.41 Å, r 5+ = 0.355 Å, rFe3+ = 0.49 Å, rCo2+ =
Bi25.6Ba Mo V O69–d, a monotonic variation of
10–2y 2y
V
0.4
0
.72 Å) [25]. The narrowest values are characteristic
parameters is observed for solid solutions of the same
symmetry. As the unit cell symmetry increases to
become monoclinic, an abrupt change is observed in the
of the series with the cobalt cation (which is larger
than the others) as the second dopant. It is known that
the Bi25.4Ba Mo O
solid solution corresponding
dependence (y ≥ 0.05 for Bi25.6Ba Mo Fe O
0.4
10 69–d
0.4
10–2y 2y 69–d
to the composition with y = 0 is the boundary of the
and y ≥ 0.1 for Bi25.6Ba Mo
V O69–d), which is
10–2y 2y
0.4
existence domain of the monoclinic modification in the
characterized by the general contraction of the unit cell,
possibly due to a slight rearrangement of the oxygen
sublattice. In this case, parameter b remains nearly
unchanged due to the higher rigidity of the structure in
Bi26–2xBa Mo O
series [17]. Introduction of even a
2x
10 69–d
small amount of the second dopant into this composition
results in the stabilization of the monoclinic structure.
The dependence of the unit cell parameters of solid
solutions containing iron or vanadium as the second
dopant is shown in Fig. 2, with the existence domains
of the triclinic and monoclinic symmetries separated
8n+
this direction, provided by the presence of [Bi12O14]
n
columns oriented along the y axis.
The IR spectra of the solid solutions
Bi25.6Ba Mo Me'' O (Me'' = Fe, V) (Fig. 3)
0.4
10–2y
2y 69–d
by a vertical bar. For Bi25.6Ba Mo
Fe O and
contain characteristic bands associated with vibrations
0.4
10–2y 2y 69–d
Table 1. Homogeneity range of solid solutions based on di- and monosubstituted bismuth molybdate Bi Mo O
10 69
2
6
Composition
Homogeneity range
Composition
Homogeneity range
Bi25.4Ba Mo
V O
y ≤ 0.6
y ≤ 0.2
y ≤ 0.05
Bi Mo
V O
y ≤ 0.75 [15]
y ≤ 0.35 [11]
y ≤ 0.2 [14]
0.4
10–2y 2y 69–d
26
10–2y 2y 69–d
Bi25.4Ba Mo
Fe O
Bi Mo
Fe O
0.4
10–2y 2y 69–d
26
10–2y 2y 69–d
Bi25.4Ba Mo
Co O
Bi Mo
Co O
2y 69–d
0.4
10–2y
2y 69–d
26
10–2y
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

856
MIKHAYLOVSKAYA et al.
a, Å
(a)
a, Å
(b)
b, Å
c, Å
b, Å
c, Å
Fig. 2. Dependence of unit cell parameters a, b, c of complex oxides (a) Bi25.6Ba0.4Mo10–2yFe O
and (b) Bi25.6Ba0.4Mo10–2yV O
2y 69–d
2y 69–d
on composition y.
Mo–O vibrations
Bi–O vibrations
ν, cm–1
Fig. 3. Examples of IR Fourier spectra of bismuth molybdates. (A) Reflectance and (ν) wave number.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

NEW COMPLEX-OXIDE MATERIALS
(b)
(c)
857
(a)
1
0 μm
10 μm
5
μm
I
(f)
Bi Mb
(d)
(e)
Bi Ma
Mo La
Ba Lb
Ba La V Ka
V La
O Ka
0
1.6
3.2
4.8 E , keV
b
5
μm
10 μm
Fig. 4. Examples of SEM micrographs of samples: (a) Bi25.6Ba0.4Mo9.8Fe0.2O69–d powder, reflected electrons, magnification 1000;
b) surface of a ceramic sample Bi25.6Ba0.4Mo9.8Fe0.2O69–d, reflected electrons, magnification 5000; (c) surface of ceramic sample
Bi25.6Ba0.4Mo9.6V0.4O69–d, secondary electrons, magnification 5000; (d) cleaved cross-section of a ceramic sample Bi25.6Ba0.4Mo9.6V0.4O69–d
(
,
secondary electrons, magnification 5000; (e) cleaved cross-section of a ceramic sample Bi25.6Ba0.4Mo9.0V1.0O69–d, secondary electrons,
magnification 5000; (f) energy-dispersive spectrum of Bi25.6Ba0.4Mo9.6V0.4O69–d powder, which demonstrates the overlapping of the
main lines of Mo and Bi. (I) Intensity and (Е ) binding energy.
