Magnetism in Mixed Valence, Defect, Cubic Perovskites: BaIn_1- Fe O_2.5+δ, x = 0.25, 0.50, and 0.75. Local and Average...

The series BaIn_1- Fe O_2.5+δ, 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, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba₂In₂O₅ (BaInO_2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba₂Fe₂O₅ (BaFeO_2.5) crystallizes in a complex monoclinic structure, P2₁/c, showing seven Fe³⁺ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm3̅m, with long-range disorder on the small cation and oxygen sites. ⁵⁷Fe Mössbauer 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 Mössbauer fraction. It appears that the Pm3̅m 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 ∼6 K [5 K] for x = 0.50 and at ∼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 χ'(max) and spin glass behavior, while for x = 0.75, χ'(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 ⁵⁷Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm3̅m model. Fe Mössbauer, 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.