Diffusion and conductivity properties of cerium niobate R.J. Packer a, ⁎ , E.V. Tsipis b , C.N. Munnings a , V.V. Kharton b , S.J. Skinner a , J.R. Frade b a Department of Materials, Imperial College London, London, SW7 2BP, United Kingdom b Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, Aveiro 3810-193, Portugal Received 14 July 2005; received in revised form 28 March 2006; accepted 28 March 2006 Abstract The tracer diffusion coefficient (D ⁎ ) and the surface exchange coefficient (k ⁎ ) provide vital information for materials used in high temperature electrochemical devices (e.g. solid oxide fuel cells or oxygen permeation membranes). These values were established for the high temperature tetragonal scheelite structured CeNbO 4+δ (monoclinic fergusonite at room temperature), which is of interest due to its wide range of oxygen stoichiometries varying from stoichiometric CeNbO 4 to CeNbO 4.33 . Measurements of D ⁎ and k ⁎ were performed by the isotopic exchange/line scan technique with SIMS (secondary ion mass spectrometry) used to determine 18 O stable isotope depth distribution. This process was carried out between temperatures of 1073 K and 1173 K at 500 mbar of 16 O/ 18 O. These measurements were then correlated with oxide ion conductivity data previously determined from four probe d.c. and e.m.f. measurements using the Nernst–Einstein relation. © 2006 Elsevier B.V. All rights reserved. Keywords: CeNbO 4 ; Scheelite; Oxide ion conductivity; Diffusion; Nernst–Einstein 1. Introduction With increasing demand for energy, coupled with the increasing volatility of crude oil prices, research into energy efficient power generation systems is of paramount importance to the world economy. One technology with great potential is the solid oxide fuel cell (SOFC), which has many advantages over existing technologies including environmentally benign byproducts, high efficiencies, quiet operation and flexible fueling choices. SOFCs are, however, limited by the performance of the ion conducting materials used in their construction, requiring high temperatures to achieve sufficient ionic conduction. Reduction of the operating temperature would mean the use of stainless steel for interconnects could be achieved and, in turn, cost- effectiveness increased. To this end, research is ongoing to find novel electrode and electrolyte materials with sufficiently high low temperature oxide ion conduction. Recently, oxygen hyperstoichiometric phase materials have generated much interest as possible SOFC components, particularly those which exhibit high oxygen interstitial mobility [1]. The first of these phases to be fully investigated were the K 2 NiF 4 -type oxides with general formula La 2 NiO 4+δ [2]. These new materials appear to have great potential as SOFC cathode materials due to high levels of mixed ionic/ electronic conduction; however, fabrication of SOFC electro- lyte components have been harder to envisage. Continuing research into developing pure ionically conducting hyperstoi- chiometric phases led to the investigation of CeTaO 4+δ , which was confirmed to contain significant oxygen interstitial content. Further investigation of the material demonstrated that CeTaO 4+δ possessed high total conductivity at low temperatures [1], but due to a number of drawbacks, namely high temperature phase instability and mixed ionic/electronic conduction [3], it was deemed unsuitable for SOFC electrolyte applications. Although CeTaO 4+δ proved to be inappropriate for use as a SOFC component the positive outcomes led to research into the incorporation of alternative group 5 transition metals on the B site. Firstly, CeVO 4+δ which exhibits a tetragonal zircon-type structure in air and transforms into a tetragonal scheelite-type structure above 1273 K. CeVO 4+δ has been shown to have conductivity of the order of 0.042 S cm - 1 at 1123 K and it is Solid State Ionics 177 (2006) 2059 – 2064 www.elsevier.com/locate/ssi ⁎ Corresponding author. Tel.: +44 20 7594 6782; fax: +44 20 7594 6757. E-mail address: robert.packer@imperial.ac.uk (R.J. Packer). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.044