b
of bismuth–oxygen and molybdenum–oxygen groups
typical of bismuth molybdates [26–29]. Absorption
bands associated with stretching and bending vibrations
(Figs. 4b–4e). Cross-sectional SEM images of cleaved
pellets (Fig. 4e) show that a dense ceramic is formed
with mostly closed nearly spherical pores, which is
indicative of the correctly chosen sample sintering mode.
The linear X-ray energy-dispersive (EDS) spectra have
overlapping lines of bismuth and molybdenum (Fig. 4f),
which gave no way of determining the quantitative ratio
of these elements. However, a composition analysis for
the metallic components at different points of the surface
and cleaved cross-section of the pellets demonstrate a
homogeneous distribution of the elements over the surface
of single-phase samples. Thus, the electron microscopic
results combined with X-ray phase analysis (XPA) data
confirmed the formation of solid solutions with certain
–1
of Bi–O bonds are observed at 570–400 cm [26–28].
–
1
The region 680–900 cm contains characteristic bands
associated with stretching vibrations of the Mo–O bond in
–1
MoO tetrahedra: the band at 830–900 cm is associated
4
with symmetric vibrations, and the bands near 680 and
–1
7
80 cm , with asymmetric vibrations of MoO . As the
4
dopant (Fe, V) concentration is raised, there is no shift
of the characteristic bands in the range of vibrations
related to the Bi–O bond. At the same time, the band
frequencies are changed by 3–10 cm–1 in that part of
the spectrum which is associated with Mo–O vibrations.
This confirms the incorporation of Fe and V into the
molybdenum sublattice, which leads to deformation of
MeOn polyhedra (Me = Mo, V/Fe) and changes the length
of metal–oxygen bonds.
homogeneity regions Bi25.6Ba Mo
Fe O
and
0.4
10–2y 2y 69–d
Bi25.6Ba Mo
V O69–d, which form a rather dense
0.4
10–2y 2y
ceramic that is satisfactory for being used as a material
for membranes in electrochemical devices.
An analysis of the particle size of doped bismuth
molybdate powders by the laser light scattering (LLS) and
SEM methods demonstrated that the average grain size is
The catalytic properties of the samples were examined
for the example of ethylene oxidation processes. Bismuth
molybdates are known as catalysts for selective oxidation
of hydrocarbons at the double bond [4], which enabled
us to expect similar properties for compounds based
0.1–20 μm (Fig. 4a). When pellets are sintered, crystallites
grow and the grain size becomes 5–10 times larger
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

858
MIKHAYLOVSKAYA et al.
exhibit a catalytic activity in the range of selective
α, %
Bi Mo O
10 69
2
6
oxidation of hydrocarbons at the double bond [4]. In this
case, the rise in the ignition temperature of oxidation to
Bi25.6Ba0.4Mo9.6V0.4O69–d
Bi25.6Ba0.4Mo9.8Fe0.2O69–d
Bi25.6Ba0.4Mo9.9Co0.1O69–d
CO and CO is a positive factor; the specific composition
2
of products requires a separate integrated study.
