F. Jeannot

Sur l'existence des oxydes rhomboe driques A(III) B(II) B′(III)O4

Abstract The symmetry group of the rhombohedral structure of the mixed oxides A(III)B(II)B′(III)O4 is R3m: A is in an octahedral coordination, B and B′ occupy the same crystallographic sites and are in a pyramidal coordination with a triangular basis. The existence of this structural type with Ni2+ and Cr3+ ions is incompatible with this coordination close to tetrahedral. The geometric conditions necessary for adoption of the rhombohedral structure have been clearly defined by comparing the cation-oxygen and oxygen-oxygen distances determined on 4 monocrystals (InCuAlO4, InFe1, 75SiO, 25O4, YbFe2O4 and YbO,5EuO,5Fe2O4). The shortest cation-oxygen distance in the pyramidal site is designated by d′: it can be calculated through the relation: d ′ = 1 2 ( r B 2+ + r B ′ 3+ ) + r O 2− by using the ionic radii of ions in tetrahedral coordination, given by R.D. Shannon. The oxygen - A3+ cation distance of the octahedron, d, is close to the sum of the ionic radii of the ions in coordinence 6. Mixed oxides ABB′O4 can crystallize in the rhombohedral system if the ratio d′/d is between 0.842 and 0.888 and these two limiting values can be refined for each couple B, B′.

Pre´paration et structure d'un polyphosphure de magnesium: MgP4

Abstract Ce compose´est obtenu par action de la vapeur de phosphore sur Mg3P2a`600°C en ampoule scelle´e. La microdiffractione´lectronique met ene´vidence une maille monoclinique primitive; un affinement par les rayons X sur diagramme de poudre conduit aux positions atomiques; il y a isotypie aver CdP4.

The MoO3WO3 and Li2OMoO3WO3 systems

Abstract The oxidation at moderate temperatures of solid solutions of the dioxides (Mo,W)O2 permits a determination of the system MoO3WO3. A monophasic region, Mo1−xWxO3 (0.35

Additional experiments on and interpretation of the role of potassium in the oxygen exchange between wustite and a carbon monoxide-carbon dioxide atmosphere

We have previously shown that, in the redox interaction between wustite and a CO-CO2 atmosphere, K+ accelerates the oxygen exchange in both directions. In view of the numerous interpretations suggested by previous workers as regards the role of potassium in the reduction of iron oxides and ores, the action of K+, Ba2+ and Li+ has been compared in order to establish the role of size and charge of the additive ions. Dealing with small wustite single crystals at 890 °C it was established that Ba2+ is nearly as active as K+, whereas Li+ is inactive (but this may also be due to its volatility). Two possible mechanisms are proposed to explain the potassium activity: it operates as a catalyst through the formation of a kind of activated complex, and/or it increases the oxygen ion mobility.

Synthese et caracterisation d'une wustite tres oxydee stabilisee par des traces de calcium

By thermal decomposition of CaFe5O7 at 1125 °C in a sealed silica tube during about ten days, we obtained the oxide Fe0.832Ca0.028O1 referred to as “Fe6O7”. Like wustite, it is iron deficient, very rich in iron (III) cations (Fe2+Fe3+ = 1.97), has the rock-salt structure (a = 429.4 pm), but the x-ray diffraction pattern does not show satellite reflections around the Bragg peaks. Like wustite, “Fe6O7” is antiferromagnetic (TN = 207 K), but the room temperature Mossbauer spectrum is different. It seems that this phase is stabilized by traces of calcium and that there is no tetrahedral interstitial iron (III).

On the possibility of a small amount of V2+ in spinels near FeV2O4

Abstract Samples of overall composition FeV 2 O 4 have been prepared at 1100°C, with Po 2 varying between 10 −14.25 and 10 −8.30 bar, and studied by quantitative X-ray diffraction, Mossbauer effect, magnetization measurements, absorption and emission spectroscopies. It is confirmed that FeV 2 O 4 does not exist in these conditions and corresponds to a mixture of V 2 O 3 and a spinel phase. From experimental and thermodynamic arguments, it is proposed that a small amount of octahedral vanadium is divalent, whereas the octahedral iron is mixed-valent with an oxidation number near to 3. It appears that nominal FeV 2 O 4 is unstable against oxidation of partially filled V 3+ 2+ band and that the Fermi energy of the stable spinel phase intersects three overlapping redox couples : the Fe 3+ 2+ couple on both tetrahedral and octahedral sites and the V 3+ 2+ couple on octahedral sites.

Kinetics of the cracking reduction of hematite

The cracking of hematite crystals during their low-temperature reduction to magnetite raises the question of the analysis of sigmoidal kinetic curves after an induction period which may be prolonged. We have recorded such curves with CO/CO2 = 2080 from 350 to 600 °C, and used different models: - the classical Shrinking Core Model which should apply only at 600 ° C and not below, - the Crackling Core Model, except for at the beginning of reaction. Surprisingly these models each yield values for the rate constant and its activation energy which are in reasonable agreement.