Magnetism In Mixed Valence, Defect, Cubic Perovskites: Bain1-Xfexo2.5+Delta, X=0.25, 0.50, And 0.75. Local And Average Structures

The series BaIn1-xFexO2.5+delta, x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mossbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba2In2O5 (BaInO2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba2Fe2O5 (BaFeO2.5) crystallizes in a complex monoclinic structure, P2(1)/c, showing seven Fe3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm (3) over barm, with long-range disorder on the small cation and oxygen sites. Fe-57 Mossbauer studies indicate a mixed valency, Fe4+/Fe3+, for both the air-fired and argon-fired samples. The increased Fe3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mossbauer fraction. It appears that the Pm (3) over barm phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at similar to 6 K [5 K] for x = 0.50 and at similar to 22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent chi'(max) and spin glass behavior, while for x = 0.75, chi'(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe3+-Fe3+ antiferromagnetic exchange and ferromagnetic Fe3+-Fe4+ exchange. The PDF and Fe-57 Mossbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm (3) over barm model. Fe Mossbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe3+ sites are largely fourfold-coordinated and Fe4+ sites are fivefold-or sixfold-coordinated.