Theelectricalconductivityofthesubstitutedbismuthmo-
lybdates was studied by the method of impedance spectros-
copy. Typical impedance hodographs (also named imped-
ance diagrams or spectra) of Bi25.6Ba Mo
Fe O
0.4
10–2y 2y 69–d
and Bi25.6Ba Mo
V O
at various temperatures
0.4
10–2y 2y 69–d
are shown in Fig. 6. The equivalent cell circuit
corresponding to the occurrence of the processes in a
certain temperature range was selected with the use of
Zview software (LSM).
T, K
Fig. 5. Dependence of the conversion α on temperature
T for Bi Mo O and a number of disubstituted
2
6
10 69
compositions based on Bi Mo O .
10 69
Two general types of impedance diagrams are observed
2
6
for the Bi25.6Ba Mo
Me'' O
(Me'' = Fe, V) series.
0.4
10–2y
2y 69–d
These diagrams correspond to the high- (~823–1073)
and low-temperature (~573–773 K) ranges (Figs. 6c,
on Bi Mo O . We studied the catalytic activity by
2
6
10 69
measuring the concentration of hydrocarbons and their
oxidation products on passing ethylene via a flow-through
reactor, with measurements made at room temperature
used as reference. The conversion of hydrocarbons (%)
was calculated by the formula
6
d and Figs. 6a, 6b, respectively). At comparatively
low temperatures (~573–773K, Figs. 6a and 6b), the
impedance spectrum is constituted by a distinctly
pronounced half-circle in the left-hand, high-frequency
part and a half-circle conjugated with a straight line in the
right-hand, low-frequency part of the plot. Depending on
the composition of a complex oxide, the conjugation of
the low-frequency half-circle with the rectilinear portion
may be more (Fig. 6a) or less (Fig. 6b) pronounced.
The patterns can be described with equivalent circuits
constituted by a parallel fragment R –CPE connected with
α = [(сRT – с )/с ] × 100,
T
RT
where c is the reagent concentration at room temperature,
RT
and c is that at the reactor outlet at temperature T.
T
Figure 5 shows examples of temperature dependences
of the conversion. It was found that the double substitution
of the columnar bismuth molybdate Bi Mo O with the
1
1
parallel fragments R –CPE and R –CPE , or a parallel
2
2
3
3
26
10 69
fragment R –W–CPE . In this case, the model element
2
2
ions under consideration raises the ignition temperature,
compared with the matrix compound, in the reaction
of ethylene oxidation to carbon oxides, which can be
attributed to the decrease in the concentration of active
centers on the catalyst surface, with just molybdenum–
oxygen polyhedra serving as these centers. However,
CPE (constant-phase element) has the impedance formula
i
P –1
ZCPE = [T(iω) ] , where T(F)is the capacitance parameter,
and P is a dimensionless power parameter. Various values
of CPE–T reflect the correspondence of the R –CPE link
1
1
to various electrochemical processes, and values of the
parameter CPE–P are used to introduce corrections for
the nonideality of the process. For example, the element
CPE corresponds to an ideal plane-parallel capacitor
at P = 1 and to a capacitor with developed surface or
complex plate configuration at P = 0.8–0.9; at P = 0.5,
CPE corresponds to a Warburg diffusion element (i.e.,
to diffusion in a semi-infinite space), at P = 0.4–0.6,
it corresponds to real diffusion processes in the near-
electrode space. For the circuit in Fig. 6a, the values of
CPE –P and CPE –P parameters lie within the range
the release of CO and CO was only recorded above
2
~
773 K, i.e., oxidation to oxygen-containing organic
compounds (e.g., ethylene oxide, acetaldehyde, alcohols,
and acids) occurs below these temperatures. When the
resulting gaseous reaction product is long passed through
a CuSO solution, a white precipitate is formed, which
4
may indicate that ethylene oxide, rather than aldehydes, is
released because it is known that a black CuO precipitate
should have been formed in the reaction with these latter
[31]. Thus, if we consider an incomplete oxidation of
2
3
–6
alkanes, it is observed at ~573–773 K, which confirms
0.55–0.65, and those of CPE –T and CPE –T are ~10 F.
