Oxide-ion-conducting phases in the Bi2MoO6-Bi 2VO5.5 system

Article abstract of DOI:10.1134/S0020168506110148

In search of new oxide-ion-conducting phases, we have synthesized a series of Bi₂V_1-x Mo _x O_5.5+x/2 ceramic samples, in which the end-members Bi₂VO_5.5 (x = 0) and Bi₂MoO₆ (x = 1) are single-layer Aurivillius phases, possess ferroelectric properties, and offer high oxide-ion conductivity. We have determined the phase composition of the samples and have investigated their electrical properties. The results indicate the formation of narrow ranges of Bi₂VO_5.5-and Bi₂MoO₆-based solid solutions (0 < x ≤ 0.1 and 0.9 ≤ x < 1, respectively). In the composition range 0.2 ≤ x ≤ 0.8, the samples consist of Bi ₂VO_5.5-or Bi₂MoO₆-based solid solutions and compounds isostructural with BiVO₄ and Bi ₆Mo₂O₁₅. Molybdenum doping stabilizes the orthorhombic phase β-Bi₂VO_5.5 to room temperature, but the solid solutions differ little in electric conductivity from Bi ₂VO_5.5. The conductivity of the Bi₂MoO ₆-based solid solutions is higher than that of undoped bismuth molybdate by about a factor of 3.

Full text of DOI:10.1134/S0020168506110148

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 11, pp. 1255–1259. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.I. Voronkova, E.P. Kharitonova, O.G. Rudnitskaya, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 11, pp. 1374–1378.
Oxide-Ion-Conducting Phases
in the Bi2MoO6–Bi2VO5.5 System
V. I. Voronkova, E. P. Kharitonova, and O. G. Rudnitskaya
Moscow State University, Vorob’evy gory 1, Moscow, 119992 Russia
e-mail: voronk@polly.phys.msu.ru
Received October 24, 2005; in final form, April 27, 2006
Abstract—In search of new oxide-ion-conducting phases, we have synthesized
a series of
Bi2V1 − xMoxO5.5 + x/2 ceramic samples, in which the end-members Bi2VO5.5 (x = 0) and Bi2MoO6 (x = 1) are
single-layer Aurivillius phases, possess ferroelectric properties, and offer high oxide-ion conductivity. We have
determined the phase composition of the samples and have investigated their electrical properties. The results
indicate the formation of narrow ranges of Bi2VO5.5- and Bi2MoO6-based solid solutions (0 < x ≤ 0.1 and
0.9 ≤ x < 1, respectively). In the composition range 0.2 ≤ x ≤ 0.8, the samples consist of Bi2VO5.5- or Bi2MoO6-
based solid solutions and compounds isostructural with BiVO4 and Bi6Mo2O15. Molybdenum doping stabilizes
the orthorhombic phase β-Bi2VO5.5 to room temperature, but the solid solutions differ little in electric conduc-
tivity from Bi2VO5.5. The conductivity of the Bi2MoO6-based solid solutions is higher than that of undoped bis-
muth molybdate by about a factor of 3.
DOI: 10.1134/S0020168506110148
INTRODUCTION
lius phases. Note that studies of oxide-ion-conducting
