The Relations between Oxygen Vacancies and Structures in the Solid Solution Systems Sr1-xMxFeO3-δ(M=Y, La, Bi and ln)

Abstract

To clarify the roles of oxygen vacancies in the, perovskite-type, compounds of ABO8, the solid solution systems (Sr, M)FeO3-δ(M=Y3+, La3+, Bi3+ and In3+) are studied from the crystallo-chemical point of view by means of X-ray diffractometry, density measurements an chemical analysis.The results are summarised as follows: (1)In the (Sr, M)FeO3-δ prepared by firing at elevated temperatures in air, the lattice constants increase linearly with the elevation of M3+ content as a result of decreasing Fe4+/Fe3+ ratio to the respective critical content of M3+, then the lattice constants decrease as a result of an additional contribution of decreasing oxygen vacancies as the M3+ content further increase. When the specimens are fired in fairly high temperatures, however, A-site-vacancies in addition to the oxygen vacancies often appear and a substitutional exchange between A-site and B-site ions also occurs in a small extent. From the fact that the all tolerance factors of (Sr, M)FeO3-δ at each maximum solubility of Ms3+ are nearly equal to 1.01, we may conclude that the stability limit of the present solid solutions to be controlled by the geometrical requirement of the constituent ions. (2)In the (Sr, M)FeO3-δ prepared by firing in vacuum, brownmillerite-type phase changes to cubic perovskite-type with the elevation of M3+ content owing to increasing incorporation of oxygen. A further incorporation of oxygen into the cubic phase results in the orthorhombic symmetry. The presence of oxygen vacancies in the solid salutians seems to play an important role for the stabilization of perayskite-type structure with cubic symmetry.