Abstract
High quality stoichiometric brownmillerite-type oxide single crystals have been successfully grown by the floating zone method using a mirror furnace. We report here on the growth conditions and structural characterization of two model compounds: Ca2Fe2O5 and Sr2ScGaO5. Both show oxygen deficiency with respect to the average perovskite structure, and are promising candidates for oxygen ion conductivity at moderate temperatures. While Sr2ScGaO5 single crystals were obtained in the cubic oxygen-deficient perovskite structure, Ca2Fe2O5 crystallizes in the brownmillerite framework. Having no cubic parent high temperature counterpart, Ca2Fe2O5 crystals were found to be not twinned. We report on structural characterization of the as-grown single crystals by neutron and X-ray diffraction, as well as scanning electron microscopy (SEM) coupled with EDX (Energy Dispersive X-Ray Spectroscopy) analysis and isotope exchange experiments.
Highlights
We focus here in particular on Ca2 Fe2 O5 (CFO) and Sr2 ScGaO5
When synthesizing Sr2ScGaO5 by classical solid state reaction at 1200 °C, the thermodynamically and unit cell parameters a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å [18]. This structure is stable phase obtained shows a brownmillerite framework with I2mb space group at room temperature characterized by two important details: (i) the B-cation shows a specific order with Sc selectively on and unit cell parameters a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å [18]
Crystal growth of brownmillerite type oxides is discussed, depending on the high temperature parent phase symmetry and related kinetics of the phase transformations, which may lead to different defect structures and associated twin domains
Summary
Brownmillerite type oxides have received considerable attention, because of their rich chemistry, structural complexity, and for their interesting properties and applications [1,2,3,4,5,6,7,8].With the general formula A2 BB0 O5 , the brownmillerite framework can be described as an oxygen-anion-deficient perovskite phase, and its structure can be derived from the cubic phase by releasing 1/6 of all oxygen atoms in an ordered way along the [110] direction of the cubic perovskite.Three different prototypes of the general formula ABO2.5 or A2 B2 O5 can all be obtained from the perovskite structure, modifying every second octahedral layer toward lower coordination transition metal polyhedra, such as a square planar coordination in LaNiO2.5 [8,9] or a square-based pyramid coordination in CaMnO2.5 [10,11]. With the general formula A2 BB0 O5 , the brownmillerite framework can be described as an oxygen-anion-deficient perovskite phase, and its structure can be derived from the cubic phase by releasing 1/6 of all oxygen atoms in an ordered way along the [110] direction of the cubic perovskite. The brownmillerite structure is characterized by tetrahedral layers containing isolated MO4 tetrahedral zigzag chains, which are separated by 1D oxygen vacancy channels, alternating with octahedral layers (see Figure 1). On top of the basic brownmillerite framework, several modulated superstructures as well as disorder in the stacking sequence have been characterized, leading in total to a more complex arrangement of the octahedral and tetrahedral layers [1,2,3,4].
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