Synthesis, crystal chemistry and physical properties of the Ruddlesden–Popper phases Sr 3Fe 2−xNi xO 7−δ (0⩽ x⩽1.0)

Abstract

The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr 3Fe 2− x Ni x O 7− 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 2 p 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 3 d n +1 L configuration with 2 p 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 T σ dominates over the O 2 p bandwith. Although magnetic measurements reveal the presence of ferromagnetic interactions that lead to magnetic frustration at T ⩽ 40 K , 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 T σ 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.