A combination of neutron powder diffraction and computer simulation techniques was performed on undoped and doped Bi 2 WO 6 Aurivillius type compounds to clarify some of the factors controlling oxygen transport in these materials. Oxygen vacancies in doped compounds are randomly distributed within the perovskite-like slab. The most favourable dopants are predicted to be NbV and TaV on WVI and LaIII on BiIII in accordance with the experimental results. The calculated migration energy of 0.63 eV is in agreement with the values deduced from impedance spectroscopy data for Ta and Nb doped Bi 2 WO 6 at T >550°C. At lower temperatures, pair clusters are predicted to form with a 0.25 eV mean binding energy, leading to a 0.88 eV activation energy for oxygen vacancy migration, in good correlation with experimental values. Finally, consideration of possible oxygen ion migration pathways in the structure showed that energy barriers to migration are lowest between adjacent apical and equatorial sites of WO 6 oxygen octahedra.The oxides of the Aurivillius family, of general formulaThe present paper deals with a neutron powder diffraction investigation of the doped phases along with computer simu-) were originally of interest for their ferroelectric properties but have recently been explored in the lation methods to clarify some of the factors controlling oxygen transport in these materials. These simulation techniques are field of oxide ion conductivity and constitute now an important new class of oxide anion conductors.1 The Aurivillius phases particularly suited to probing ion migration on the atomic scale and have been successfully applied to other anionic consist of an intergrowth between (Bi 2 O 2 )2+ sheets and (A m−1 B m O 3m+1 )2− perovskite-like layers containing from m= conductors such as perovskite-based oxides29 and brownmillerite-structured Ba 2 In 2 O 5 .30 In this work the energetics of 1-8 layers.2-4 Anionic conductivity is the result of oxygen vacancies, either intrinsic as in Bi 2 VO 5.5 and related extrinsic defects and of various mechanisms or pathways for oxygen migration have been explored and compared with the BIMEVOX compounds,5-18 or extrinsic when these vacancies are introduced by appropriate dopants with lower valence experimental structural and conductivity data. states.19-23 We recently succeeded in preparing Bi 2 WO 6 (m=1) oxygen deficient phases by substitution for WVI with NbV and TaV, Methods thus leading to Bi 2 W 1−x M x O 6−x/2 solid solutions within the Experimental composition range 0 < x <0.15. Maximum oxide ion conductivities were obtained when x=0.05 for both phases and Polycrystalline samples were prepared by solid state reactions reached conductivity values of 10−3 V−1 cm−1 at ca. 450°C.from appropriate amounts of stoichiometric mixtures as These performances are intermediate between those of the high previously described.24 They were characterised by X-ray anion conductor BIMEVOX phases and those of doped higher diffraction with a Guinier De Wolff focusing camera using Cumembers of...

The BICOVOX family of compounds (cobalt-doped Bi4V20,,) has been characterised combining chemical, structural and electrical approaches. The compositional extent of the single-phase BICOVOX solid solution, stable at ambient temperature, was determined. Single-crystal and powder X-ray diffraction data, combined with density measurements, enabled us to formulate the solid solution as Bi2(V, -x-yCoxBi,,)Oz. The maximum x value (0.25) is reached when y=O.O4, and the maximum y value (0.06), when x=O.lO. The evolution of the Arrhenius conductivity plots vs. x and y has been studied within the tetragonal domain. The maximum ~7 value is roughly independent of composition for Paper 5/01932J;

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