Doping and defect association in AZrO 3 (A  Ca, Ba) and LaMO 3 (M  Sc, Ga) perovskite-type ionic conductors † M. Saiful Islam,* Peter R. Slater, Julian R. Tolchard and Tim Dinges ‡ Materials Chemistry Laboratory, Chemistry Division, University of Surrey, Guildford, UK GU2 7XH. E-mail: m.islam@surrey.ac.uk Received 20th February 2004, Accepted 5th April 2004 First published as an Advance Article on the web 23rd July 2004 Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO 3 (A = Ca, Ba) and LaMO 3 (M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant–defect association at the atomic level. The energetics of dopant incorporation in AZrO 3 show strong correlation with ion size. We predict Y 3+ to be one of the most favourable dopants for BaZrO 3 on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy–dopant pairs in BaZrO 3 are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO 3 is via a curved pathway around the edge of the ScO 6 octahedron. Dopant–vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO 3 , but very low energies in LaGaO 3 . This points to greater trapping of the oxygen vacancies in doped LaScO 3 , perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data. 1 Introduction Oxygen ion and proton conductivity in perovskite-structured oxides have attracted considerable attention owing to the range of electrochemical applications (e.g. fuel cells, gas sensors, cer- amic membranes) and the fundamental fascination with trans- port phenomena in solid state materials. A range of perovskite ceramics, particularly cerates (ACeO 3 ) and zirconates (AZrO 3 ), exhibit significant proton conductivity. 1 An important example is the development of a potentiometric gas sensor for hydrogen in molten metal based upon doped CaZrO 3 as the proton-con- ducting electrolyte. 2 In terms of defect chemistry, the oxide is typically doped by a trivalent cation at the Zr site resulting in the formation of charge-compensating oxygen vacancies, which are readily filled by hydroxy ions in the presence of water vapour. More recently, there has been renewed interest in acceptor- doped BaZrO 3 whose combination of high proton conductivity coupled with good chemical stability, 3–7 makes this material a promising candidate for solid oxide fuel cell (SOFC) appli- cations. Bohn and Schober 5 find that the proton mobility in Y-doped BaZrO 3 is among the highest ever reported for a per- ovskite-type proton conductor. The Y-doped BaZrO 3 system has the potential to operate at lower temperatures than the con- ventional SOFC electrolyte, and hence recent research has involved attempts to optimise the materials’ properties. An electrochemical reactor with a ceramic proton-conducting membrane based on doped BaZrO 3 has also been used to study the electrochemical promotion of catalysis. 6 Also in this field of conducting solids, the oxygen transport properties of the LaGaO 3 -based perovskite have been widely investigated, 8–18 owing to the higher oxygen ion conductivity than the conventional Y/ZrO 2 electrolyte at lower temperatures. The incorporation of cation dopants to form the system La 1-x - Sr x Ga 1-y Mg y O 3-δ (often termed LSGM) gives rise to the highly mobile oxygen vacancies that are responsible for the observed † Based on the presentation given at Dalton Discussion No. 7, 5–7th July 2004, University of St Andrews, UK. ‡ Present address: Fachhochschule Gelsenkirchen, D45665 Reckling- hausen, Germany. ionic conductivity. More recently, the oxygen ion and proton conduction properties of the related LaScO 3 material have been investigated, particularly systems doped with alkaline-earth dopants at either La or Sc sites. 19–21 For example, Lybye and Bonanos 19 have investigated the La 0.9 Sr 0.1 Sc 0.9 Mg 0.1 O 3-δ material and showed mixed conductivity at low oxygen partial pressure; proton conduction was dominant at temperatures below 700 °C while above 800 °C oxygen ion conduction became increasingly dominant with temperature. More recently, Kato et al. 21 have investigated the electrical conductiv- ity of Al-doped La 1-x Sr x ScO 3 as a potential SOFC material. It has become increasingly clear that the investigation of defect phenomena and atomistic diffusion mechanisms under- pins the fundamental understanding of macroscopic behaviour. However, there is often limited atomic-scale information on complex ceramic oxides, such as lattice defects, dopant-site selectivity and the extent of defect-dopant clustering. There is also debate as to whether there is any significant interaction between the dopant ion and the protonic defect leading to possible proton “trapping”. This study attempts to provide further insight into these problems by using computer simulation techniques, which are now well established tools in solid state chemistry. The reliabil- ity of such an approach has been demonstrated by our simu- lation studies of defects, ion transport and surface structures of other perovskite oxides (e.g. LaMnO 3 , LaCoO 3 ). 22–25 This paper presents recent computational studies of topical oxygen ion and proton-conducting perovskites based upon LaScO 3 and BaZrO 3 , with direct comparison with related work on LaGaO 3 (ref. 23) and CaZrO 3 (ref. 22), respectively. Emphasis here is placed on probing dopant site-selectivity, defect association and oxygen ion migration, which have assisted in the further understanding of these complex oxides on the atomic-scale. 2 Computational methods Our description of the computational techniques will be brief since comprehensive reviews are given elsewhere. 26–29 In this paper, two main classes of technique have been employed in the study of the perovskite materials: atomistic (static lattice) and quantum mechanical (ab initio) methods. DOI: 10.1039/ b402669C This journal is © The Royal Society of Chemistry 2004 Dalton Trans. , 2004, 3061–3066 3061 Downloaded by University of Bath on 12 November 2012 Published on 23 July 2004 on http://pubs.rsc.org | doi:10.1039/B402669C View Online / Journal Homepage / Table of Contents for this issue