phases in the similar system Bi2WO6–Bi2VO5.5
revealed limited series of solid solutions which had a
substantially higher conductivity in comparison with
the end-members [9].
The compounds Bi2MoO6 (BM) and Bi2VO5.5 (BV)
belong to the large family of layered bismuth-contain-
ing perovskite-related compounds (Aurivillius phases)
in which the perovskite slab consists of only one octa-
hedral layer. The Aurivillius phases are ferroelectric
superionics with a high oxide-ion conductivity and are
potentially attractive as solid electrolytes for advanced
power sources [1]. Both Bi2MoO6 and Bi2VO5.5 exhibit
complex polymorphism. At 604°C, bismuth molybdate
undergoes an irreversible phase transition from a low-
temperature, polar orthorhombic phase (γ), which has
an Aurivillius structure, to a tetragonal phase (γ'') and
then to a high-temperature, nonpolar monoclinic phase
(γ'), which does not have a layered perovskite-like
structure [2]. Bismuth vanadate has recently attracted a
great deal of attention owing to its high oxide-ion con-
ductivity [3], which attains 0.1 S/cm at 550°C. Bismuth
vanadate undergoes two phase transitions, at 440 and
570°C, associated with a ferroelectric transition from a
monoclinic polar phase (α) to an orthorhombic nonpo-
lar phase (β) and from the β phase to a tetragonal phase
(γ), also nonpolar [4, 5]. The high-temperature, γ phase
offers high conductivity and can be stabilized to room
temperature by doping with transition metals, resulting
in the formation of the large BIMEVOX family of
oxide-ion conductors [6].As shown byVannier et al. [7]
and Thery et al. [8], molybdenum substitution for up to
The objective of this work was to synthesize Auriv-
illius phases in the Bi2MoO6–Bi2VO5.5 system with the
aim of extending the range of oxide-ion conductors and
to investigate their electrical properties.
EXPERIMENTAL
We prepared two batches of ceramic samples with
nominal compositions of Bi2V1 – xMoxO5.5 + x/2 by solid-
state reactions in appropriate mixtures of extrapure-
grade Bi2O3, MoO3, and V2O5 at 800 and 500°C. Two
firing temperatures were used because the Aurivillius
phase of Bi2VO5.5 requires higher synthesis tempera-
tures (on the order of 800°C) than that of Bi2MoO6,
which exists, as mentioned above, only below 604°C.
This restricts the firing temperature of BM-rich compo-
sitions.
In the first batch, x was varied from 0 to 1 in
0.1 steps. The samples were fired first at 600 and then
at 800°C, with intermediate grinding and pressing at
0.1 GPa. After synthesis, the samples were quenched to
room temperature. At 800°C, equilibration took 3 days,
as checked by x-ray diffraction (XRD). To obtain
20% of the vanadium leads to the formation of Aurivil- orthorhombic BM (γ phase) with a layered structure,
1255

