Novel composition above the limit of Bi:Zr solid solution; synthesis and physical properties of Bi1.33Zr0.67O3+δ

Article abstract of DOI:10.1016/j.materresbull.2004.06.011

This paper presents an increase to x = 0.67 of the zirconium content in the conductive Bi_2-xZr_xO_3+δ solid solution. Complete incorporation of Zr in the β_III-Bi₂O ₃ structure, confirmed by X-ray powder diffraction, has produced a phase with a lower volume and superior conductivity than those predicted by an earlier study. The observed β_III-δ Bi_2-xZr _xO_3+δ phase transition around 730δC has been characterised for the first time and shows a segregation of a mixture of predominantly γ-Bi₂O₃ and approximately 30% of the ZrO₂, before total reincorporation of the Zr in the high temperature δ-phase.

Full text of DOI:10.1016/j.materresbull.2004.06.011

Materials Research Bulletin 39 (2004) 1841–1847
www.elsevier.com/locate/matresbu
Novel composition above the limit of Bi:Zr solid solution;
synthesis and physical properties of Bi Zr O
1.33 0.67 3+d
a
Iratxe de Meatza , Jon P. Chapman , Fabrice Mauvy , Jos e´ I. Ruiz de Larramendi ,
a
b
a
c
Mar ´ı a I. Arriortua , Te o´ filo Rojo
a,
*
a
Departamento de Qu ´ı mica Inorg a´ nica, Facultad de Ciencia y Tecnolog ´ı a, Universidad del Pa ´ı s Vasco,
Apdo. 644, E-48080 Bilbao, Spain
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux (ICMCB-CNRS), 33608 Pessac Cedex, France
b
c
Departamento de Mineralog ´ı a y Petrolog ´ı a, Facultad de Ciencia y Tecnolog ´ı a,
Universidad del Pa ´ı s Vasco, Apdo. 644, E-48080 Bilbao, Spain
Received 18 November 2003; received in revised form 25 March 2004; accepted 29 June 2004
Abstract
This paper presents an increase to x = 0.67 of the zirconium content in the conductive Bi2 Zr O solid
x 3+d
ꢀx
solution. Complete incorporation of Zr in the b -Bi O structure, confirmed by X-ray powder diffraction, has
III
2 3
produced a phase with a lower volume and superior conductivity than those predicted by an earlier study. The
observed b -d Bi Zr O phase transition around 730 8C has been characterised for the first time and shows a
III
2ꢀx
segregation of a mixture of predominantly g-Bi O and approximately 30% of the ZrO , before total reincorpora-
x 3+d
2
3
2
tion of the Zr in the high temperature d-phase.
2004 Elsevier Ltd. All rights reserved.
#
Keywords: A. Oxides; C. Impedance spectroscopy; C. X-ray diffraction; D. Ionic conductivity; D. Phase transitions
1
. Introduction
The high oxide ion conductivity observed in Bi O and related systems has led to a considerable
2
3
number of publications regarding these phases [1,2], mainly due to their potential application in gas
sensors and solid oxide fuel cells (SOFC). Among the polymorphs of Bi O , the fluorite related, cubic
2
3
* Corresponding author. Tel.: +34-946012458; fax: +34-946013500.
E-mail address: qiproapt@lg.ehu.es (T. Rojo).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2004.06.011

