Magnetic Properties of the Oxygen Deficient Perovskites La0.5Ba0.5Co1−xFexO3−δ

Abstract

New perovskites characterized by a complex mixed valence of transition elements have been isolated: La0.5Ba0.5Co1−xFexO3−δ(0 ≤x≤ 1 and 0.05 ≤ δ ≤ 0.07) with the cubic symmetry, and La0.5Ba0.5Co0.5Fe0.5O2.65with the tetragonal symmetry (a≈ap,c≈ 2ap). The structure of the latter has been described as YBaFeCuO5-type 90° oriented domains distributed in a stoichiometric perovskite matrix La0.5Ba0.5Co0.5Fe0.5O3. The Mössbauer study of these phases has allowed the two species Fe(III) and Fe(IV) to be evidenced: no proof of disproportionation of Fe(IV) into Fe(III) and Fe(V) has been obtained. The cobalt mixed valence Co(III)–Co(IV) has been deduced from the Mössbauer study coupled with the chemical analysis for the oxidized samples La0.5Ba0.5Co1−xFexO3−δ, whereas the reduced phase La0.5Ba0.5Co0.5Fe0.5O2.65is characterized by the mixed valence Co(II)–Co(III). The magnetic susceptibility measurements of La0.5Ba0.5Co1−xFexO3−δshow an evolution from an antiferromagnetic behavior forx≥ 0.50 withTNranging from 142 to 218 K, to a ferromagnetic behavior forx< 0.50 withTcranging from 120 to 150 K. The tetragonal phase La0.5Ba0.5Co0.5Fe0.5O2.65is antiferromagnetic withTN≈ 480 K. All the antiferromagnetic phases exhibit a second transition atTB<TN. The origin of the latter is not explained for the cubic phases and may correspond to a change of spin orientation of the transition element species in the case of the tetragonal phase.