Hyperfine interactions in LaFe1−xNixO3−d, La0.4Sr0.6Co0.2Fe0.8O3−d and La1−xSrxCo1−y−zFeyNizO3−d perovskites

The paper presents results of Mössbauer effect measurements performed at 300 K and 80 K on series of perovskite structured, ABO3 type oxides of the (La–Sr)(Co–Fe–Ni)O3 compositions. The nature of hyperfine interactions (i.e. of dominating electric quadruple or alternatively, magnetic split) within the B sublattice was found to be dependent on which sublattice (A or B) has diverse occupancy. Hyperfine magnetic interactions split the spectra at low temperatures, what enables to distinguish discrete valences of iron ions and show that in the La0.4Sr0.6Co0.2Fe0.8O3−d and LaFe0.8Ni0.2O3−d cathode materials there is a mixed-valence Fe3+/Fe5+ rather than Fe3+/Fe4+ as could be expected. The presence of the mixed valence Fe3+/Fe5+, what seems to be microscopic, electronic reason for high catalytic activity toward oxygen reduction, was attributed to charge disproportionation reaction 2Fe4+ → Fe3+ + Fe5+ caused by substitution of Fe3+ with Ni2+ ions. Spin–spin relaxation phenomena in magnetically ordered state of LaFe0.8Ni0.2O3−d and La0.4Sr0.6Co0.2Fe0.8O3−d samples were observed and the appearance of the magnetic order was attributed to decrease of the relaxation rate of the magnetic moment of iron.