Synthesis and properties of phases in the system BaCO3-PbO- Nb2O5

Article abstract of DOI:10.1023/B:INMA.0000023968.67549.54

The phase formation kinetics and phase relations in the BaCO ₃-PbO-Nb₂O₅ system are studied. Barium lead niobates are prepared by solid-state reactions, and the composition ranges of solid solutions with different structur

Full text of DOI:10.1023/B:INMA.0000023968.67549.54

Inorganic Materials, Vol. 40, No. 4, 2004, pp. 415–419. Translated from Neorganicheskie Materialy, Vol. 40, No. 4, 2004, pp. 486–490.
Original Russian Text Copyright © 2004 by Shtin, Podkorytov, Zhukovskii, Sokolova, Kudakaeva.
Synthesis and Properties of Phases
in the System BaCO3–PbO–Nb2O5
S. A. Shtin, A. L. Podkorytov, V. M. Zhukovskii,
E. V. Sokolova, and S. R. Kudakaeva
Ural State University, pr. Lenina 51, Yekaterinburg, 620083 Russia
e-mail: anatoliy.podkorytov@usu.ru
Received April 15, 2003
Abstract—The phase formation kinetics and phase relations in the BaCO3–PbO–Nb2O5 system are studied.
Barium lead niobates are prepared by solid-state reactions, and the composition ranges of solid solutions with
different structures are determined. All of the materials are examined by x-ray diffraction, and their stability in
acid media is assessed. The electrical conductivity of the Pb-containing niobates is measured. The results are
used to analyze property–structure–composition relations. Based on the phase-equilibrium data and the chem-
ical stability and electrical properties of the niobates, the structure types and compositions of niobates are iden-
tified which are the most attractive for producing ion-selective materials.
INTRODUCTION
mixtures during heating to 1050°C, using a Q-1500D
thermoanalytical system (Table 3) interfaced with an
AMD K6-2-333 PC for measurement automation and
data processing. The reaction kinetics were followed
using isothermal gravimetry. The experimental setup
included a VLA-200M analytical balance, heater,
power supply, temperature controller, and temperature-
measuring system. Conversion was inferred from
weight loss data; the rate-limiting step was identified by
the reduced coordinate method [5].
Our previous studies [1–3] showed that niobates are
potential materials for ion-selective electrodes. Clearly,
precise knowledge of their chemical, structural, and
electrochemical properties is critical for this applica-
tion. Detailed investigation of Ni- and Pb-containing
strontium niobates [4] culminated in the fabrication of
ion-selective electrodes with good performance param-
eters [2, 3]. The objective of this work was to prepare
new electrode materials based on Pb-containing alka-
line-earth niobates for ion-selective meters.
Samples for conductivity measurements were pre-
pared by pressing powdered materials into pellets
10 mm in diameter, which were sintered first at 800°C
and then at the highest synthesis temperature for 4 h.
EXPERIMENTAL
The starting reagents used in solid-state synthesis
are listed in Table 1. The reagents were precalcined in
order to remove residual moisture and obtain stable
modifications.
After grinding the faces of the pellets, electrical
contacts were formed by depositing ammonium
hexachloroplatinate, which was then decomposed
above 300°C to give fine-particle platinum:
The starting mixtures were placed in alundum cruci-
bles and reacted in air in Silit furnaces with multiple
intermediate grindings in 2-propanol. During synthesis,
the temperature was raised stepwise. The temperature
was maintained with an accuracy of 5°ë and was
measured by Pt/Pt–Rh thermocouples connected to an
F-295-4 voltmeter.
The phase composition of the samples and the sto-
ichiometry ranges of solid solutions were determined
by x-ray diffraction (XRD) on a DRON-2.0 powder dif-
fractometer (CuKα radiation). The final synthesis tem-
peratures (ts) and solid-solution ranges are listed in
Table 2.
(NH4)2PtCl6
2NH3↑ + 2HCl↑ + 2Cl2↑ + Pt.
Electrical conductivity was measured by a two-
probe technique at 1 kHz during cooling in air, using an
Table 1. Starting reagents and calcination temperatures
(calcination time, 3 h)
Calcination
temperature, °C
Reagent
Grade
Nb2O5
BaCO3
PbO
OSCh 7-3
1200
600
OSCh 7-2
The reaction onset temperature (t0) and the temper-
ature range of active reaction (∆t) were determined by
thermal analysis (TG + DTA) of BaCO3 + PbO + Nb2O5
Analytical grade
500
0020-1685/04/4004-0415 © 2004 MAIK “Nauka/Interperiodica”

