Phase stability of SrFeCo 0.5 O y under synthesis and annealing conditions B.J. Mitchell a , J.W. Richardson Jr. a, *, C.D. Murphy a , B. Ma b , U. Balachandran b , J.P. Hodges c , J.D. Jorgensen c a Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA b Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA c Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Received 13 October 1999; received in revised form 29 April 2001; accepted 20 May 2001 Abstract Dense ceramic tubes of the multi phase mixed ionic/electronic conductor SrFeCo 0.5 O y (SFC2) have been .synthesized by solid- state reaction. Stability of the component phases of SFC2 was studied by insitu neutron diffraction in the temperature range of 900– 1200  C in air and Ar environments. In air between 900 and 1050  C, the material is stable, with Sr 2 (Fe,Co) 3 O y (236) being the major phase. Above 1050  C, 236 undergoes decomposition into perovskite and rocksalt phases, and at 1200  C, only a small fraction of the 236 phase is stable. In Ar, the 236 phase is completely stable at 900  C but is completely decomposed by 1100  C, whereupon only the perovskite and rocksalt phases remain. Rietveld analysis indicates that the 236 and perovskite phases become more Fe-rich as decomposition occurs, while the perovskite phase lattice parameter and oxygen content vary readily as temperature and gas environment are changed. # 2002 Published by Elsevier Science Ltd. Keywords: Membranes; Neutron diffraction; Phase composition; Stability; SrFeCo 0.5 O y 1. Introduction Recently, the mixed-conducting oxide SrFeCo 0.5 O y (SFC2) has been identified as a potential dense ceramic membrane that can be used to separate gas at elevated temperatures. Indeed, Balachandran et al. 1 demon- strated that extruded membrane tubes of SFC2 can be used for partial oxidation of methane to produce syngas (CO+H 2 ) in a methane conversion reactor operating at  850  C. The oxygen flux obtained from the separation of air in this reactor is commercially feasible, and the use of this technology could significantly reduce the cost of oxygen separation. 2 Ceramic oxides have been considered for membrane applications for many years. Teraoka et al. 3,4 initially showed that certain ceramic materials, including some perovskite-based oxides, exhibit oxygen permeability at high temperatures while remaining impenetrable to other gases. Typically, ABO 3 perovskite materials 5 8 have been studied for oxygen diffusion properties or membrane applications, and, as-synthesized, these are single-phase materials. SFC2 is different from the ABO 3 perovskites in that it is not a single-phase material under standard synthesis conditions. Four known 9,10 phases, Sr 2 Fe 3a Co a O 6.5 (236), SrFe 1b Co b O 3 0 (perovskite/brownmillente), Co 3c Fe c O 3 (rock salt), and Co 3d Fe d O 4 (spinel), are certainly identifiable; with their presence dependent upon synthesis condi- tions. Guggilla and Manthiram 11 and Ma et al. 12 have shown that substitution of Co for Fe in SrFe 1.5x Co x O y produces single-phase materials only over a limited Co range. This finding suggests that Co 3+ is not particu- larly stable within the 236 phase. The enhanced perfor- mance of SFC2 relative to that of the individual phases, as reported by Ma et al., 12 is not yet understood. The layered structure of Sr 2 (Fe,Co) 3 O 6.5 (see Fig. 1) is formed from perovskite and double-layer inter- growths. The Fe/Co atoms are assumed to be in +3 oxidation state and occupy three distinct environments: octahedral (6-fold), trigonal bi-pyramidal (distorted 5-fold), and square pyramidal (distorted 5-fold). The 0955-2219/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0955-2219(01)00367-3 Journal of the European Ceramic Society 22 (2002) 661–671 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: jwrichardson@anl.gov (J.W. Richardson Jr.).