Local electronic and geometrical structure of LaNi1-xMnxO3+δ perovskites determined by x-ray-absorption spectroscopy -: art. no. 144409

Local and electronic structures of Ni and Mn atoms in the LaNi1-xMnxO3+delta series were studied by means of x-ray-absorption spectroscopy. The Ni and Mn K edges and Mn L-3/L-2 edges were analyzed. The local structure, determined by extended x-ray-absorption fine-structure spectroscopy, shows a distorted MnO6 octahedron in LaMnO3. This distortion decreases when Ni replaces Mn in the unit cell. In samples with the same Mn/Ni ratio, the octahedron distortion is smaller in oxidized samples. This result shows that the holes induced by oxidation belong mainly to the Mn sublattice. In the LaNi1-xMnxO3+delta series, a contraction of the MnO6 octahedron is coupled to an expansion of the NiO6 octahedron. This result is well correlated with the changes in the oxidation states deduced from x-ray-absorption near-edge spectroscopy (XANES). The Mn valence state continuously changes from the formal 3+ state in LaMnO3 to nearly a 4+ state in LaNi0.5Mn0.5O3. The Ni valence state instead shifts from Ni3+ in LaNiO3 to Ni2+ in LaNi0.5Mn0.5O3. Therefore, Ni2+ and Mn4+ are present in LaNi0.5Mn0.5O3+delta. Moreover, the detailed analysis of the XANES spectra points to an important mixing between the 3d orbitals of both cations and the 2p orbitals of oxygen atoms.