Synthesis, Crystal Chemistry and Physical Properties of the Ruddlesden-Popper Phases Sr3Fe2-xNixO7-d (0< x <1.0).

L. MOGNI; F. PRADO; H. ASCOLANI; M. ABBATE; M.S. MORENO; A. MANTHIRAM; A. CANEIRO

JOURNAL OF SOLID STATE CHEMISTRY

ElSEVIER B. V.

Año: 2005 vol. 178 p. 1559 - 1568

0022-4596

The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr3Fe2-xNixO7-d with 0 £ x £1.0 have been studied. X-ray diffraction and selected area electron diffraction data indicate that the samples have a tetragonal (Space group I4/mmm) structure. Electron diffraction 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 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 Ts dominates over the O 2p 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 ³ 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 Ts value. Negative magnetoresistance (~ -24% for x = 0.6 and –7% for x = 1.0 at 20 K and 9T) was observed for the Ni containing samples.