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842
I. de Meatza et al. / Materials Research Bulletin 39 (2004) 1841–1847
ꢀ
1
ꢀ1
d-phase shows high oxide ion conductivity (1 V cm at 800 8C) but limited to a range from 730 to
25 8C (melting point) with the disadvantage of its instability against reduction. This highly conducting
8
phase can be obtained below 730 8C by solid solution formation with other oxides [3,4].
Due to the stability and well known high conductivity of zirconia, several papers have been published
regarding the Bi O –ZrO system [5–9]. The phase diagram by Levin et al. [6] indicated a narrow solid
2
3
2
solution range for a high temperature cubic phase, with phase separation at room temperature, indicating
a small solubility of ZrO in the various forms of Bi O . Phase diagrams presented by Hund [7] suggested
a solid solution range of x = 0–0.70 ZrO with tetragonal Bi O structure but have not, to the authors’
2
2 3
2
2 3
knowledge, been confirmed experimentally. Contrarily, Sorokina and Sleight [8] reported that, despite
Hund’s assertation, at temperatures above 750 8C only ZrO and b-Bi1.84Zr0.16O3.08 are stabilised. A
2
recent study of Abrahams et al. [9] on the Bi O -rich end of the Bi Zr O system sets the limit of
2
3
2ꢀx
x 3+x/2
solid solution at x = 0.17 despite their x = 0.15 simultaneous X-ray and neutron refinement clearly
showing a second phase. Here we report the synthesis of a single phase sample of composition above that
limit as suggested by Hund. The compound Bi1.33Zr0.67O3+d, prepared after high temperature calcination
(
800 8C), shows tetragonal b -structure [9] and higher conductivity than predicted for this large fraction
III
of Zr.
2
. Experimental
A sample of nominal composition Bi1.33Zr0.67O3+d was prepared by solid state techniques from
analytical grade Bi O and ZrO(NO ) instead of conventional zirconia. The starting materials were
2
3
3 2
ground with acetone in an agate mortar, pelleted, placed in an alumina crucible and heated in air at 500 8C
for 5 h then 800 8C for 60 h with one intermediate grinding. Fast cooling of this sample results in an
orange polycrystalline powder. X-ray powder diffraction data, collected using a Philips X’Pert-MPD
diffractometer (Bragg-Brentano geometry; Cu Ka; secondary monochromator; range 10–1158 2u; step
0
.018 2u; 15 s per step), were fitted by the Rietveld method using GSAS [10]. A variable temperature X-
ray study, from 200 to 800 8C for every 5 8C was also carried out using the above mentioned
diffractometer with an Anton Paar Physica TCU2000 variable temperature chamber. The experimental
density was measured by Archimedes method using a Mettler Toledo AG135 analytical balance with
density kit. A sample for impedance measurements was prepared as a circular pellet with thickness, L =
2
0
.118 cm and surface area, S = 0.7014 cm , sintered at 800 8C for 2 h and slow cooled (12 8C/h). The
pellet density was 95.8% of the theoretical density. AC impedance data were collected using an Eco-
Chemie Autolab PGSTAT30 system, in the frequency range 10 mHz to 1 MHz over several temperatures
ranging from 300 to 745 8C in air and over one cycle of heating and cooling.
3
. Results and discussion
The use of ZrO(NO ) as reagent, in addition to a fast cooling of the sample, appears to be an important
3
2
factor in the attainment of a meta-stable single phase sample of Bi1.33Zr0.67O3+d. The room temperature
X-ray powder diffraction pattern, fitted with a single tetragonal phase (see Fig. 1), is consistent with the
P4 /nmc structure of b -Bi1.85Zr0.15O3.075 refined from neutron powder diffraction data by Abrahams et
2
III
3
˚
˚
˚
al. [9] yielding refined parameters of a = 7.7113(2) A, c = 5.6325(2) A and V = 334.93(2) A . Contrary to

I. de Meatza et al. / Materials Research Bulletin 39 (2004) 1841–1847
1843
Fig. 1. Fitted diffraction profile for Bi1.33Zr0.67
(
O
at 298 K showing observed (crosses), calculated (line) and difference
3ꢂd
2
lower) profiles. Markers indicate reflection positions. Inset: detail of 27.808 < 2u < 28.308 showing absence of ZrO .
˚
the tendency of their parameters to increase from values of a = 7.7206(8) A, c = 5.6370(6) A and V =
˚
3
36.0(1) A for x = 0.15 with increasing zirconium (and therefore oxygen) content, ours are more
˚
3
comparable to those for their x = 0.11 sample. The fitted profile also confirms the absence of any Zr
ꢀ
3
impurity. In addition, the measured density of 7.90(7) g cm is in agreement with a density value of
ꢀ
3
.922(4) g cm obtained from X-ray diffraction. This result represents a significant increase (x = 0.67)
7
of the Zr content in the Bi2 Zr O solid solution, contrary to the published results that limited the solid
ꢀx
x 3+d
solution to x = 0.17 [9].
The variable temperature X-ray diffraction is shown in Fig. 2. On raising the temperature of the single
phase Bi1.33Zr0.67O3+d, there are small additional diffraction peaks which are consistent with the P4 /nmc
2
symmetry but are not seen in the room temperature diffraction pattern. Around 450 8C there appears a
minute fraction (<1.9%) of an I4/mmm polymorph of Bi O (PDF no. 76-2477). At 550 8C the P4 /nmc
2
2.3
2
Bi1.33Zr0.67O3+d begins to decompose yielding the meta-stable g-Bi O (PDF no. 74-1375). Further
decomposition leads to a mixture of ZrO , g-Bi O and a further orthorhombic phase (o-Bi O , space
2
3
2
2
3
2 2.3
group Immm) related to the above mentioned tetragonal polymorph of Bi O , with part of the Zr still
2
2.3
incorporated in the Bi O phases. The phase segregation and solubility limit change with temperature can
3
2
be described as a chemical reaction:
ꢁ
650
Bi1:33Zr0:67
O
ꢀ! C0:30ZrO þ 0:70 g ꢀ Bi1:64Zr0:36O þ 0:10 oꢀðBi; ZrÞ O
3þd
2
3
2
2:3
Above 730 8C, a single cubic phase, isostructural with d-Bi O is observed. No additional reflections
2
3
are observed, indicating complete incorporation of x = 0.67 Zr in this structure. Another measurement
after fast cooling the sample to room temperature shows the initial bIII single phase, indicating the

