Electronic structure of CaMn1-xNbxO3 (x=0.02, 0.04, 0.06 and 0.08) perovskites. Self-organization of terminal layers.

Electronic structure of niobium doped CaMn 1- x Nb x O 3 (x = 0.02, 0.04, 0.06 and 0.08) powder and fresh chip manganites was studied by x-ray absorption near edge spectroscopy (XANES) at Mn K -edge, and by x-ray photoelectron spectroscopy (XPS) of Mn 3 s -, Mn 2 p -, O 1 s -, Ca 2 p - and Nb 3 d -core levels. These XPS spectra for all the studied perovskites are very similar. The O 1 s and Ca 2 p spectra for powders differ from those for fresh chips because of the quick oxidation and carbonization of powder surface. The Mn 3 s , Mn 2 p and Nb 3 d spectra for the powders and fresh chips are almost the same. According to Mn 3 s spectra the formal valence of surface manganese ions is 3.82 for all studied oxides indicating the presence of oxygen vacancies in the samples. Niobium is in pentavalent state as evidenced by XPS measurements, but the negative chemical shift of the Nb 3 d 5 / 2 core level with respect to Nb 2 O 5 oxide is observed. High similarity of the XPS spectra of the samples with different Nb doping is probably a result of the self-organization of terminal layers. • Niobium doping decreases manganese valence and introduces oxygen vacancies. • Pentavalent state of niobium is evidenced by position and shape of Nb 3 d lines. • Negative chemical shift of Nb 3 d 5 / 2 line with respect to Nb 2 O 5 oxide is observed. • Effect of oxidation and carbonization of surface is underlined. • Self-organization of terminal layers is revealed. [ABSTRACT FROM AUTHOR]