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Structure–property relationships in fluorite-type Bi2O3–Yb2O3–PbO solid-electrolyte materials

Published online by Cambridge University Press:  21 November 2014

Nathan A. S. Webster ,
Chris D. Ling  and
Frank J. Lincoln
Nathan A. S. Webster*
Affiliation:
CSIRO Mineral Resources Flagship, Private Bag 10, Clayton South, VIC 3169, Australia
Chris D. Ling
Affiliation:
School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia
Frank J. Lincoln
Affiliation:
School of Chemistry and Biochemistry, University of Western Australia, Crawley, WA 6009, Australia
*
a)Author to whom correspondence should be addressed. Electronic mail: nathan.webster@csiro.au
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Abstract

New quenched-in face-centred cubic fluorite-type materials were synthesised in the Bi2O3–Yb2O3–PbO system. After annealing in air at 500 °C for up to 200 h, each material underwent a conductivity-lowering structural transformation, thus making them unsuitable for use as solid electrolytes in solid-oxide fuel cells. For example, (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 underwent a fluorite- to Bi17Yb7O36-type orthorhombic transformation, indicative of long-range cation ordering, and (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 underwent a fluorite- to β-Bi2O3-type tetragonal transformation, indicative of long-range 〈001〉 oxide-ion vacancy ordering.

Keywords

Bi2O3-based oxide ion conductorsX-ray diffractionstructure and conductivity evolution
Type
Technical Articles
Information
Powder Diffraction , Volume 29 , Issue S1 , December 2014 , pp. S73 - S77
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Copyright
Copyright © International Centre for Diffraction Data 2014 

I. INTRODUCTION

Bi2O3-based solid-electrolyte materials with the face-centred cubic (fcc) fluorite-type structure (space group Fm $\overline 3$m) have attracted interest for potential use in solid-oxide fuel cells (SOFCs) owing to their exceptionally high oxide-ion conductivity. The need to develop oxide-ion conductive materials with high conductivity and excellent structural stability over a wide temperature range has underpinned research directed at solid-electrolyte materials (Kanatzidis and Poeppelmeier, Reference Kanatzidis and Poeppelmeier2008). For pure Bi2O3, the fluorite-type phase (δ-Bi2O3) is the most highly conductive oxide-ion conductor known (Krok et al., Reference Krok, Abrahams, Wrobel and Kozanecka-Szmigiel2006), with Fuda et al. (Reference Fuda, Kishio, Yamauchi and Fueki1985) describing the oxide-ion sublattice as “liquid-like”. The high conductivity is attributed to the Bi3+, with its highly polarisable 6s2 lone-pair electrons (Mairesse, Reference Mairesse and Scrosati1993), and significant occupational and positional disorder on the oxide-ion sublattice (Battle et al., Reference Battle, Catlow and Moroney1987; Jiang et al., Reference Jiang, Buchanan, Stevenson, Nix, Li and Yang1995; Boyapati et al., Reference Boyapati, Wachsman and Chakoumakos2001a, Reference Boyapati, Wachsman and Jiang2001b) However, δ-Bi2O3 is stable only between 730 °C and its melting point (825 °C) and it cannot be preserved at room temperature (Levin and Roth, Reference Levin and Roth1964). Below 730 °C Bi2O3 exists either as a monoclinic α (P21/c), tetragonal β (P $\overline 4$21c), or in the body-centred cubic (bcc) γ (I23) phase (Sammes et al., Reference Sammes, Tompsett, Näfe and Aldinger1999; Drache et al., Reference Drache, Roussel and Wignacourt2007).

By doping with some rare earth (Sm to Lu, including Y) and transition metal (e.g. V, Nb, Ta, and W) oxides, the fluorite-type phase can be quenched-in. This work has been comprehensively reviewed (Sammes et al., Reference Sammes, Tompsett, Näfe and Aldinger1999; Drache et al., Reference Drache, Roussel and Wignacourt2007). It is also possible to use a combination of metal oxides – “double doping” – to quench-in the Bi2O3-based fluorite-type phase. Examples include the ternary oxide systems Bi–Dy–W (Jiang et al., Reference Jiang, Wachsman and Jung2002), Bi–Er–W (Watanabe and Sekita, Reference Watanabe and Sekita2005), Bi–Ln–Te (Mercurio et al., Reference Mercurio, El Farissi, Frit, Reau and Senegas1990), Bi–Ln–V (Portefaix et al., Reference Portefaix, Conflant, Boivin, Wignacourt and Drache1997), Bi–Ca–Pb (Drache et al., Reference Drache, Conflant and Boivin1992), Bi–Y–Pb (Borowska-Centkowska et al., Reference Borowska-Centkowska, Liu, Holdynski, Malys, Hull, Krok, Wrobel and Abrahams2014), and Bi–La–Pb (Webster et al., Reference Webster, Hartlieb, Saines, Ling and Lincoln2011).