416
SHTIN et al.
Table 2. Structure, composition, and stoichiometry ranges of solid solutions in the BaCO3–PbO–Nb2O5 system
Structure type, symmetry
Cryolite, cubic
Chemical composition
Stoichiometry range
ts, °C
Ba6 – xPbxNb2O11
Ba3 – xPbxNb2O8
Ba4 – xPbxNb2O9
Ba5 – xPbxNb4O15
Ba1 – xPbxNb2O6
Ba2 – xPbxNb10O27
Pb3 – xBaxNb2O8
0 < x ≤ 0.5
0 < x < 0.1
0 < x ≤ 0.5
0 < x ≤ 0.4
0 < x ≤ 0.1
0 < x ≤ 0.2
0 < x ≤ 0.5
1450
1200
1400
1400
1250
1150
1200
Perovskite, cubic
Hexagonal
Calcium tantalate, orthorhombic
Tetragonal tungsten bronze (TTB), tetragonal
Pyrochlore, tetragonal
RLC meter and a standard cell with platinum current are more stable in acid media than Pb-substituted stron-
leads, which were pressed against the contacted faces.
The temperature was monitored with a Pt/Pt–10% Rh
thermocouple connected to an F-295-4 voltmeter.
Readings were taken every 10°C. Repeated measure-
ments showed good reproducibility of results. The
accuracy in our conductivity measurements was 1%.
The total ionic transference numbers Σti in the constit-
uent niobates were evaluated earlier from emf measure-
ments [6].
tium niobates, which is of importance in pH measure-
ments. The highest stability in acidic aqueous solutions
is offered by niobates with the TTB structure.
Figure 2 shows the Arrhenius plots of conductivity
for Ban – xPbxNb2yO5y + n. For comparison, Fig. 3 shows
the composition dependences of conductivity for the
parent barium niobates. Table 5 compares the conduc-
tivities of niobates and the activation energies for con-
duction above and below the break point.
RESULTS AND DISCUSSION
The conductivity of Pb-containing barium niobates
decreases with increasing Pb content (Fig. 3). The high-
est conductivity is offered by the Ba6 – xPbxNb2O11 (cry-
olite structure) and Ba2 – xPbxNb10O27 (TTB structure)
solid solutions. The conductivity of Ba4 – xPbxNb2O9,
Ba5 – xPbxNb4O15, and Ba1 – xPbxNb2O6 is somewhat
lower. A similar relationship holds for the undoped bar-
ium niobates. For all of the solid solutions studied, we
determined the activation energy for conduction
(Table 5).
According to our results, BaCO3 does not decom-
pose below 1050°C (Table 3). Therefore, any weight
losses in BaCO3-containing reaction mixtures below
this temperature are due to CO2 release by virtue of
chemical interaction with other components.
The introduction of PbO into reaction mixtures con-
taining Nb2O5 and BaCO3 typically reduces t0. More-
over, increasing the PbO content reduces ∆t (Table 3).
The introduction of PbO notably accelerates solid-
state reactions. The reduced coordinate method (Fig. 1)
was used to identify the rate-limiting process at differ-
ent reaction stages. In all of the reaction mixtures, we
revealed a transition, at different conversions, from
direct chemical reaction to diffusion control (Table 4),
which is well described by the Ginstling–Brownshtein
equation under the assumption that the reactants consist
of spherical particles [5],
Table 3. Thermal analysis data for reaction mixtures
Composition
t0, °C
∆t, °C
BaCO3
>1050*
745
–
4BaCO3 + 2PbO + Nb2O5
5BaCO3 + PbO + Nb2O5
4.8BaCO3 + 0.2PbO + 2Nb2O5
0.8BaCO3 + 0.2PbO + Nb2O5
2BaCO3 + 5Nb2O5
820–1050
970–1050
950–1050
945–1050
990–1050
880–1050
2
3
2Kτ
2/3
--
D(α) = 1 – α – (1 – α)
810
= --------- = Kτ,
R2
845
where α is conversion, K is the reaction rate constant,
R is the particle radius of the component being covered,
and τ is the reaction time.
Some of the Pb-containing barium niobates have a
broader homogeneity range (Table 2) than their stron-
tium analogs [4]. Moreover, Pb-doped barium niobates
890
900
1.8BaCO3 + 0.2PbO + 5Nb2O5
* Thermolysis onset.
760
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SYNTHESIS AND PROPERTIES OF PHASES IN THE SYSTEM BaCO3–PbO–Nb2O5
417
A
–logσ [S/cm]
R
α
D
4.0
4.5
5.0
5.5
6.0
6.5
0.2
0.1
0
900°C
950°C
5
1
4
2
3
0.7
0.8
0.9
1.0
1.1
1.2
103/T, K
1
2 τ/τ0.2
Fig. 1. Reaction kinetics in a 3.5BaCO + 0.5PbO + Nb O
2 5
3
mixture. The solid lines represent theoretical fits for differ-
ent rate-limiting processes: (A) nucleation, (R) chemical
reaction, (D) diffusion through the layer of the reaction
product.
Fig. 2. Arrhenius plots of conductivity for
(1) Ba Pb Nb O , (2) Ba Pb Nb O , (3) Ba Pb Nb O ,
4
2
2
11
3.5 0.5
2
9
0.9 0.1
2 6
(4) Ba Pb Nb O , and (5) Ba Pb Nb O .
4.6 0.4
4
15
1.8 0.2
10 27
Some of the Pb-containing barium niobates have a
higher conductivity than their strontium analogs [4] and
are, therefore, of practical interest. In particular, there is
electrodes. Some of the niobates studied here meet the
main requirements for such materials [7–9]. The attrac-
tive properties of niobates ceramics, such as high phase
considerable interest in new materials for ion-selective purity, stability in acid media in a broad pH range,
Table 4. Isothermal gravimetry data for BaCO3–PbO–Nb2O5 samples
Composition
t, °C
αmax, %
Type of control
0.8BaCO3 + 0.2PbO + Nb2O5
850
900
950
800
900
950
800
900
950
850
900
950
800
850
900
19
41
65
10
32
62
8
D
D
R (α < 25), D (α > 30)
3BaCO3 + PbO + Nb2O5
D
D
D
3.5BaCO3 + 0.5PbO + Nb2O5
4.8BaCO3 + 0.2PbO + Nb2O5
4BaCO3 + 2PbO + Nb2O5
D
23
42
9
D
RD (α < 15), D (α > 15)
D
27
73
13
71
76
D
R (α < 20), D (α > 20)
D
R (α < 35), D (α > 35)
R (α < 40), D (α > 40)
Note: D denotes diffusion control, described by the Ginstling–Brownshtein equation, and R denotes kinetic control, described by the con-
tracting-sphere equation [5].
INORGANIC MATERIALS Vol. 40 No. 4 2004

