Crystal Structure and Disorder of the Fast Oxide-ion

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Crystal structure and disorder of the fast oxide-ion

conductor cubic Bi2O3

Masatomo Yashima *, Daiju Ishimura

Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering,

Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama-shi 226-8502, Japan

Received 4 July 2003; in final form 4 July 2003

Published online:

Abstract

Accurate nuclear density distribution of cubic bismuth oxide (d-Bi2O3) has been obtained by the maximum entropy

method (MEM)-based pattern fitting combined with the Rietveld method using neutron-powder-diffraction data. The

result shows for the first time that the oxide ions have a complicated disorder spreading over a wide area and shift to the

h1 1 1i directions from the ideal fluorite site. These features are responsible for the high oxide-ion conductivity. 2003 Elsevier B.V. All rights reserved.

1. Introduction

Solid oxides with the fluorite-type structure

such as zirconia-, ceria- and bismuth oxide-based

materials are fast oxide-ion conductors, which

enables the use as good electrolyte materials for

applications to solid oxide fuel cells and oxygen

sensors [17]. To develop better electrolyte mate-

rials requires a better understanding of the con-duction mechanism, and crucial to this is the

knowledge of the disorder of mobile ions at high

temperatures where the materials work efficiently.

The bismuth oxide d-Bi2O3 with the fluorite-type

structure has the highest oxide-ion conductivity in

the known compounds. The crystal structure of

the d-Bi2O3 has been the subject of a number of

investigations for almost 100 years [4,5,812]. But

until now, the accurate disorder of oxide ions has

not been derived yet. Here we report the accurate

disorder in the d-Bi2O3. Determination of the

disorder was carried out by a combined technique

of Rietveld refinement [13,14] and a maximum

entropy method [15,16]-based pattern fitting [17]

of the neutron-powder-diffraction data measured

at 778

C. The oxide ions have a complicateddisorder spreading over a wide area and shift to

the h1 1 1i directions from the ideal fluorite site.This feature would be responsible for the fast

oxide-ion conduction of d-Bi2O3.

2. Experimental procedure

Commercial bismuth-oxide powders were cal-

cined at 400 C for 12 h to remove the carbonates

Chemical Physics Letters 378 (2003) 395399

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*Corresponding author. Fax: +81-45-924-5630.

E-mail address: [email protected](M. Yashima).

0009-2614/$ - see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2003.07.014
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and then pressed into pellets. Neutron powder

patterns of the d-Bi2O3 with good counting

statistics were obtained at 778.3 1.4 and759.4 0.4 C in air using a furnace with MoSi2heaters [18,19] and a diffractometer with a 150-

multi-detector system, HERMES [20], at the JRR-

3M reactor. The wavelength of incident neutron

was 1.8207 AA. The powder patterns were measured

in the range from 2h 20 to 148. The presentneutron-powder-diffraction pattern of the d-Bi2O3(Fig. 1) was very similar with that in the literature

[10,11]. The maximum intensity was 29 610 counts

for the data at 778 C. All the peaks were indexed

by the fluorite-type structure withFm3msymmetryand by a small amount of the second phase from

an alumina stage. The Sillen model (Space group

Pn3m) [8,10] did not yield indices for some ob-

served peaks, indicating no possibility of this

model.

The experimental data were analyzed by a

combination of Rietveld analysis [13,14] and the

maximum entropy method [15,16] (MEM)-based

pattern fitting [17]. It is well known that the MEM

can produce a nuclear density distribution map

from the neutron-diffraction data [17,21]. In MEM

analysis, any kind of complicated nuclear density

distribution is allowed as long as it satisfies thesymmetry requirements.

3. Results and discussion

First, Rietveld analysis was done using three

structural models shown in Figs. 2ac. The iso-

tropic harmonic model was used for the atomic

displacement parameters of Bi at 4a site and the

oxygen at 8c position, while the anisotropic dis-

placement parameters were refined for the oxygenat 32f site (Table 1). The Battle model (Fig. 2c)

gave the best fit with the observed neutron-dif-

fraction data, comparing with other models. The

present weighted reliability factors (R-factors)

based on the Rietveld analyses, Rwp were 4.75%,

3.81% and 3.77% for the Gattow and Schroder [9],

Harwig [10] and Battle et al. [11] models, respec-

tively. The calculated profile with the Battle model

agreed well with the observed one (Fig. 1). The

refined crystal parameters based on the Battle

30000

25000

20000

15000

10000

5000

0

Intensity

1501401301201101009080706050403020

2 /

111

Fig. 1. Rietveld fitting result for neutron-diffraction data of Bi2O3 measured at 778 C. Red crosses (+ symbols) and green line denote

observed and calculated intensities, respectively. Up and down short verticals indicate the positions of possible Bragg reflections of d-

Bi2O3 specimen and the alumina phase of the sample stage, respectively. The 1 1 1 is the index of the peak at the lowest 2h position.

