New quenched-in fluorite-type materials in the Bi 2 O 3 –La 2 O 3 –PbO system: Synthesis and complex phase behaviour up to 750 8C Nathan A.S. Webster a, *, Karel J. Hartlieb a , Paul J. Saines b,1 , Chris D. Ling b , Frank J. Lincoln a a School of Biomedical, Biomolecular and Chemical Sciences, M313, The University of Western Australia, Crawley, WA 6009, Australia b School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia 1. Introduction Bi 2 O 3 -based solid electrolyte materials with the fcc fluorite- type structure (space group Fm ¯ 3m) are of interest for use in solid oxide fuel cells (SOFCs) due 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 underpins most of the research directed at solid electrolyte materials [1]. For pure Bi 2 O 3 , the fluorite-type phase (d-Bi 2 O 3 ) with a conductivity of 1 S cm 1 at 750 8C [2] is the most highly conductive oxide ion conductor known [3], and Fuda et al. [4] described the oxide ion sublattice as being ‘‘liquid- like’’. The high conductivity is attributed to the Bi 3+ cation, with its 6s 2 lone pair electrons, being highly polarisable [5], and the highly disordered nature of the oxide-ion sublattice (occupancy and positional disorder) [6–14]. However, d-Bi 2 O 3 is stable only between 730 8C and its melting point, 825 8C, and it cannot be preserved to room temperature [15]. Below 730 8C, Bi 2 O 3 exists as either a monoclinic a (P2 1 /c), tetragonal b (P2 1 /c) or bcc g (I23) phase, depending on the temperature and cooling rate of the sample [16,17]. By doping with some rare earth (Sm to Lu, including Y) [16–23] and transition metal (e.g. V, Nb, Ta and W) [24,25] oxides, the fluorite-type phase can be quenched-in. Doping with La 2 O 3 [26] and Nd 2 O 3 [21] typically produces b 1 /b 2 rhombohedral (space group R ¯ 3m, hexagonal setting) layered solid solution phases at room temperature, rather than the fluorite-type phase [27]. b 1 is the high temperature phase and can be preserved to room temperature by quenching, while the b 2 phase forms below 650 8C by slow cooling or annealing [16]. It is also possible to use a combination of metal oxides – ‘‘double doping’’ – to quench-in the Bi 2 O 3 -based fluorite-type phase. Examples include the ternary oxide systems Bi–Dy–W [28,29], Bi–Er–W [30], Bi–Ln–Te (Ln = La, Sm, Gd, Er) [31], Bi–Y–Nb [32], Bi–Ln–V (Ln = La–Yb) [33,34], Bi– La–U [35], Bi–Ca–Pb [36,37] and Bi–Y–Pb [38]. A partial air-quenchable domain of the fluorite-type phase in the Bi 2 O 3 –Er 2 O 3 –PbO system has also been reported [12]. Pb 2+ , with its stereochemically active 6s 2 lone pair and aliovalent nature, was expected to have interesting effects on conductivity and structure. Several of these Bi 2 O 3 –Er 2 O 3 –PbO fluorite-type materi- als displayed high oxide ion conductivity, and differential thermal analysis experiments demonstrated that the fluorite-type struc- ture was stable between room temperature and 700 8C for a heating rate of 208 min 1 . However, annealing at 500 and 600 8C resulted in various symmetry-lowering phase transformations taking place, thus making them unsuitable for use as solid electrolytes [39]. For example, the materials with composition (BiO 1.5 ) 0.80 (ErO 1.5 ) 0.20x (PbO) x , x = 0.03, 0.06, 0.09, underwent a fluorite-type to b-Bi 2 O 3 -type tetragonal transformation during annealing at 500 8C. b-Bi 2 O 3 has a fluorite-type superstructure Materials Research Bulletin 46 (2011) 538–542 ARTICLE INFO Article history: Received 25 August 2010 Received in revised form 17 December 2010 Accepted 21 December 2010 Available online 30 December 2010 Keywords: A. Ceramics A. Oxides C. X-ray diffraction D. Phase transitions ABSTRACT New quenched-in fluorite-type materials with composition (BiO 1.5 ) 0.94x (LaO 1.5 ) 0.06 (PbO) x , x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 8C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO 1.5 ) 0.92 (LaO 1.5 ) 0.06 (PbO) 0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a b-Bi 2 O 3 -type tetragonal phase around 400 8C. In the range 450–700 8C, a-Bi 2 O 3 -type monoclinic, Bi 12 PbO 19 -type bcc and b 1 /b 2 -type rhombohedral phases, and what appeared to be a e-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 8C. ß 2010 Elsevier Ltd. All rights reserved. * Corresponding author. Current address: CSIRO Process Science and Engineering, Box 312, Clayton South, VIC 3169, Australia. Tel.: +61 3 9545 8635; fax: +61 3 9562 8919. E-mail address: nathan.webster@csiro.au (Nathan A.S. Webster). 1 Current address: Department of Materials Science and Metallurgy, The University of Cambridge, Cambridge CB3 0FY, UK. Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.12.035