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

Abstract

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 Sr ₃ Fe ₂ O ₇ . For this purpose Ruddlesden-Popper-type phases Sr ₃ Fe _2–x Ti _x O _7–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, ⁵⁷ Fe Mössbauer 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. Mössbauer 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 Sr ₃ Fe ₂ O ₇ . 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.