(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

396 M. Yashima, D. Ishimura / Chemical Physics Letters 378 (2003) 395399


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model are shown in Table 1. The present unit-cell

and positional parameters agreed with those in the

literature [11].

Second, the MEM analysis was done using the

structural factors obtained by the Rietveld analysis

with the Battle model. Number of structure factors

derived in the analysis was 12. The present results

of the MEM analyses would be reliable, because

the 1 1 1 peak intensity at the lowest 2h position

that is the most important information for the

Table 1

Structure parameters of d-Bi2O3 measured at 778 C

Atom Site g x y z U (AA2)

Bi 4a 1 0 0 0 0.078(2)

O1 8c 0.23(22)a 0.25 0.25 0.25 0.08(5)

O2 32f 0.13(5)a 0.335(21) x(O2) x(O2) 0.121b

Space group: Fm3m; a 5:65499 AA, Rwp 3:77%, Rp 2:69%, Re 1:62%, Goodness of fit, Rwp=Re 2:33, RI 4:56%,RF 4:54%. g, occupancy; B, isotropic atomic displacement parameter; x, y andz, fractional coordinates.

aThe total oxygen content was fixed to be 6 in a unit cell: 8gO1 32gO2 6.bEquivalent isotropic atomic displacement parameter calculated using the refined anisotropic atomic displacement parameters,

U11O2 U22O2 U33O2 0:122 AA2; U12O2 U23O2 U31O2 0:00730 AA

2.

Fig. 2. Structural models for d-Bi2O3. (a) Gattow model based on an ideal fluorite-type structure: average distribution of six oxygen

atoms at the site (8c) ofFm-3m(xxx, x 1=4); (b) Harwig model: average distribution of six oxygen atoms at the site (32f) ofFm-3m

(xxx,x 1=4 d); (c) Battle model: six oxygen atoms are distributed at the 8 cand 32fsites; and (d) scattering amplitude distribution ofd-Bi2O3 at 778 C with equicontour surface at 0.3 fm/AA

3 obtained by the combination technique of Rietveld refinement and the

maximum entropy method-based pattern fitting. To simplify only the region of 06y6 1=2 is drawn. Red and yellow spheres denote thebismuth and oxygen atom sites, respectively.

M. Yashima, D. Ishimura / Chemical Physics Letters 378 (2003) 395399 397


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MEM analysis was measured (Fig. 1). The MEM

calculations were done using a computer program

PRIMARIMA [22], with 64 64 64 pixels. To reduce

the bias imposed by the simple structural model,an iterative procedure named by REMEDYcycle

[17] was employed after the MEM analysis. In the

sixth REMEDY cycle, the R-factor based on the

Bragg intensities, RI was improved from 4.56% to

1.82%. In the MEM analysis, the R-factor based

on the structure factors, RF was also improved by

the REMEDY cycle from 1.54% to 0.89%. The

MEM map obtained after the sixth REMEDY

cycle provided much information on the compli-

cated disorder of oxide ions (Fig. 2d), comparing

with the previous models (Figs. 2ac). The previ-

ous simple models consisted of atom spheres are

no longer appropriate to describe the positional

disorder of oxide ions.

To visualize the structural disorder at 778 C,

the MEM nuclear density distribution map on the

(1 1 0) plane is shown in Fig. 3. The present study

reveals for the first time that the oxide ions in the

d-Bi2O3 have a complicated disorder and spread

over a wide area, compared with Bi ions. Two

maxima of the nuclear density are clearly seen atthe 32fsites near an ideal 8c position of the oxide

ion on the (11 0) plane (arrows in Fig. 3). The

distance between the two maxima is estimated to

be 1.36 AA. Bulges in theh1 1 1idirections indicate apossible diffusion mechanism [23] where the oxide

ions move through a face of the surrounding Bi

tetrahedron to the octahedral cavity and then re-

entering a neighboring anion site. Bulges in the

[0 0 1] directions suggest another diffusion path

where the oxide ions move along the c axis. This

feature was also confirmed in the MEM analysis of

the same sample measured at a different tempera-

ture 759 C. Previous neutron-powder-diffraction

works [10,11] suggested some disorder of oxide

ions assuming four or five split sites for an ideal 4c

position of the oxide ion as shown in Figs. 1b and

c, but failed to indicate the accurate positional

Fig. 3. Scattering amplitude distribution on (1 1 0) plane ofd-Bi2O3at 778 C with white contours in the range from at 0.3 to 4.0 fm/AA3

(0.2 fm/AA3 step). Oxide ions have a disorder along [1 1 1] directions. The possible conducting paths of the oxide ion are seen along

h0 0 1i and h1 1 1i directions.

