Synthesis and characterization of the K2NiF4 phases La1+xSr1-xCo0.5Fe0.5O4-δ (x=0, 0.2)

The K2NiF4 phases LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, and their reduced forms LaSrCo0.5Fe0.5O3.75 and La1.2Sr0.8Co0.5Fe0.5O3.85, have been successfully prepared by solid-state reactions, followed by reduction in 10% H2/N2 in order to produce oxygen-deficient materials. All materials crystallize in a tetragonal K2NiF4 structure (space group I4/mmm) with Co and Fe randomly distributed over the B-sites of the structure. Moessbauer spectra have confirmed the trivalent state of Fe in these materials. In the reduced materials, oxide ion vacancies are confined to the equatorial planes of the K2NiF4 structure and the Co is present almost entirely as Co2+ ions; low-temperature neutron powder diffraction data reveal that these reduced phases are antiferromagnetically ordered with a tetragonal noncollinear arrangement of the moments. The Co3+ ions, present in stoichiometric LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, inhibit magnetic order and are assumed to be in the low-spin state. - Graphical abstract: The reduced K2NiF4 phases LaSrCo0.5Fe0.5O3.75 and La1.2Sr0.8Co0.5Fe0.5O3.85 accommodate disordered oxide ion vacancies confined to equatorial planes of the structure. Magnetic exchange results in AFM order at low temperature, which can be represented by a tetragonal noncollinear model for the moments. Co3+ ions, present in stoichiometric LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, inhibit magnetic order and are assumed to be in the low-spin state