A partial air-quenchable domain of the fluorite-type phase in the Bi2O3–Er2O3–PbO system has also been reported (Webster et al., Reference Webster, Ling, Raston and Lincoln2007). Pb2+, which also has a stereochemically active 6s2 lone pair, had a significant effect on conductivity and structure, and several of these Bi2O3–Er2O3–PbO fluorite-type materials displayed high oxide-ion conductivity. Pb2+ was shown to occupy face-centred positions of the fluorite-type structure. In another work, annealing at 500 and 600 °C resulted in various symmetry-lowering phase transformations, with associated decays in conductivity (Webster et al., Reference Webster, Ling, Raston and Lincoln2008). For example, the materials with composition (BiO1.5)0.80(ErO1.5)0.20−x(PbO)x (x = 0.03, 0.06, and 0.09) underwent a fluorite- to β-Bi2O3-type tetragonal transformation during annealing at 500 °C. β-Bi2O3 has a fluorite-type superstructure resulting from ordering of oxide-ion vacancies along the 〈001〉 directions, with the 6s2 lone pair directed towards the vacancies (Blower and Greaves, Reference Blower and Greaves1988; Ducke et al., Reference Ducke, Trömel, Hohlwein and Kizler1996), and the fluorite- to β-Bi2O3-type transformation was attributed to attractions between the oxide-ion vacancies and Pb2+. In the present work, the synthesis and characterisation of the quenched-in fluorite-type materials in the Bi2O3–Yb2O3–PbO system is reported. In addition, their structural and conductivity evolution during annealing at 500 °C, which will determine their suitability for practical use as solid electrolytes in SOFCs, is described.

II. EXPERIMENTAL

A. Sample preparation

Powders of Bi2O3, Yb2O3, and PbO (99.9%, Aldrich) were weighed in the desired proportions and milled with zirconia balls in polypropylene vials for 4 h. The mixed powders were then heated in alumina crucibles in air for 24 h, ground using a mortar and pestle, reheated for a further 24 h followed by air quenching, and then ground again to produce the final product. (BiO1.5)0.80(YbO1.5)0.20 and (BiO1.5)0.70(YbO1.5)0.30 were heated at 850 °C, in accordance with the Bi2O3–Yb2O3 equilibrium phase diagram proposed by Drache et al. (Reference Drache, Roussel, Wignacourt and Conflant2004). Most of the other materials were air quenched from 700 °C, following the results of Webster et al. (Reference Webster, Ling, Raston and Lincoln2007) for the Bi2O3–Er2O3–PbO system. The exceptions were (BiO1.5)0.70(YbO1.5)0.10(PbO)0.20 and (BiO1.5)0.80(YbO1.5)0.05(PbO)0.15, which were quenched from 675 °C, and also (BiO1.5)0.70(YbO1.5)0.05(PbO)0.25 and (BiO1.5)0.80(YbO1.5)0.02(PbO)0.18, which were quenched from 650 °C to avoid melting. Each quenched-in fluorite-type material was annealed in air at 500 °C for up to 200 h. The decay in conductivity (σ) during annealing was measured using the procedure described by Webster et al. (Reference Webster, Ling, Raston and Lincoln2008).

