J. Pérez-Cacho

Study of the phase transitions in

has been synthesized under high oxygen pressure from a precursor prepared by means of a sol-gel method. The material obtained is of single phase and has an orthorhombic crystal structure at room temperature. The magnetic, electrical and thermal properties have been measured over a wide temperature range. This compound exhibits a metal-insulator and a paramagnetic-antiferromagnetic transition at 400 K and at 220 K, respectively. The metal-insulator transition is of first order while the magnetic ordering is a second order transition as can be inferred from calorimetric measurements. In addition, we observe an anomaly in the resistivity near the Neel temperature indicating a change in the conduction mechanism at this temperature. Our results support local antiferromagnetic correlations above the Neel temperature for this compound.

Local structure and oxidation state of Mn and Ni atoms in LaNi1-xMnxO3+delta perovskites.

X-ray absorption spectroscopy has been used to study LaNi1-xMnxO3+ perovskites (x = 0.1, 0.25, 0.5, 0.75, 0.9). We have analysed both, the oxidation state and the local structure around the transition metal atoms. XANES spectra of these samples suggest a mixed valence state for both transition metal atoms. The Mn K-edge is shifted to high energies as the Ni content is increased up to x=0.5. The opposite behavior is found instead, for the Ni K-edge. Samples with x=0.5 and x=0.25 present the Mn K -edge at the same energy but the absorption features are different for both samples. The local structure has also been studied by means of the EXAFS technique. The first coordination shell nicely reflects the structural change observed in this series from rhombohedral perovskites (x>0.5) to orthorhombic perovskites (x<0.5). The changes in the local structure for Ni and Mn atoms agree with the variation of the oxidation state observed in the XANES spectra.