Oxygen deficiency in Sr 2 FeO 4- x : electrochemical control and impact on magnetic properties.

The oxygen-deficient system Sr 2 FeO 4- x was explored by heating the stoichiometric Fe 4+ oxide Sr 2 FeO 4 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 K 2 NiF 4 -type crystal structure (space group I 4/ 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 Sr 3 Fe 2 O 7- x and suggests removal of oxygen atoms from FeO 2 lattice planes. The magnetic properties were studied by magnetization, 57 Fe 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.