B. Data collection and analysis

X-ray diffraction (XRD) was used to determine the phases present at room temperature after air quenching and annealing. Room-temperature XRD data were collected over the range 10°–90° in 2θ with a step size of 0.02° in 2θ using a Siemens D5000 diffractometer with Bragg–Brentano geometry fitted with a Cu tube and operated at 40 kV and 35 mA. Synchrotron XRD (S-XRD) data were collected on the Debye Scherrer diffractometer at the Australian National Beamline Facility, Beamline 20B at the Photon Factory, Tsukuba, Japan (Sabine et al., Reference Sabine, Kennedy, Garrett, Foran and Cookson1995). Samples were held in 0.3 mm quartz capillaries and rotated continuously during data collection. The wavelength used was 0.9384 Å, determined using a silicon standard (NIST SRM 640C). At room temperature data were recorded on three 20 × 40 cm2 Fuji image plates spanning 5°–125° in 2θ. For an in situ annealing experiment, the sample was heated quickly to 500 °C in a furnace and allowed to equilibrate for 5 min before acquiring patterns every 15 min at 500 °C. Two image plates spanning 5°–85° in 2θ were used for these measurements. The maximum number of patterns on the image plates was 28, corresponding to 7 h annealing time. Fitting of S-XRD data was performed using TOPAS (Bruker, Reference Bruker2009).

III. RESULTS AND DISCUSSION

A. Partial air-quenchable domain of the fluorite-type phase

Materials with composition (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x (x = 0, 0.03, 0.06, and 0.09), (BiO1.5)0.70(YbO1.5)0.30−x(PbO)x (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.22), and (BiO1.5)0.85(YbO1.5)0.15−x(PbO)x (x = 0.03), had the fluorite-type structure after quenching as evidenced by their laboratory XRD patterns containing only the typical fluorite-type reflections (PDF 27-0052) (ICDD, Reference Soorya2010) for δ-Bi2O3. Figure 1 shows the partial air-quenchable domain of the fluorite-type phase in the Bi2O3–Yb2O3–PbO system. β-Bi2O3-type tetragonal (PDF 27-0050) as well as mixed Bi6Pb2O11-type monoclinic (PDF 41-0404) and Bi12PbO19-type bcc (PDF 39-0837) phases, were also quenched-in.

Figure 1. (Color online) The partial air-quenchable domain of the fluorite-type phase in the Bi2O3–Yb2O3–PbO system. The * symbols indicate the samples for which annealing behaviour was investigated.

B. Structure evolution during annealing

Only the behaviour of quenched-in fluorite-type materials in the (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x series is discussed. Those with x = 0.03, 0.06, and 0.09 had all undergone phase transformations after annealing at 500 °C for 100 h, and the XRD patterns are shown Figure 2(a). They are, therefore, not suitable for use as solid electrolytes at this temperature. The XRD pattern for (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 displayed splitting of the fluorite-type reflections, and annealing for a further 100 h increased this splitting. The XRD pattern for the 200 h annealed material is also shown in Figure 2(a) and is indicative of a phase similar to the orthorhombic [space group Pmmm, a = 16.245(6), b = 10.713(4), and c = 5.493(2) Å] material with composition Bi17Yb7O36 reported by Drache et al. (Reference Drache, Roussel and Wignacourt2005). In contrast, the material (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 had transformed to the β-Bi2O3-type tetragonal phase, and (BiO1.5)0.80(YbO1.5)0.14(PbO)0.06 was a mixture of the Bi17Yb7O36-type orthorhombic, β-Bi2O3-type tetragonal, and Bi12PbO19-type bcc phases.

Figure 2. (Color online) (a) Ex situ laboratory XRD patterns for (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x, x = 0, 0.03, 0.06, and 0.09, annealed at 500 °C for 100 h, and for x = 0.03 annealed for 200 h, and (b) fit to ex situ S-XRD data collected for (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 annealed for 200 h.

The Bi17Yb7O36-type orthorhombic structure is essentially a 3a × 2a × 1a supercell of the fcc fluorite-type structure, caused by ordering of Bi3+ into the face-centre positions and Yb3+ into the corner positions. This scheme of cation ordering is similar to that proposed in the short-range order models of Verkerk et al. (Reference Verkerk, Van de Velde, Burggraaf and Helmholdt1982) for (BiO1.5)0.80(ErO1.5)0.20 and Battle et al. (Reference Battle, Catlow and Moroney1987) for (BiO1.5)0.80(YbO1.5)0.20. Figure 2(b) shows the output of a Pawley (Reference Pawley1981) refinement (in Pmmm) of S-XRD data collected for the 200 h annealed (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03, indicating refined unit-cell parameters of a = 16.4521(5), b = 10.8405(3), and c = 5.5736(2) Å. Additional work is required to determine further structural parameters such as atomic positions, site occupancies, and displacement parameters.

