Magnetic properties of the oxygen deficient perovskites La0.5Ba0.5Co1-xFexO3-delta

New perovskites characterized by a complex mixed valence of transition elements have been isolated: La0.5Ba0.5Co1-xFexO3-delta (0 less than or equal to x less than or equal to 1 and 0.05 less than or equal to delta less than or equal to 0.07) with the cubic symmetry, and La0.5Ba0.5Co0.5Fe0.5O2.65 with the tetragonal symmetry (a approximate to a(p), c approximate to 2a(p)), The structure of the latter has been described as YBaFeCuO5-type 90 degrees oriented domains distributed in a stoichiometric perovskite matrix La0.5Ba0.5Co0.5Fe0.5O3. The Mossbauer 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 Mossbauer study coupled with the chemical analysis for the oxidized samples La0.5Ba0.5Co1-xFexO3-delta, whereas the reduced phase La0.5Ba0.5Co0.5Fe0.5O2.65 is characterized by the mixed valence Co(II)-Co(III), The magnetic susceptibility measurements of La0.5Ba0.5Co1-xFexO3-delta show an evolution from an antiferromagnetic behavior for x greater than or equal to 0.50 with T-N ranging from 142 to 218 K, to a ferromagnetic behavior for x < 0.50 with T-c ranging from 120 to 150 K. The tetragonal phase La0.5Ba0.5Co0.5 Fe0.5O2.65 is antiferromagnetic with T-N approximate to 480 K. All the antiferromagnetic phases exhibit a second transition at T-B < T-N. 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. (C) 1996 Academic Press, Inc.