Synthesis, crystal chemistry and physical properties of the Ruddlesden-Popper phases Sr3Fe2-xNi xO7-δ (0≤x≤1.0)

Abstract

The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr3Fe 2-xNixO7-δ with 0≤x≤1.0 have been studied. X-ray diffraction and selected area electron diffraction (ED) data indicate that the samples have a tetragonal (Space group I4/mmm) structure. ED patterns and high-resolution images reveal the presence of a regular stacking along the c-axis for the x=1.0 sample. The lattice parameters, oxygen content, and average oxidation state of iron and nickel decrease with increasing Ni content. The electronic structure of the x=1.0 sample was studied by M 2p X-ray photoelectron spectroscopy (XPS). An analysis of the spectra using the cluster model indicates that this material is in the negative charge-transfer regime and the ground state is dominated by the 3d n+1L configuration with 2p holes (L) in the oxygen band. The insulator states are stabilized due to a p-p type band gap that arises because the p-d transfer integral Tσ dominates over the O 2p bandwith. Although magnetic measurements reveal the presence of ferromagnetic interactions that lead to magnetic frustration at T≤40K, no long-range magnetic order was observed for the samples with x≥0.3. The electrical resistivity decreases with increasing Ni content as the p-p band gap tend to close due to the reduction of the Tσ value. Negative magnetoresistance (∼-24% for x=0.6 and -7% for x=1.0 at 20 K and 9 T) was observed for the Ni containing samples. © 2005 Elsevier Inc. All rights reserved.