C. Conductivity decay

Annealing/conductivity experiments were performed on the quenched-in fluorite-type materials in the (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x series and the results, with relative conductivity (σ/σ 0, where σ 0 is the initial conductivity at 500 °C) plotted against annealing time are shown in Figure 3(a). The conductivity decay for (BiO1.5)0.80(YbO1.5)0.20 is similar to that reported previously and is attributed to long-range occupancy ordering of the oxide-ion sublattice, where oxide-ion vacancies order along the 〈111〉 direction (Wachsman et al., Reference Wachsman, Ball, Jiang and Stevenson1992, Reference Wachsman, Boyapati, Kaufman and Jiang2000; Jiang et al., Reference Jiang, Buchanan, Stevenson, Nix, Li and Yang1995; Jiang and Wachsman, Reference Jiang and Wachsman1999; Boyapati et al., Reference Boyapati, Wachsman and Chakoumakos2001a, Reference Boyapati, Wachsman and Jiang2001b). This ordering has been evidenced directly in the past by superlattice reflections in selected area electron diffraction patterns, and indirectly by a reduction in the unit-cell parameter (but with the fcc unit cell being maintained), which is symptomatic of the oxide-ion vacancy ordering. As is shown in Figure 2(a), the XRD pattern for annealed (BiO1.5)0.80(YbO1.5)0.20 contained only the typical fcc reflections. Interestingly, the conductivity of both (BiO1.5)0.80(YbO1.5)0.20 and (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 decayed to a similar σ/σ 0 value of ~0.09 after 30 h, which implies similar structural modifications to the original fluorite-type material in the early stages of annealing when most of the conductivity decay occurs. The rate of decay was noticeably higher for the Pb2+-doped material, and is attributed to the greater proportion of highly polarisable cations.

Figure 3. (Color online) (a) Relative conductivity, σ/σ 0, as a function of annealing time at 500 °C, for (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x, x = 0, 0.03, 0.06, and 0.09, and (b) a comparison between the σ/σ 0 plots obtained here for (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 and by Webster et al. (Reference Webster, Ling, Raston and Lincoln2008) for (BiO1.5)0.80(ErO1.5)0.11(PbO)0.09.

The conductivity decay results for (BiO1.5)0.80(YbO1.5)0.14(PbO)0.06 and (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 are also shown in Figure 3(a), and a comparison between the results obtained here for (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 and in the previous work of Webster et al. (Reference Webster, Ling, Raston and Lincoln2008) for (BiO1.5)0.80(ErO1.5)0.11(PbO)0.09 (which also underwent the fcc fluorite-type to β-Bi2O3-type tetragonal transformation during annealing) are shown in Figure 3(b). Not surprisingly, the conductivity of both (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 and (BiO1.5)0.80(ErO1.5)0.11(PbO)0.09 had decayed to a similar σ/σ 0 value (~0.06) after 30 h. The σ/σ 0 value for (BiO1.5)0.80(YbO1.5)0.14(PbO)0.06 after 30 h was lower (0.05), which is attributed to the multiphase nature of this annealed material.

Revisiting the conductivity decay exhibited by (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03, an annealing S-XRD experiment was performed where S-XRD patterns were collected at 500 °C for 15 min each, continuously over a period of 7 h (28 patterns), after which time most of the conductivity decay for this material had occurred [Figure 3(a)]. Figure 4(a) shows the observed, calculated, and difference profiles for the data from the first image plate, following Rietveld refinement against the 28th dataset. The crystal structure of (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03, determined using neutron powder diffraction data (Webster, Reference Webster2008), was used as the input. The inset in Figure 4(a) shows a magnified view of the fit for the (311) fluorite-type reflection. As is shown in Figure 2(a), orthorhombic splitting of this reflection is characteristic of a fluorite-type to Bi17Yb7O36-type orthorhombic transformation; orthorhombic splitting was not observed.

