LILIANA MOGNI, FERNANDO PRADO, HUGO ASCOLANI, MIGUEL ABBATE, MARIO S. MORENO, ARUMUGAM MANTHIRAM AND ALBERTO CANEIRO

JOURNAL OF SOLID STATE CHEMISTRY

Elsevier

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 (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 Ts 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 gaptend to close due to the reduction of the Ts value. Negative magnetoresistance (~-24% for x = 0:6 and -7% for x = 1:0 at 20K and 9T) was observed for the Ni containing samples.3Fe2-xNixO7-d 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 Ts 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 gaptend to close due to the reduction of the Ts value. Negative magnetoresistance (~-24% for x = 0:6 and -7% for x = 1:0 at 20K and 9T) was observed for the Ni containing samples.