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X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel–copper spinel manganites

2000; Elsevier BV; Volume: 2; Issue: 5 Linguagem: Inglês

10.1016/s1466-6049(00)00047-7

1878-5689

Christophe Drouet, Christel Laberty, J.L.G. Fierro, Pierre Alphonse, André Rousset,

Ferroelectric and Piezoelectric Materials

The surface of non-stoichiometric nickel and nickel–copper spinel manganites has been investigated by X-ray Photoelectron Spectroscopy (XPS). The oxidation states of the nickel, copper and manganese cations present on the surface of the samples were determined from the analysis of the M 2p3/2 core levels (M=Ni, Cu, Mn). In particular, both Cu2+ and Cu+ were evidenced in the structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards lower binding energy in the Ni 2p3/2 region, which was explained by the displacement of some Ni2+ cations from tetrahedral to octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni/Mn for nickel manganites, Ni/(Mn+Cu) and Cu/(Mn+Ni) for nickel–copper manganites, determined from XPS data, were compared to the ratios corresponding to the bulk composition. This study shows in all cases a nickel enrichment at the surface which is not affected by the copper content of the oxide. On the contrary, the ratio Cu/(Mn+Ni) was found to be lower than the corresponding bulk value.