Oxygen vacancy ordering

In the brownmillerite structure, the oxygen vacancies order in such a way that half the iron of CaFe02.5 are octahedrally coordinated and half are tetrahedrally coordinated. This ordering is an expression of the tetrahedral-site stability of Fe ions. In a system like Lai yCayFe03 x < y/2, there is a tendency to form intergrowths of perovskite and brownmillerite structures the oxygen vacancies do not remain randomly distributed. Mossbauer spectra at 4.2 K for this complex system exhibit three sextets they have been interpreted in terms of 2 Fe and one Fe " ", or of an Fe and the disproportiona-... 

Fig.2. Structures of (a) Ca2Fe2Os (brown-millerite) and (b) Ca2Mn2Os. Oxygen vacancy ordering in the a-b plane is also shown.

Figure 3.7 Brownmillerite structure as an example of long-range oxygen vacancy ordering. O — octahedral formed by oxygen (open circles) and B cations (closed circles). T — tetrahedra involving B-site cations. The squares denote oxygen vacancies. For clarity, the A-site cations are not shown.

C. Paulmann, Study of oxygen vacancy ordering in mullite at high temperature, Phase Trans. 59, 77-90 (1996). 

The change in the direction of the oxygen vacancy ordering implies a perpendicular orientation of the CX1O4 groups from one domain to the adjacent one. Possible models of the junction between the domains, with or without mirror plane (110) are drawn in figure 8 only one layer out of three polyhedral layers has... 

A structure for Lax.gBa .5Cu307 3 had been previously proposed on the basis of powder x-ray diffraction data (1). The proposed structure had the same basic cation positions as in the 1-2-3 structure, but the oxygen vacancy ordering was much different. Furthermore, it was proposed that the cell edge was a>/2 despite the fact that there was apparently no evidence for this larger cell. A 100 reflection was listed in a table, but no observed d value was given. Our attempts to refine our data with this proposed structure met with no success, and we thus conclude that the proposed structure (1) is incorrect. 

X 2-y/2ap phase are based on oxygen-vacancy ordering in the CuO chain plane of the compound [7.22, 7.12, 7.22, 7.41-7.46], we have first tried to construct a model based on an oxygen-vacancy ordering in the CuOi- plane. Some of these models even included vacancies in the Ba layer, in order to reduce the oxygen content of the model structure below 1 — d = 0.25. The models constructed were all consistent with the determined plane group, but diffraction pattern simulations for these models as well as for those proposed in the literature, all produced unsatisfactory matches with the experiment. 

A model for the displacements in the BaO layer adjacent to the oxygen vacancy ordered CuOi g layer follows from electrostatic considerations. The ordering in the CuOi g plane in alternating full and empty CuiOi chains leads... 

Klie R.F., Ito Y., Stemmer S., Browning N.D. Observation of oxygen vacancy ordering and segregation in perovskite oxides. J. Solid State Ionics 2000 130 289-302. 

L. M. Claus, H. (1988). Oxygen vacancy ordering and superconductivity in YBajCujO,-. Physica, C153-155, 578-81. 

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