Oxygen deficiency in Sr₂FeO_4-: electrochemical control and impact on magnetic properties

The oxygen-deficient system Sr₂FeO_4- was explored by heating the stoichiometric Fe⁴⁺ oxide Sr₂FeO₄ in well-defined oxygen partial pressures which were controlled electrochemically by solid-state electrolyte coulometry. Samples with x up to about 0.2 were obtained by this route. X-ray diffraction analysis reveals that the K₂NiF₄-type crystal structure (space group I4/mmm) of the parent compound is retained. The lattice parameter a slightly decreases while the c-parameter increases with increasing x, which is in contrast to the Ruddlesden-Popper system Sr₃Fe₂O_7- and suggests removal of oxygen atoms from FeO₂ lattice planes. The magnetic properties were studied by magnetization, ⁵⁷Fe Mössbauer, and powder neutron diffraction experiments. The results suggest that extraction of oxygen atoms from the lattice progressively changes the elliptical spiral spin ordering of the parent compound to an inhomogeneous magnetic state with coexistence of long-range ordered regions adopting a circular spin spiral and smaller magnetic clusters.