2 3
the assumption that compounds based on Bi Mo O
In the case of a circuit with a Warburg element (Fig. 6b)
2
6
10 69
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

NEW COMPLEX-OXIDE MATERIALS
859
R1
R2
R3
Z'', kΩ
R1
R2
R3
(a)
CPE1
CPE2
Z'', kΩ
–300
CPE1 CPE2 CPE3
–
15
R1
–150
0
20
R1
40
Z', kΩ
(
c)
0
150
300 Z', kΩ
Z'', kΩ
(d)
R0
R1
R2
R0
R2
Z'', kΩ
CPE2
CPE1
CPE1 CPE2
–
50
–
1000
0
100
Z', kΩ
0
1500
3000 Z', kΩ
(e)
(f)
–
1
–log σ [S cm–1]
–
log σ [S cm ]
Bi Mo O
Bi Mo O
2
6
10 69
26
10 69
Bi25.6Ba0.4Mo O
Bi25.6Ba0.4Mo O
10 69–d
10
69–d
Bi25.6Ba0.4Mo9.9Co0.1O69–d
Bi25.6Ba0.4Mo9.8V0.2O69–d
Bi25.6Ba0.4Mo9.6V0.4O69–d
Bi25.6Ba0.4Mo9.9Fe0.1O69–d
Bi25.6Ba0.4Mo9.8Fe0.2O69–d
Bi25.6Ba0.4Mo9.6Fe0.4O69–d
1
.0
1.2
1.4
03/T, K–1
1.6
1.0
1.2
1.4
1.6
1
10 /T, K
3
–1
Fig. 6. Examples of Z ''–Z '' impedance curves: (a) Bi25.6Ba0.4Mo9.8V0.1O69–d, 623 K; (b) Bi25.6Ba0.4Mo9.8Fe0.2O69–d, 623 K;
c) Bi25.6Ba0.4Mo9.8V0.1O69–d, 1023 K; and (d) Bi25.6Ba0.4Mo9.8Fe0.2O69–d, 1023 K. Equivalent circuits are shown; with square symbols
represent experimental data and the solid line is the simulated line. Arrhenius dependences for selected compositions of the series
Bi25.6Ba0.4Mo10–2yMe'' O : Me = (e) Co, V and (f) Fe. (σ) Electrical conductivity and (T) temperature.
(
2y 69–d
10–11–10–10 F, which demonstrates that the parameter R1
uniquely corresponds to the total sample resistance [32].
the value of CPE –P is within the range 0.060–0.68,
2
–
6
and that of CPE –T and CPE –T is ~10 F. Thus, on the
2
3
one hand, the equivalent circuit 6b is a particular case of
the circuit 6a, which is more convenient for use in the
course of simulation. On the other hand, it can be seen,
irrespective of the chosen equivalent circuit, that the
corresponding links describe one and the same process,
i.e., the hindered diffusion in the near-electrode space
As the temperature is raised, the diffusion hindrance
becomes less pronounced, and the formation of a single
clearly asymmetric half-circle spaced apart from the zero
value along the abscissa axis is seen in the impedance
curve (Figs. 6c and 6d). The equivalent circuit describing
this spectrum is constituted by a series resistor R0
corresponding to the total resistance of the electrolyte
[32] and two parallel fragments R –CPE and R –CPE
[
32]. The fragment R1–CPE is characterized in both
1
circuits by the values CPE –P = 0.85–0.97 and CPE –T ≈
2
3
1
1
2
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

8
60
Table 2. Electrical conductivity of solid solutions based on bismuth molybdate Bi Mo O
10 69
MIKHAYLOVSKAYA et al.
