Crystal Structures of Mixed-Conducting Oxides Present in The Sr-Fe-Co-O System
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Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
In order to develop and understand an oxygen permeation mechanism for SrFeCo0 _03. precise information concerning the crystal structures formed and oxygen nonstoichiometry at various temperatures and oxygen partial pressures is required. We have therefore initiated a wide ranging crystallographic study of the SrFeCo 0503_6 material and related phases. Quantitative analysis of powder X-ray and neutron diffraction data from SrFeCo 0503.6, performed at Argonne, has revealed its multiphase nature. The major phase, present at a wt. fraction of - 70%, is of the form Sr 4(Fe,Co) 60 1 3 and is isotypic in crystal structure to Sr 4Fe 60 3 [6,7]. Present also are, an oxygen deficient perovskite of the form Sr(Fe,Co)0 3.6 with a wt. fraction of - 25% and CoO as a minor phase at - 5% wt. fraction. We have therefore undertaken a study of the two Sr-Fe-Co oxide systems present in the SrFeCo 0503.5 material. In this paper we report preliminary structural findings on the SrFe1 .xCoxO 3_5 (0 • x < 0.3) system. EXPERIMENTAL
Synthesis Polycrystalline samples of SrFe-xCoxO 3.5 (x = 0, 0.1 , 0.2 and 0.3) were prepared by solid-state reaction of SrCO 3, Fe 20 3 and Co 30 4. The starting materials were mixed under n-amyl alcohol in a ball-mill for 3 hr. After drying, these mixtures were initially fired at 900'C for 12 hr., cooled to room temperature and reground in an agate mortar. Further firings at 1050'C, 1100°C and 1150'C for 12 hr. durations were carried out to complete the reaction. Subsequently, each SrFe1 xCoxO 3., sample was split into three and annealed under the following conditions, flowing Ar at 1050'C, air at 550'C or flowing 02 at 350'C, all for 8 hr. periods. Overall twelve samples in the SrFe1.xCox0 3 ., system were prepared with x = 0, 0.1, 0.2 and 0.3 combined with 8 = 0.12, 0.25 and 5 = 0.5 from the 02, air, and Ar annealing stages, respectively. Characterization Powder X-ray and time-of-flight neutron diffraction data were collected on all samples using a Scintag diffractometer and the Special Environment Powder Diffractometer at Argonne National Laboratory's Intense Pulsed Neutron Source. Rietveld profile analysis of the powder neutron diffraction data was performed using the GSAS suite of programs [8]. RESULTS AND DISCUSSION For the end member SrFeO 3.,, Takeda et al. [9] have determined that four distinct phases, with differing crystal structures, exist. The phases are defined by the ideal compositions SrFeO 3-1/n, where n = oo, 8, 4 and 2. The n = ,, and 2 members, SrFeO 3 and SrFeO 25, possess simple primitive cubic perovskite and brownmillerite crystal structures respectively see Figure 1. In the brownmillerite structure, oxygen vacancies are ordered into lines along the c-axis, which is the [11 O]p direction of the parent cubic perovskite. These lines of vacancies are located in every other (001)p plane, giving rise to alternate layers of FeO4 tetrahedra and FeO 6 octahedra. Electron diffraction measurements performed on the n = 8 and
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