Synthesis, crystal chemistry and physical properties of the Ruddlesden–Popper phases Sr3Fe2−xNixO7−δ (0⩽x⩽1.0)

The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr3Fe2−xNixO7−δ with image 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 image sample. The lattice parameters, oxygen content, and average oxidation state of iron and nickel decrease with increasing Ni content. The electronic structure of the image 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 3dn+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 image dominates over the O 2p bandwith. Although magnetic measurements reveal the presence of ferromagnetic interactions that lead to magnetic frustration at image, no long-range magnetic order was observed for the samples with image. The electrical resistivity decreases with increasing Ni content as the p–p band gap tend to close due to the reduction of the image value. Negative magnetoresistance (∼−24% for image and −7% for image at 20 K and 9 T) was observed for the Ni containing samples.