1256
VORONKOVA et al.
BiVO4
*
Bi6Mo2O15
x = 0
α
α
x = 0.1
x = 0.2
β
*
*
*
*
x = 0.5
x = 0.8
*
*
*
*
*
*
*
x = 0.9
x = 1.0
10
20
30
40
50
60
2θ, deg
Fig. 1. XRD patterns of Bi V
Mo O
samples synthesized at 800°C (batch I); α and β mark superlattice reflections from
5.5 + x/2
2
1 – x
x
α-BV and β-BV, respectively; also shown are fundamental reflections from additional phases isostructural with BiVO and
4
Bi Mo O .
6
2 15
we prepared a second batch of samples, with x varied
from 0.5 to 1 in 0.1 steps, by firing at 500°C, followed
by quenching. At this temperature, equilibration
required 6 days.
XRD patterns were collected at room temperature
on Rigaku D-max/RC and DRON-2.0 diffractometers
(CuKα radiation). Temperature-dependent electrical
conductivity and dielectric permittivity were measured
INORGANIC MATERIALS Vol. 42 No. 11 2006

OXIDE-ION-CONDUCTING PHASES IN THE Bi2MoO6–Bi2VO5.5 SYSTEM
1257
x = 1.0
x = 0.9
x = 0.8
*
*
*
* *
30
*
20
40
50
60
2θ, deg
Fig. 2. XRD patterns of Bi V
tional phases.
Mo O
samples synthesized at 500°C (batch II); the asterisks mark reflections from addi-
2
1 – x
x
5.5 + x/2
at 1 MHz from 30 to 750°C using a Tesla BM 431E tively) decreases. Solid solutions based on the mono-
bridge and samples with platinum (batch I) or silver clinic phase γ'-Çå exist in the range 0.9 ≤ x < 1. At
(batch II) electrodes.
intermediate Mo contents, 0.2 ≤ x ≤ 0.8, two additional
RESULTS AND DISCUSSION
Figures 1 and 2 show XRD patterns of batch I and II
BM–BV samples, synthesized at 800 and 500°C,
respectively. As seen in Fig. 1, there is a limited series
of BV-based solid solutions in the range 0 < x ≤ 0.1. The
solid-solution range is narrower than that reported ear-
lier [7, 8]. At room temperature, the x = 0.1 solid solu-
tion is isostructural with the orthorhombic phase β-BV,
as evidenced by the presence of the characteristic
superlattice reflection at 2θ = 25° in its XRD pattern
[4, 5]. Increasing the molybdenum content within the
solid-solution range reduces the lattice parameter a of
the solid solutions from 5.597 to 5.594 Å and increases
the c parameter from 15.396 to 15.433 Å. Concurrently,
the orthorhombic distortion (020 + 200, 024 + 204, and
133 + 313 doublets at 2θ ꢀ 32°, 40°, and 55°, respec-
Solid solutions
based on Bi2VO5.5
BiVO4,
Solid solutions
based on Bi2MoO6,
BiVO4, and
,
and Bi6Mo2O15
Bi Mo2O15
6
0
0.2
0.4
0.6
0.8
1.0
Bi2VO5.5
x
Bi2MoO6
Fig. 3. Composition stability ranges of phases in the
Bi V Mo O system.
2
1 − x
x 5.5 + x/2
INORGANIC MATERIALS Vol. 42 No. 11 2006

1258
VORONKOVA et al.
ε
–3.0
300
–3.5
–4.0
–4.5
200
100
2
1
2
–5.0
–5.5
1
10
15
20
25
104/T, K–1
0
200
400
600
800
t, °C
Fig. 5. Arrhenius plots of conductivity for (1) Bi MoO and
Fig. 4. Dielectric permittivity as a function of temperature
2
6
for (1) Bi MoO and (2) Bi V Mo
O
.
(2) Bi V Mo O
.
2
6
2
0.1
0.9 5.95
2
0.1
0.9 5.95
ε
1400
2
1
1200
1000
800
–1
–2
–3
–4
600
400
1
2
200
10
15
20
25
104/T, K–1
0
100
200
300
400
500
t, °C
Fig. 7. Arrhenius plots of conductivity for (1) Bi VO and
Fig. 6. Dielectric permittivity as a function of temperature
for (1) Bi VO and (2) Bi V Mo
2
5.5
O
.
(2) Bi V Mo
O
.
2
5.5
2
0.9
0.1 5.55
2
0.9
0.1 5.55
phases emerge, isostructural with BiVO4 and
Bi6Mo2O15 [10, 11]. Figure 3 shows the composition
stability ranges of the phases present in the samples
synthesized at 800°C (batch I). In the samples with
0.5 ≤ x ≤ 1.0 synthesized at 500°C (batch II), the Auriv-
illius phase based on the orthorhombic phase γ-Çå
exists in the range 0.9 ≤ x ≤ 1. The lattice parameters of
Figures 4 and 5 present the temperature depen-
dences of dielectric permittivity and electrical conduc-
tivity for bismuth molybdate and Bi2V0.1Mo0.9O5.95
,
isostructural with the orthorhombic phase of BM (syn-
thesis at 500°C). At ꢀ600°ë, the curves show strong
anomalies due to the ferroelectric phase transition from
the orthorhombic phase γ-BM to the monoclinic phase
the solid solutions depend very little on vanadium γ'-BM. Vanadium doping reduces the transition temper-
content.
ature by 20°C and increases the conductivity by a factor
INORGANIC MATERIALS Vol. 42 No. 11 2006

OXIDE-ION-CONDUCTING PHASES IN THE Bi2MoO6–Bi2VO5.5 SYSTEM
1259
of about 3 over the entire temperature range studied, up
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ACKNOWLEDGMENTS
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This work was supported by the Russian Foundation
for Basic Research, grant no. 04-03-32432.
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INORGANIC MATERIALS Vol. 42 No. 11 2006