Effects of A-site substitution on the structure and magnetic properties of Bi_(0.15)Sr_(0.85)Ae_yCo_(1-x)Fe_xO_(3-delta) perovskites

摘要

The effects of partial substitution of Sr~(2+) by Ca~(2+) and Ba~(2+) on the A-site of oxygen-deficient perovskites, Bi_(0.15)Sr_(0.85-y)Ae_yCo_(1-x)Fe_xO_(3-delta), where y = 0.28 for Ae = Ba and y = 0.17 for he = Ca, and 0.0 <= x <= 1.0, have been investigated. The differing ionic size of the Ca~(2+) and Ba~(2+) cations influences both the crystal structure and the properties of the materials. The smaller Ca~(2+) cation favoured formation of an oxygen vacancy ordered perovskite superstructure (I4/mmm, a = 2a_p, c = 4a_p), meanwhile the presence of the larger Ba~(2+) cation promoted a disordered simple cubic structure (Pm3-barm, a = a_p) that was also found for all Fe containing samples, i.e. x >= 0.25. The samples were studied with PXRD, NPD, TGA, electron microscopy and magnetic susceptibility measurements. All as-prepared samples exhibited long range G-type antiferromagnetic ordering. The effect of oxygen annealing was dramatic for the Bi_(0.15)Sr_(0.68)Ca_(0.17)Co_(1-x)Fe_xxO_(3-delta) series, with a disappearance of magnetic order for x >= 0.25 linked to increasing spin-glass properties. The oxygen content of the Bi_(0.15)Sr_(0.57)Ba_(0.28)Co_(1-x)Fe_xO_(3-delta) as-prepared materials was generally higher than their Ca substituted counterparts, and the long range antiferromagnetic order was more resistant to oxygen annealing.