418
SHTIN et al.
Table 5. Electrical properties of niobates
–logσ [S/cm]
ε, eV
tbreak, °C
( 5°C)
Composition
1150°C
650°C
t > tbreak
t < tbreak
Ba6Nb2O11
2.70
3.75
1.78
3.45
4.45
3.90
4.37
2.60
3.82
3.91
3.99
4.02
2.77
4.32
6.05
3.08
4.90
5.86
6.30
6.68
5.60
–
0.80
1.26
1.38
0.90
1.85
1.92
1.33
2.78
2.37
1.90
2.87
1.73
1.20
1.00
1.00
1.16
0.90
0.34
1.19
0.49
0.72
1.45
0.74
0.80
1.18
0.70
640
640
700
–
Ba4Pb2Nb2O11
Pb3NiNb2O9
Ba4Nb2O9
Ba3.5Pb0.5Nb2O9
Ba5Nb4O15
880
1020
650
960
930
750
750
760
1110
Ba4.6Pb0.4Nb4O15
BaNb2O6
Ba0.9Pb0.1Nb2O6
Ba2Nb10O27
6.86
6.08
5.75
4.26
Ba1.8Pb0.2Nb10O27
PbNb2O6
Pb2.5Ba0.5Nb2O8
mixed conductivity, framework–channel structure niobates (with polystyrene as an inert matrix) is sensi-
favorable for reversible exchange processes, and good tive to the concentration of Pb ions in solution [10].
mechanical strength, make them potential materials for
membranes of ion-selective electrodes.
According to preliminary tests, the potential of thin-
film ion-selective electrodes based on Pb-containing
ACKNOWLEDGMENTS
This work was supported by the Russian Foun-
dation for Basic Research (grant R2002 URAL,
no. 02-03-96457) and the US Civilian Research and
Development Foundation (grant EK-005-XI).
logσ [S/cm]
–3
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INORGANIC MATERIALS Vol. 40 No. 4 2004

SYNTHESIS AND PROPERTIES OF PHASES IN THE SYSTEM BaCO3–PbO–Nb2O5
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