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Se han conseguido preparar monocristales de 10 oxidos mixtos de la serie LnMn2O5 (Ln= Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho y Er) mediante sintesis hidrotermal optimizada, en un unico paso y sin tratamientos termicos posteriores partiendo de las correspondientes sales metalicas en disolucion acuosa a 240 oC. Los monocristales obtenidos son relativamente grandes, de varias micras y su morfologia varia a lo largo de la serie: es poligonal en el caso de los compuestos de los lantanidos del inicio de la serie (Ln= Pr, Nd, Sm, Eu y Gd) y acicular en el caso de los compuestos de Y y de los lantanidos del final de la serie (Ln= Tb, Dy, Ho y Er). Tras el analisis de los productos obtenidos empleando distintas condiciones de sintesis atribuimos la diferente morfologia a una mayor velocidad de crecimiento cristalino a lo largo del eje c cuando intervienen los iones mas pequenos (Y, Tb, Dy, Ho y Er) debido a la mejor adaptacion de estos ultimos a la estructura cristalina del compuesto.

In this work, we study the dielectric properties of the layered perovskites La 3/2 Sr 1/2 NiO 4 and La 5/3 Sr 1/3 NiO 4 in the frequency range 20 Hz-1 MHz and in the temperature range 110-350 K. For this study, we have synthesized single phase polycrystalline samples by a conventional solid-state reaction. These nickelates show very high values of the dielectric constant at room temperature (e' r ∼ 10 5 ), that make them very attractive for potential applications, provided that their rather high losses can be minimized. The obtained results reveal that even if their dielectric behavior is strongly enhanced by extrinsic polarization effects, there is also a very interesting correlation between their dielectric response and their electronic state.

In this work we have prepared, by a sol-gel method, LiCo 1-x M x O 2 compounds (M= Ni, Al and Mg), in order to study the doping effect in their electrochemical behaviour as cathodes in lithium-batteries. We have studied the influence of the synthesis conditions (using various chelating agents for the formation of the gel) on their morphologic, structural and electrochemical properties. We have obtained monophasic materials: LiCo 1-x Ni x O 2 (0≤x≤0.8), LiCo 1-x Mg x O 2 (0≤x≤0.05), LiCo 1-x Al x O 2 (0≤x≤0.3) and LiCo 0.5 Ni 0.5-x Al x O 2 (0≤x≤0.3). In general, the samples obtained with succinic acid have better ordered lithium layers than malic samples.The capacity of the Li//LiCo 1-x M x O 2 batteries decrease upon doping. However, more stable charge-discharge cycling performances have been obtained as compared to those displayed by the native oxides. In LiCo 1-x Mg x O 2 , small amounts of MgO appear as secondary phases for 0.05

By using the nitrate decomposition method in the presence of KNO 3 , we have been able to enlarge the compositional range of the cobalt-layered compounds Ln 1-x Sr 1+x CoO 4 (Ln: La, Nd, Gd) to 0≤x≤0.40 in the case of lanthanum, to 0≤x≤0.30 for neodymium, and to 0≤x≤0.20 in the case of gadolinium. In this work we present the results of their structural characterization by means of powder X-ray diffraction (PXD) and Rietveld analysis as a function of composition (x) and the rare earth (Ln), information that has been complemented by infrared spectroscopy (IR), electron diffraction (ED) and electron microscopy.

In this work, the behaviour of the real and imaginary components of the dielectric permittivity of layered perovskite La 1 5 Sr 0 . 5 CoO 4 have been studied, in the frequency range 20 Hz-1 MHz and in the temperature range 115-350 K. Polycrystalline samples were prepared by the ceramic method. The powder diffraction results showed the pattern of the desired phase together with a small quantities of La 2 O 3 impurities. A comparative study of the electrical and magnetic properties of this charge-ordered compound (T C O ∼750 K) has revealed a basically semiconductor behaviour with a complex bidimensional antiferromagnetism which can be explained on the basis of a thermally activated evolution of the spin state of the Co 3 + ions. A detailed investigation of the dielectric properties of this system has allowed us to identify two superimposed relaxation processes: the first one in the low frequency range (f