Perovskites vacancy-ordered structures

Anion-deficient perovskites and vacancy-ordered structures. Anion-vacancy... 

Table 1—Hexagonal Perovskite Oxides Exhibiting B-Site Vacancy-ordering Structural details...

Anion-Deficient Perovskites and Vacancy-Ordered Structures... 

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-... 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Up to now, very few authors have dealt with vacancy ordering in ternary oxides AM03 y derived from the perovskite structure. Some have been prompted to study more carefully vacancy-ordering in some phases of this type and to correlate it to significant physical properties. 

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