Figure 4. (Color online) (a) Rietveld fit to the final in situ S-XRD dataset collected during annealing of (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 at 500 °C. Experimental data are shown as plus signs, the calculated pattern as a solid line, and the difference pattern as solid lines below. Crystal structure data: a = 5.5268(2) Å, space group Fm $\overline 3$m, R wp = 4.21%, R (F 2) = 6.23%. (b) Refined lattice parameter as a function of annealing time. The error bars are indicative of the error in the fit between the calculated and observed intensities.

Similar refinements were performed for the other S-XRD patterns, and the variation in the refined fcc unit-cell parameter with annealing time is shown in Figure 4(b). The decrease in unit-cell parameter during annealing was evidence that oxide ion sublattice ordering had occurred. It is likely that short-range cation ordering would occur in this material during the early stages of annealing, and, therefore also in (BiO1.5)0.80(YbO1.5)0.20, given the similar decay behaviour. Indeed, the presence of short-range cation ordering in (BiO1.5)0.80(YbO1.5)0.20 was shown by Battle et al. (Reference Battle, Catlow and Moroney1987) and it is worth noting that the material prepared by Battle et al. was annealed at 500 °C for 24 h. With further annealing the cation ordering becomes long-range in the case of (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03, since orthorhombic splitting of the fluorite-type reflections was observed in XRD patterns for the 100 and 200 h annealed materials.

IV. CONCLUSION

New quenched-in fcc fluorite-type materials in the Bi2O3–Yb2O3–PbO system were synthesised by solid-state reaction. However, long-term annealing at 500 °C, to test for structural stability, resulted in conductivity-lowering structural transformations, making these materials unsuitable for practical use as solid electrolytes in SOFCs. The nature of the structural transformations and magnitude of the conductivity decay in the (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x series varied with x, with (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 undergoing a fluorite-type to Bi17Yb7O36-type orthorhombic transformation, (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 undergoing transformation to a β-Bi2O3-type tetragonal phase, and (BiO1.5)0.80(YbO1.5)0.14(PbO)0.06 transforming to a mixture of Bi17Yb7O36-type orthorhombic, β-Bi2O3-type tetragonal, and Bi12PbO19-type bcc phases. Using S-XRD the structural transformation (〈111〉 oxide-ion vacancy ordering) responsible for the conductivity decay in the early stages of annealing of (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 was established, with cation ordering becoming long-range after extended annealing.

ACKNOWLEDGEMENTS

The authors thank Professor Colin Raston for support of this research, and the Australian Institute for Nuclear Science and Engineering (AINSE) for the provision of an AINSE Postgraduate Award. Work performed at the Australian National Beamline Facility was supported by the Australian Synchrotron Research Programme, which was funded by the Commonwealth of Australia under the Major National Research Facilities Programme, and was performed with the help of Dr. James Hester.

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Figure 1. (Color online) The partial air-quenchable domain of the fluorite-type phase in the Bi2O3–Yb2O3–PbO system. The * symbols indicate the samples for which annealing behaviour was investigated.

Figure 1

Figure 2. (Color online) (a) Ex situ laboratory XRD patterns for (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x, x = 0, 0.03, 0.06, and 0.09, annealed at 500 °C for 100 h, and for x = 0.03 annealed for 200 h, and (b) fit to ex situ S-XRD data collected for (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 annealed for 200 h.

Figure 2

Figure 3. (Color online) (a) Relative conductivity, σ/σ0, as a function of annealing time at 500 °C, for (BiO1.5)0.80(YbO1.5)0.20−x(PbO)x, x = 0, 0.03, 0.06, and 0.09, and (b) a comparison between the σ/σ0 plots obtained here for (BiO1.5)0.80(YbO1.5)0.11(PbO)0.09 and by Webster et al. (2008) for (BiO1.5)0.80(ErO1.5)0.11(PbO)0.09.

Figure 3

Figure 4. (Color online) (a) Rietveld fit to the final in situ S-XRD dataset collected during annealing of (BiO1.5)0.80(YbO1.5)0.17(PbO)0.03 at 500 °C. Experimental data are shown as plus signs, the calculated pattern as a solid line, and the difference pattern as solid lines below. Crystal structure data: a = 5.5268(2) Å, space group Fm$\overline 3$m, Rwp = 4.21%, R (F2) = 6.23%. (b) Refined lattice parameter as a function of annealing time. The error bars are indicative of the error in the fit between the calculated and observed intensities.