2
6
Electrical conductivity, S cm–1
Composition
Bi Mo O
9
73 K
673 K
1.67 × 10–3
3.25 × 10–3
4.62 × 10–3
3.94 × 10–3
3.81 × 10–3
4.68 × 10–5
9.12 × 10–5
9.27 × 10–5
16.66 × 10–5
9.41 × 10–5
2
6
10 69
Bi25.6Ba Mo O
0.4
10 69–d
Bi25.6Ba Mo V O
0.4
9.8 0.2 69–d
Bi25.6Ba Mo Fe O
0.4
9.9 0.1 69–d
Bi25.6Ba Mo Co O
0.1 69–d
0.4
9.9
(
Figs. 6c and 6d). In the case under consideration, CPE–T
for the example of Bi25.6Ba Mo
Concentration dependences of this kind are inherent in
the family of complex oxides based on Bi Mo O
V O
(Fig. 6e).
0.4
10–2y 2y 69–d
–4
is ~10 F and CPE–P is ~0.7–0.85 at high temperatures
for all Bi25.6Ba Mo Me'' O compositions, which
0.4
10–2y
2y 69–d
26
10 69
describes electrochemical reactions at the electrode–
[6, 10, 11, 14].
electrolyte interface [32]. Thus, R was taken for the
1
sample resistance at low temperatures, and R , at
CONCLUSIONS
0
high temperatures. These data were used to construct
Arrhenius dependences of the electrical conductivity for
A number of disubstituted bismuth molybdate solid
solutions of compositions Bi25.6Ba Mo Me'' O69–d,
Bi25.6Ba Mo
Me'' O
(Figs. 6e and 6f), which
0.4
10–2y
2y 69–d
0.4
10–2y
2y
are linear.
where Me'' are d-elements (Co, Fe, V) were obtained. A
narrowing of the homogeneity regions and stabilization
of the monoclinic modification (space group P2/C) of
bismuth molybdate are observed for the solid solutions
under study with respect to similar monosubstituted
compounds, which is correlated with the size of ions
being substituted. IR spectroscopy confirmed that Fe
and V are incorporated into the molybdenum sublattice,
It can be seen that a characteristic break is observed for
the matrix compound in the –log σ–1000/T dependence
in the phase-transition region (triclinic to monoclinic
modification). For doped bismuth molybdates, there is
no clearly pronounced break because the temperature
of the possible transition lies below the temperature of
impedance measurements.Acomparison of composition
series with mono- and disubstitution demonstrated
that, in all cases, the electrical conductivity of samples
increases relative to that of the monosubstituted
composition (Bi25.6Ba Mo O69–d). The highest electrical
which leads to deformation of MeO polyhedra (Me =
n
Mo, V/Fe) and to a change in the length of metal–oxygen
bonds. The double substitution of the columnar bismuth
molybdate Bi Mo O with ions of barium and d-metals
26
10 69
0.4
10
makes higher the ignition temperature in the reaction of
ethylene oxidation to carbon oxides. Specific features of
impedance diagrams and Arrhenius dependences of the
electrical conductivity of Bi25.6Ba0.4Mo10–2yMe2''yO69–d
were noted. The maximum conductivity is observed for
compositions lying at the concentration boundary of
the existence domains of the triclinic and monoclinic
modifications.
conductivity is observed for the composition lying at the
concentration boundary of the existence domain of the
triclinic and monoclinic modifications (Figs. 6e and 6f),
i.e., the doping of the molybdate with small amount of
substituents is the most effective for modifying their
properties. The electrical conductivities of a number of
compositions are listed in Table 2. The increase in the
electrical conductivity of iron-substituted compositions
relative to other similar samples at low temperatures may
be due to the contribution from the electronic constituent
of conductivity. In the general case, defects in the oxygen
sublattice become more ordered with increasing dopant
concentration. This is accompanied by a decrease in
the unit cell volume and leads to decrease in the carrier
mobility and in the electrical conductivity, which is seen
ACKNOWLEDGMENTS
The study was carried out under a State assignment
from the Ministry of Education and Science of the Russian
Federation (no. 4.2288.2017/4), with financial support
from the Russian Foundation for Basic Research (project
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 90 No. 6 2017

NEW COMPLEX-OXIDE MATERIALS
861
1
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C, 1997, vol. 53, no. 3, pp. 269–272.
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