Synthesis and Characterisation of La 095 Sr 005 GaO 3d , La 095 Sr 005 AlO 3 and Y 095 Sr 005 AlO 3 P. S. Anderson, a G. C. Mather, b F. M. B. Marques, b * D. C. Sinclair a and A. R. West a a Chemistry Department, University of Aberdeen, Aberdeen AB24 3UE, UK b Ceramics and Glass Engineering Department, UIMC, University of Aveiro, 3810 Aveiro, Portugal (Received 3 August 1998; accepted 26 October 1998) Abstract The oxide-ion conducting phases La 095 Sr 005 GaO 3d (LSG), La 095 Sr 005 AlO 3d (LSA) and Y 095 Sr 005 AlO 3d (YSA) have been synthesised, either by solid state reaction or through a chemical route. Structural and microstructural characterisation was carried out using XRD and SEM/EDS techniques and a.c. impe- dance spectroscopy (250±1000 C) was employed to determine the electrical properties. Although low- temperature impedance results were strongly dependent on phase purity and microstructure, high temperature electrical conductivity data could be used to compare the electrical conductivities of LSG, LSA and YSA. Chemical modi®cation from the well- known Sr-doped lanthanum gallate via replacement of dierent cations on the A and/or B site decreased the conductivity, increased the activation energies and decreased the ionic conductivities. From the results, there is no simple correlation between oxygen ion conductivity and geometric aspects related to unit cell parameters and cation radii. Instead, cation polarisability seems to play an extremely important role in designing improved oxygen ion conductors based on the perovskite structure. # 1999 Elsevier Science Limited. All rights reserved Keywords: electrical properties, perovskites, microstructure: ®nal, lanthanum gallate, ionic conductivity. 1 Introduction Potential applications of oxygen ion conductors are numerous, including uses such as components in oxygen sensors, oxygen pumps and oxygen- permeable membranes. They have also been studied for application in solid oxide fuel cells (SOFC), which oer a clean, pollution-free alternative energy source for the electrochemical generation of electricity at high eciency. The most widely researched oxide ion con- ductors are those with the ¯uorite structure which contain tetravalent cations, such as zirconium (ZrO 2 ) and cerium (CeO 2 ). 1 Currently, Y 2 O 3 -sta- bilised ZrO 2 (YSZ) is the most common electrolyte used in SOFCs, however, the oxide ion con- ductivity in YSZ is rather low for fuel cell applica- tions. Due to the low conductivity, SOFCs which use YSZ electrolytes must operate at around 1000 C, which leads to problems associated with high costs and the ability of the materials to func- tion at the operating temperature. In particular, there are problems associated with phase stability of the various component materials and their compatibility at ca 1000 C. These constraints have led to the investigation of other possible oxide-ion conducting materials, including ABO 3 oxides based on the perovskite structure. Perovskites oer numerous advantages, including the stability of the crystal structure, the variety of elements which can be accommodated in the crystal lattice, and the ease with which oxygen vacancies can be produced by partial substitution of the A- and/or B-site cations with lower valence cations. The suitability of a wide range of per- ovskitic oxides as potential oxide ion conductors have been analysed. 2±7 Recently, LaGaO 3 -based perovskite-type oxides have been shown to exhibit extremely high oxide-ion conductivity. 2±6 Before this discovery, research into oxide ion conducting perovskite-based materials had led to disappoint- ing results. 7 The LaGaO 3 -based perovskite-type oxides have ionic transport numbers close to unity, Journal of the European Ceramic Society 19 (1999) 1665±1673 # 1999 Elsevier Science Limited Printed in Great Britain. All rights reserved PII:S0955-2219(98)00263-5 0955-2219/99/$ - see front matter 1665 *To whom correspondence should be addressed. Fax: +351- 34-25300; e-mail: fmarques@cv.ua.pt