The recent discovery of colossal magnetoresistance [1] in mixed oxides of manganese has stimulated the research of this property in related compounds. Ferromagnetic oxides with Mn in a mixed-valence state seem to be good candidates to show magnetoresistance. LaM1-xMnxO3 (M = Ni, Co) compounds are ferromagnets. Some authors explained their magnetic ordering in terms of interactions between Ni2+(Co2+) and Mn4+. Other authors claim instead for a homovalent substitution between Mn3+ and Ni3+ and ascribed these differences to synthetic details. Moreover a recent spectroscopic study on LaMn1-xCoxO3 compounds suggests a mixture of Mn3+ and Mn4+.

We have performed an X-ray-absorption spectroscopy study of LaNi1-xMnxO3+ series at Mn and Ni K-edges in order to solve the controversy about the electronic state of the transition metals. The measurements were carried out at beamlines ID26 and BM29. The aim of this work is to extract the local geometric structure around each transition metal atom by EXAFS and to study the electronic state and local geometry by XANES. Samples were prepared from different synthetic routes [2] to check their effect on the local and electronic structures.


Fig. 49: Modulus of the room temperature Fourier transform of the Mn and Ni K-edge EXAFS spectra for selected LaNi1-xMnxO3+ samples.

The local structure, determined by EXAFS, shows a tetragonal distorted MnO6 octahedron in LaMnO3. The replacement of Mn by Ni decreases both the tetragonal MnO6 distortion and the average Mn-O distance. This can be seen in Figure 49 (Mn K-edge), where the first peak (Mn-O shell) of the Fourier Transform (FT) for the x = 0.5 sample is higher than for LaMnO3. Samples (x 0.5) with the same Mn/Ni ratio and different oxygen content were also studied. The octahedron distortion is smaller in oxidised samples suggesting an increase in holes (oxidation) in the Mn sublattice. Opposite effects were observed in the local structure around the Ni atom. EXAFS shows that the Ni-O distance increases as the Mn content does. Figure 49 (Ni K-edge) displays the shift of the FT first peak toward higher values for the x = 0.5 sample. No changes were observed in the Ni environment in oxidised samples. Therefore, 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 XANES. The Mn valence state continuously changes from the formal 3+ state in LaMnO3 to a nearly 4+ state in LaNi0.5Mn0.5O3. Thus, the energy position of the absorption edge for LaNi0.5Mn0.5O3 is close to the position for CaMnO3 with formal 4+ state (Figure 50). The Ni valence state instead shifts from Ni3+ in LaNiO3 to Ni2+ in LaNi0.5Mn0.5O3 (inset of Figure 50). Therefore, Ni2+ and Mn4+ are the most suitable ionic approximation for LaNi0.5Mn0.5O3+.


Fig. 50: Normalised Mn K-edge XANES spectra of LaNi1-xMnxO3+ (x = 1 and 0.5) samples compared to CaMnO3. Inset: Normalised Ni K-edge XANES spectra of the LaNi0.5Mn0.5O3.08 sample compared to LaNiO3.

In conclusion, the solid solution cannot be considered as homovalent. The addition of Ni3+ to LaMnO3 would produce an oxidation of the Mn sublattice (coupled to the reduction of Ni3+). In the same way, the incorporation of Mn3+ into LaNiO3 would lead to a reduction of Ni3+ (and to an oxidation of Mn3+). Moreover, these results do not seem to depend on synthetic details. Oxygen excess is accommodated as cationic vacancies implying an oxidation of the Mn sublattice.

[1] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys., 48, 167-293 (1999).
[2] J. Blasco, M.C. Sánchez, J. Pérez-Cacho, J. García, G. Subías, J. Campo, J. Phys. Chem. Solids, 9, 781- 792 (2002).

Principal publication and Authors
M.C. Sánchez (a), J. García (a), J. Blasco (a), G. Subías (b), J. Perez-Cacho (a), Phys. Rev. B 65, 1444091-1444099 (2002).
(a) ICMA-Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, Zaragoza (Spain)
(b) ESRF