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disorder. In this work, we have successfully ob-

tained the accurate disorder of oxide ions in d-

Bi2O3 by means of the MEM-based pattern fitting

of neutron powder data. The above complicateddisorder of oxide ions would be responsible for the

fast oxide-ion conduction.

Acknowledgements

We acknowledge very much to Dr. K. Ohoy-

ama and Mr. K. Nemoto for the use of the

HERMES diffractometer, and to Mr. A. Sakai and

Mr. M. Mori for experimental assistance. Figs. 2

and 3 were drawn using the computer program

VENUSENUS developed by Dilanian and Izumi [22].

This work was partly supported by the Grant-in-

Aid for Scientific Research (B) of The Ministry of

Education, Culture, Sports, Science and Technol-

ogy of Japan.

References

[1] E.C. Subbarao, in: A.H. Heuer, L.W. Hobbs (Eds.),

Advances in Ceramics Vol. 3, Science and Technology of

Zirconia I, American Ceramic Society, Columbus, OH,1981, p. 1.

[2] H. Inaba, H. Tagawa, Solid State Ionics 83 (1996) 1.

[3] T. Takahashi, H. Iwahara, J. Appl. Electrochem.3 (1972) 65.

[4] P. Shuk, H.-D. Wiemhoofer, U. Guth, W. Goopel, M.

Greenblatt, Solid State Ionics 89 (1996) 179.

[5] N.M. Sammes, G.A. Tompsett, H. Naafe, F. Aldinger,

J. Eur. Ceram. Soc. 19 (1999) 1801.

[6] B.C.H. Steele, in: T. Takahashi (Ed.), High Conductivity

Solid Ionic Conductors, Recent Trends and Applications,

World Scientific, Singapore, 1989, p. 402.[7] J.C. Boivin, G. Mairesse, Chem. Mater. 10 (1998)

2870.

[8] L.G. Sillen, Ark. Kem. Mineral. Geol. 12A (1937) 1.

[9] G. Gattow, H. Schroder, Z. Anorg. Allg. Chem. 318 (1962)

176.

[10] H.A. Harwig, Z. Anorg. Allg. Chem. 444 (1978) 151.

[11] P.D. Battle, R.A. Catlow, J. Drennan, A.D. Murray,

J. Phys. C 16 (1983) L561.

[12] K. Koto, K. Suda, N. Ishizawa, H. Maeda, Solid State

Ionics 72 (1994) 79.

[13] H.M. Rietveld, J. Appl. Crystallogr. 2 (1968) 65.

[14] F. Izumi, T. Ikeda, Mater. Sci. Forum 321324 (2000) 198.

[15] D.M. Collins, Nature 298 (1982) 49.

[16] M. Takata et al., Nature 377 (1995) 46.

[17] F. Izumi, S. Kumazawa, T. Ikeda, W.-Z. Hu, A. Yamam-

oto, K. Oikawa, Mater. Sci. Forum 378381 (2001) 59.

[18] O. Yokota, M. Yashima, Y. Yamaguchi, M. Ohashi, S.

Hotta, T. Oketani, H. Naitoh, in: M. Nishi, Y. Matsushita

(Eds.), Activity Report on Neutron Scattering Research,

Vol. 4, Neutron Scattering Laboratory, Institute for Solid

State Physics, University of Tokyo, Tokai, Japan, 1997,

p. 170.

[19] M. Yashima, J. Am. Ceram. Soc. 85 (2002) 2925.

[20] K. Ohoyama, K. Nemoto, M. Ohashi, T. Kajitani, Y.

Yamaguchi, Jpn. J. Appl. Phys. Part 1 37 (1998) 3319.

[21] M. Sakata, T. Uno, M. Takata, C.H. Howard, J. Appl.

Crystallogr. 26 (1993) 159.[22] F. Izumi, R.A. Dilanian, in: Recent Research Develop-

ments in Physics, vol. 3, Transworld Research Network,

Trivandrum, 2002, p. 699.

[23] M. Lerch, H. Boysen, P.G. Radaelli, J. Phys. Chem. Solids

58 (1997) 1557.

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Crystal Structure and Disorder of the Fast Oxide-ion

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