Local electronic and geometrical structure of LaNi1-xMnxO3+Φ perovskites determined by x-ray-absorption spectroscopy

Local and electronic structures of Ni and Mn atoms in the LaNi 1 - x Mn x O 3 + Φ 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 MnO 6 octahedron in LaMnO 3 . 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 LaNi 1 - x Mn x O 3 + Φ series, a contraction of the MnO 6 octahedron is coupled to an expansion of the NiO 6 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 LaMnO 3 to nearly a 4+ state in LaNi 0 . 5 Mn 0 . 5 O 3 . The Ni valence state instead shifts from Ni 3 + in LaNiO 3 to Ni 2 + in LaNi 0 . 5 Mn 0 . 5 O 3 . Therefore, Ni 2 + and Mn 4 + are present in LaNi 0 . 5 Mn 0 . 5 O 3 + Φ . 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.