Charge disproportionation in iron(IV) oxides:: electronic properties and magnetism in Sr3Fe2-xTixO7-y annealed at high oxygen pressures

It has been studied how partial substitution of formally 3d(4) Fe-IV ions by 3d(0) Ti-IV ions influences the charge-disproportionation state and magnetism in Sr3Fe2O7. For this purpose Ruddlesden-Popper-type phases Sr3Fe2-xTixO7-y have been synthesized from the oxides and, in order to reach high oxygen contents, annealed at oxygen pressures up to 70 MPa. The materials were investigated by X-ray powder diffractometry, Fe-57 Mossbauer spectroscopy, magnetic susceptibility, and electrical resistance measurements. From the a-lattice parameters it is derived that in spite of annealing the samples at high oxygen pressures a certain oxygen deficiency occurs which gives rise to an Fe-III fraction increasing with x. Mossbauer spectra evidence a charge disproportionation of Fe-IV in materials with x = 0.2 and 0.5, whereas for x = 1.0 and 1.5 a mixed-valence state is formed with an Fe-III/Fe-IV ratio as derived from sample composition. The isomer shifts and magnetic hyperfine fields suggest a collective electronic state with partial transfer of charge and spin density between the Fe sites in the charge-disproportionation phases. In the samples with x = 0.2 and 0.5 the degree of charge and spin separation is increased in comparison with Sr3Fe2O7. The electrical conductivity decreases with increasing x. The magnetism of all the materials is governed by ferro- and antiferro-magnetic exchange interactions which lead for decreasing temperature to a broad transition to an antiferromagnetic state for x = 0.2 and to spin-glass behavior for materials with larger x.