Influence of cation vacancies on structural and magnetic properties of La_1-xMnO_3+δ nanopowders

La_1-xMnO_3+δ (x=0-0.3) nanopowders were prepared at low temperature (825 °C) from a sol-gel method. As results of chemical analyses and X-ray diffraction of products, two ranges of La/Mn ratios can be distinguished: for La/Mn<0.9, a phase separation into La_0.9MnO₃ and Mn₃O₄ occurs while single phases with vacancies on both La and Mn sites are obtained for La/Mn>0.9. La and Mn vacancy concentrations were determined from combined Rietveld refinements of X-ray and neutron diffraction data. Correlations between these structural characteristics and magnetic properties were made. Curie temperatures remain constant at a very high value of 295 K for La/Mn<0.9, while they decrease for increasing ratios of La/Mn>0.9. The decrease of Curie temperatures can be related to an increasing amount of Mn vacancies up to a limit corresponding to a La_0.93Mn_0.93O₃ compound with La/Mn=1. For this compound a cluster-glass behaviour is observed. Finally, neutron diffraction was performed as a function of temperature in order to determine the nature of the magnetic interactions.