Nanosized Sodium-Doped Lanthanum Manganites: Role of the Synthetic Route on their Physical Properties

In this paper we present the results of the synthesis and characterisation of nanocrystalline La1-xNaxMnO3+delta samples. Two synthetic routes were employed: polyacrylamide-based sol-gel and propellant synthesis. Pure, single phase materials were obtained with grain size around 35 nm for the sol-gel samples and around 55 nm for the propellant ones, which moreover present a more broaden grain size distribution. For both series a superparamagnetic behaviour was evidenced by means of magnetisation and EPR measurements with peculiar features ascribable to the different grain sizes and morphology. Preliminary magnetoresistivity measurements show enhanced low-field (< 1 T) magnetoresistance values which suggest an interesting applicative use of these manganites.Comment: 31 Pages 10 Figures to appear in Chem. Mate

Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure

Apport de l’EXAFS à l’étude de l’effet alcalin mixte dans les verres silicates

Nous avons enregistré des spectres de photoabsorption sur des verres appartenant au système ternaire SIO2 - Cs2O - Rb2O au voisinage des seuils de la raie LIII du césium (5012 eV) et de la rale K du rubidium (15200 eV). La comparaison des spectres EXAFS décrit la variation de l’environnement de l’ion alcalin lorsqu’on remplace progressivement un Ion alcalin par un autre

Frittage de YBaCuO, implications du diagramme de phases

The motivations of this experimental work are to underline the implications between the phases diagram constitution and the sintering of YBaCuO superconductors. This preliminary work is focussed on the solid → liquid transformations of this system, in the vicinity of the (123) phase. Two transformations are observed at 915 and 935 °C depending of the composition of the compound. They both have an important role on the sintering process and the chemical homogeneity of the ceramic. No such transformations seems to occur in the domain (123)-(211)-BaCuO2, the sintered sample has therefore a better chemical homogeneity.Les motivations de ce travail sont d'examiner les implications du diagramme de phases du système pseudo-ternaire Y2O3-CuO-BaO sur le frittage de composés proches de la composition supraconductrice YBa 2Cu3O7-p. Dans ce travail préliminaire, on s'est intéressé à mettre en évidence les transformations solide → liquide de ce système, au voisinage de la composition (123). Deux transformations apparaissent à 915 et 935 °C, selon la composition du produit, elles jouent un rôle important sur le frittage et l'homogénéité chimique du produit fritté. Il ne semble pas y avoir de transformations liquides dans le domaine (123)-(211)-BaCuO2, il en résulte une homogénéité chimique plus grande des échantillons frittés

Local Order in LiNbO 3 , in the Paraelectric and Molten States

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