1
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I. de Meatza et al. / Materials Research Bulletin 39 (2004) 1841–1847
Fig. 2. Temperature dependant X-ray diffraction patterns for Bi1.33Zr0.67O3+d
.
reversibility of this transition. The low temperature b - and high temperature d-structures are both able
III
to accommodate a Zr substitution of x = 0.67 with the intermediate g-structure unable to incorporate more
than x = 0.36.
The impedance spectra in both low and high temperature regions are shown in Fig. 3. For low
temperatures the Nyquist plots are characterised by a slightly depressed semi-circle for the high
frequency contribution and straight lines at lower frequencies (see Fig. 3a). The high frequency part
represents the bulk contribution from the Bi1.33Zr0.67O3+d with the straight line being associated with
interfacial processes at the platinum blocking electrodes. The bulk contribution was fitted with a simple
parallel combination of a resistance, R, and a capacitance, C. The mean value of the calculated
ꢀ
11
bulk capacitance was 4 ꢃ 10
F, in good agreement with that usually found for bulk response
11] meaning that the resistance, R, of the high frequency curves represents the intra-granular response of
[
the ceramic. The bulk conductivity, s, is then calculated from the bulk resistance R according to the
relation, s = (1/R)ꢄ(L/S).
At higher temperatures the electrode impedance becomes more significant than the bulk contribution.
The high frequency part represents bulk impedance associated with the inductive contribution of the
platinum wires used as current collectors. Medium and low frequency contributions can be attributed to
the complex electrode processes (charge transfer, oxygen exchange, solid state diffusion, gas-phase
diffusion) (see Fig. 3b).
The Arrhenius plot of the conductivity is shown in Fig. 4. There are two regions in the plot, separated
by a large increase in conductivity for temperatures above 700 8C corresponding to the d- and bIII-
polymorphs of Bi1.33Zr0.67O3+d observed in the variable temperature X-ray diffraction. The obtained

I. de Meatza et al. / Materials Research Bulletin 39 (2004) 1841–1847
1845
Fig. 3. Complex plane plots of Bi1.33Zr0.67O3+d at (a) T = 347 and (b) T = 745 8C.
ꢀ
1
ꢀ1
ꢀ6 ꢀ1 ꢀ1
cm for lower
temperatures. The activation energy observed in the low temperature range is 0.91 eV and is almost
conductivity values were s745 = 0.026 V cm and s300 = 1.18 ꢃ 10
V
equal to that of yttrium stabilised zirconia (ꢅ1.0 eV). In the high temperature domain, d-Bi O , the
2
3
activation energy of the conductivity seems to decrease as previously observed by Shuk et al. [1]. As the
oxygen partial pressure of the surrounding atmosphere is high, it can be assumed that the conductivity is
mainly ionic [1]. The s300 conductivity is comparable with the highest value reported by Abrahams et al.
[9] in their entire solid solution and is also far superior to that predicted in his work for higher zirconium
contents.

1
846
I. de Meatza et al. / Materials Research Bulletin 39 (2004) 1841–1847
3+d
Fig. 4. Arrhenius plot of total conductivity for Bi1.33Zr0.67O .
4
. Conclusions
In conclusion, a single phase sample of composition Bi1.33Zr0.67O3+d was synthesised using both
reagents and thermal treatments different from those previously used in the literature. The limit of solid
solution, x = 0.17 of Zr, previously reported in b -Bi O has been increased. The existence of this
III
2 3
elevated level of Zr substitution is also evidenced in the cubic, high temperature d-Bi O structure seen
2
3
above 720 8C. The intermediate, meta-stable g-Bi O is unable to accommodate more than x = 0.36
2
3
substitution of Zr, leading to a segregation of phases during the transition. The high conductivity of
Bi1.33Zr0.67O3+d can be explained by a high density with respect to the theoretical crystallographic
density, which is itself higher than predicted by a previous study. This highlights the importance of
sample processing conditions when preparing materials for use as components of solid oxide fuel cells.
Acknowledgements
I.d.M. gratefully acknowledges the Eusko Jaurlaritza/Gobierno Vasco for her predoctoral studentship
PI-1999-82). J.P.C. thanks the MCYT (MAT200-10064) for funding.
(
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