Tetrahedral Chain Order in the Sr₂Fe₂O₅ Brownmillerite

Abstract

The crystal structure of the Sr 2 Fe 2 O 5 brownmillerite has been investigated using electron diffraction and high resolution electron microscopy. The Sr 2 Fe 2 O 5 structure demonstrates two-dimensional order: the tetrahedral chains with two mirror-related configurations (L and R) are arranged within the tetrahedral layers according to the -L-R-L-Rsequence, and the layers themselves are displaced with respect to each other over 1/2[111] or 1/2[111 j ] vectors of the brownmillerite unit cell, resulting in different ordered stacking variants. A unified superspace model is constructed for ordered stacking sequences in brownmillerites based on the average brownmillerite structure with a ) 5.5298(4)Å, b ) 15.5875(12)Å, c ) 5.6687(4)Å, and (3 + 1)-dimensional superspace group I2/m(0βγ)0s, q ) βb* + γc*, 0 e β e 1/2, 0 e γ e 1.

Local and Average Structures and Magnetic Properties of Sr2FeMnO5+y, y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the Effect of the A-Site Cation

“…15 The Fe(Mn)ÀO bond length, 1.94539(2) Å, is very close to the average FeÀOa n dM n ÀOd i s t a n c e si n A 2 Careful inspection of the neutron diffraction data revealed shoulders appearing on almost every major peak, shifted systematically to smaller d spacings; see the inset in Figure 2b. This signaled the presence of a slight amount of another cubic phase with similar composition but smaller cell parameters, which was confirmed by a two-phase Rietveld refinement.…”

“…It is noteworthy that, in the above cases, the change in A-site cation results in a change in space group, whereas the general structural type remains the same. Both Ca 2 (99.9% Cerac) that were mixed and pressed into pellets. The pellets were annealed at 1250°C in an argon atmosphere for a total time of ∼100 h and were then slowly cooled to room temperature.…”

“…For the (8) octahedra, the four bonds in the aÀc plane with an average distance of 1.92 Å are significantly shorter than the two bonds out of the plane, 2.22 Å, whereas for the tetrahedra, the four bond lengths range from 1. 12,13 There does not appear to be a simple, straightforward explanation for the remarkable absence of tetrahedral chains in the long-range sense, which is equivalent to the destruction of oxide vacancy order, in proceeding from Ca 2 FeMnO 5 to Sr 2 X-ray diffraction and refers to the average structure. Our NPDF and M€ ossbauer results show that the local structure is more Figure 5.…”

Investigation of the microstructural features of SrCo0.8Fe0.2O3−δ perovskite

“…It is known that different ordering of tetrahedral chains is characteristic of the brownmillerite structure, which in turn leads to different space symmetry groups: Icmm, Ibm2, and Pnma [17]. Figure 8 shows the results of modeling the diffraction patterns using different space groups.…”

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

“…This was challenged, however, by transmission electron microscopy results showing clear intralayer alternation of tetrahedral chains. 33 Additionally, more recent neutron diffraction experiments on single crystals grown by the floating zone method have indicated that P bcm is indeed the preferred structure type for Sr 2 Fe 2 O 5 . 18,19 Furthermore, all of these recent experiments are supported by the aforementioned results of our ab initio calculations; the small energy difference between the P bcm and I2bm structures, however, indicate that intergrowths of the phases is not out of the question.…”