Cation Distribution in a Titanium Ferrite Fe2.75Ti0.25O4Measured byin-SituAnomalous Powder Diffraction Using Rietveld Refinement

1998

Many ferrites contain different cations with various valence states and location in the spinel structure. In compounds such as these, only a combination of different techniques such as Mussbauer spectroscopy, IR analysis, and thermogravimetry allows the distribution of cations to be obtained. For very complicated distributions, the mathematical decomposition of derivative thermogravimetric curves (DTG) leading to quantitative distribution is uncertain. In this paper, we present an alternative technique based on resonant diffraction. The anomalous scattering of each cation in the crystalline material is used to determine its amount and position by Rietveld refinement. Since the energy for su…

Synthesis and Stability Region of Stoichiometric Nanocrystalline Vanadium−Iron Spinel Powders

2000

Vanadium-iron spinels with nanometric sizes have been synthesized by a soft chemistry route. This way of elaboration consists of a coprecipitation followed by thermal treatments at low temperatures. The last thermal annealing is performed under a reducing atmosphere in order to obtain the exact oxygen stoichiometry (four oxygen atoms for three metal atoms in AB 2 O 4 ). Because of the low temperatures used, very low oxygen partial pressures (between 10 -20 and 10 -30 Pa) have to be applied. For this purpose, a reducing setup has been used. It consists of H 2 /N 2 /H 2 O gas mixtures, creating a reducing atmosphere with low oxygen partial pressure. This atmosphere can be applied to a sample …

Experimental set up for determining the temperature-oxygen partial pressure conditions during synthesis of spinel oxide nanoparticles

1997

Abstract Nanometric spinel oxide powders, Fe 3− x M x O 4 , where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe 3− x M x O 4+δ where δ is the deviation from stoichiometry so that further annealing at low temperatures around 450°C and low oxygen partial pressure around 10 −25 Pa given by N 2 /H 2 /H 2 O gas mixtures is required: this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial p…

Cation Distribution in Ferrites with Spinel Structure Measured by Anomalous Powder Diffraction

1998

Utilisation de la diffraction résonnante pour déterminer la distribution cationique d'un ferrite de titane nanométrique

1998

Les proprietes magnetiques des ferrites de structure spinelle dependent, entre autres, de la distribution cationique dans la maille. La connaissance de cette repartition a pu etre obtenue dans le cas d'un ferrite substitue au titane en utilisant la diffraction anomale de poudre couplee avec un affinement structural de type Rietveld, methode qui est le fruit des dernieres avancees effectuees autour du rayonnement synchrotron.

Réactivité vis-à-vis de l'oxygène de spinelles de fer-vanadium de taille nanométrique et distribution cationique

1997

Resume L'etat de division des spinelles de fer-vanadium nanometriques V x Fe 3− x O 4 (0 ⩽ x ⩽ 2 ) permet d'oxyder au sein-meme de la structure spinelle les ions du fer et du vanadium. Les analyses calorimetriques et thermogravimetriques, ainsi que la spectroscopie infrarouge, montrent que les ions Fe B 2+ ,V B 3+ et Fe A 2+ sont oxydes successivement en ions Fe 3+ et V 5+ a des temperatures inferieures a 450 °C. Les spinelles lacunaires a valence mixte qui en resultent, ont un taux en lacunes croissant avec x, qui peut etre superieur a ceux determines jusqu'alors pour ce type d'oxydes.