Brownmillerite Phase Research Articles

Oxygen Permeation Through Cobalt-Containing Perovskites: Surface Oxygen Exchange vs. Lattice Oxygen Diffusion

The oxygen permeation fluxes from to across cobalt-containing perovskite ceramic membranes and were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes for uncatalyzed and for were obtained when vs. was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed and Application of a thin layer of catalytically active on the feeding-gas surface of under the condition of a fixed atm and a varied not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of on the permeate side has little catalytic effect, especially at low range, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained. © 2001 The Electrochemical Society. All rights reserved.

Structural transformation in fluorinated LaACuGaO5 (A=Ca, Sr) brownmillerites

The brownmillerite type phases LaSrCuGaO5 and LaCaCuGaO5 were fluorinated using XeF2 and the resulting materials were investigated using X–ray diffraction, high resolution electron microscopy (HREM) and electron diffraction. For low content of XeF2, both materials undergo a decrease in orthorhombicity maintaining an aO≈4aP, bO≈aP2, cO≈aP2 unit cell with space group Ima2. On increasing the amount of XeF2, LaSrCuGaO5 transforms to a tetragonal structure with cell parameters, aT≈aP, cT≈2aP (space group P4/mmm). The model for the tetragonal structure was proposed on the basis of HREM observations. The anion exchange reaction results in the replacement of one oxygen anion by two fluorines in the (GaO□) layers. This reaction is accompanied by a random occupation of the anion positions in the Ga-layers by oxygen, fluorine and anion vacancies, leading to a tetragonal symmetry. Different superstructures, due to local ordering of anions and anion vacancies, were observed by electron microscopy.

A neutron diffraction study of the temperature dependence of Ca 2Fe 2O 5

The crystal structure of Ca 2Fe 2O 5 has been studied as a function of temperature up to 1000°C. Cell parameters vary linearly with temperature while a structural phase transition from Pcmn to Icmm takes place at about 700°C. The transition induces a change in the tilting of the octahedra and displacements of the oxygen ions in the tetrahedra that build up the structure in alternating layers. Increasing disorder along the chains of tetrahedra and ordering of the octahedra and coordination polyhedron around Ca are observed in the high temperature structure. The magnetic structure was refined in space group Pcm′ n′. Comparisons are made to the structures of Sr 2Fe 2O 5 and other doped brownmillerite phases.

Recent developments in perovskite-based oxide ion conductors

Recent advances in the study of oxygen-deficient perovskites and brownmillerite phases as oxygen ion conductors are reviewed. The structures of these phases containing oxygen vacancies are surveyed and the role of the oxygen vacancies with respect to the ionic conductivity in these materials is discussed in detail.

A neutron diffraction study of two strontium cobalt iron oxides

The structures of the perovskite and brownmillerite phases of the nonstoichiometric oxide SrCo 0.8Fe 0.2O 3-δ have been refined by Rietveld analysis of powder neutron diffraction data. SrCo 0.8Fe 0.2O 2.78 : M r = 190.575, cubic, space group Pm 3 m (No. 221), α = 3.8491(2)Å, V = 57.025(4)Å 3, Z = 1, R p = 8.37%, R wp = 11.52. Sr 2Co 1.6Fe 0.4O 5: M r = 371.87, orthorhombic, space group Icmm (No. 74), α = 5.5981(3)Å, b = 15.7832(8)Å, c = 5.4692(3)Å, V = 483.23(7)Å 3, Z = 4, R p = 6.23%, R wp = 7.52%.

Nonstoichiometry and phase relationship of the SrFeO 2.5[sbnd]SrFeO 3 system at high temperature

In order to make clear the phase relationships and nonstoichiometry of the SrFeO 2.5 SrFeO 3 system at elevated temperatures, measurements were made using high-temperature gravimetry and high-temperature powder X-ray diffraction (XRD) analysis under the controlled oxygen partial pressure of 1–10 −5 atm at temperatures above 400°C. At 400–900°C, the system was found to consist of the SrFeO 2.5+δ phase of the brownmillerite-type structure with small oxygen excess-type nonstoichiometry and the SrFeO 3−δ phase of the cubic perovskite-type structure with large oxygen deficient-type nonstoichiometry. At temperatures above 900°C, the brownmillerite phase transformed to the perovskite-type and the system was composed of only the cubic perovskite-type phase.

Electrical and magnetic properties of nonstoichiometric Ca 2Mn xFe 2−xO 5+ δ

Electrical resistance and magnetic susceptibility of nonstoichiometric Ca 2Mn xFe 2-xO 5+ δ were measured in the range from room temperature to 1100°C and from the liquid helium temperature to 600°C, respectively. The high electrical resistance and activation energy for the brownmillerite phase changed scarcely with increase of δ. The Néel temperature decreased slightly. Electrical and magnetic properties changed abruptly in the phase transition from brownmillerite-type to ordered-type which was derived from the slight increase of δ. The ordered phase had properties similar to those of oxygen-deficient perovskite-type ferrates (i.e. low electrical resistance, low activation energy, very low Néel temperature). The range of two-phase mixture was discussed from the result of electrical resistance measurement.

Studies on nonstoichiometry of Ca 2Mn xFe 2−xO 5+δ

The structure of Ca 2Mn xFe 2−xO 5+δ (0 ≤ x ≤ 0.5), in which δ was determined by thermogravimetric and chemical analyses, was investigated by means of X-ray diffraction and Mőssbauer effect measurements. Increase of δ was accompanied with increase of both tetravalent manganese ion and octahedral site and decreased the lattice parameter b o. in the brownmillerite structure. Anomalous variation of the b o with δ was explained to be due to the Jahn-Teller distortion caused by Mn 3+. Above a certain value of δ, the brownmillerite phase changed into an ordered phase which has the lattice parameters a o = a dical, b o = 2b dical and C o = 2C dical. In the phase portion of iron ions was tetravalent.