Proprietes magnetiques de l'oxyde de vanadium V3O7

Magnetic susceptibility and magnetization measurements on V₃O₇ from 4,2 to 600 K characterize a paramagnetic behaviour above 18 K : the samples follow a Curie-Weiss law with a Curie constant in good agreement with a spin only value $\text{S}\text{=}\text{1}\text{2}$ per V⁴⁺. At lower temperature the magnetic ordering may be attributed to a metamagnetic behaviour resulting from the layer type structure.     

Etude au microscope electronique des phenomenes de transition de phases dans des monocristaux d'oxyde V2O3 pur et dope au chrome

An investigation by electron microscopy confirms the low temperature transition in single crystal of V₂O₃ and V_2?xCr_xO₃ (x = 0,048). The twin planes were identified in the corundun structure as (1 1 2 0)_M. An explanation is proposed.     

Proprietes magnetiques de l'oxyde Eu2O3 monocristallin

Single crystals of europium oxide Eu₂O₃ with monoclinic B structure were prepared by the Verneuil process. The magnetic properties of these crystals have been studied. Paramagnetic susceptibility measurements have been performed for various orientations of the crystal with respect to the magnetic field at constant temperatures (5,77 and 300 K) as well as at increasing temperatures (5 – 1000 K). They show an anisotropy. An interpretation of this phenomenon is proposed.     

Transition metal-isolant dans l'oxyde V6O11: Etude structurale,magnetique et calorimetrique

Structural, magnetic and thermal properties have been carried out on V₆O₁₁ mixed valence oxide. It has been shown that the metal-insulator transition which occurs at 163 K is associated to a crystallographic modification. On the basis of the already known low temperature structure of V₄O₇ a scheme is proposed in order to describe the M-I transition. It involves both electron localization and pair formation, and agrees well with the magnetic susceptibility data in the paramagnetic region.     

Synthese et etude structurale de l'oxyde double Dy5Re2O12

The unit cell of Dy₅Re₂O₁₂ is monoclinic, space group P2_l/m, with $\text{a}\text{= 12.425(8),}\text{b}\text{= 7.511(5),}\text{c}\text{= 5.653(5)}\text{A}\text{?}\text{, γ = 107.8(2)°,}\text{Z}\text{= 2}$.The crystal structure has been determined from single crystal diffractometer data. However, the presence of a polysynthetic twinning prevents a correct refinement.This structure is built from infinite chains of octaedra ReO₆ along c axis which are bridged along b axis by isolated octaedra DyO₆. These two types of octaedra from sheets along (100) between them are located the other Dy atoms which assume the cohesion of this tridimensionnal network.     

Presence de l'eau dans V2O5 non cristallin

Non cristalline pentoxide V₂O₅ obtained from molten V₂O₅ using quenching method contains a few amount of water. Thermogravimetry, thermal analysis, mass spectrometer, aqueous dissolution demonstrate that it exists a relation between non cristalline structure and water bond. At 190°C, the oxide crystallises at the same time that the water is eliminated.     

Phenomene de croissance en spirale de l'oxyde ReO2 monoclinique

During the study of polymorphic transformation from monoclinic to orthorhombic ReO₂, particular crystal growth of monoclinic oxide has been observed. Electronic microscopy results lead up to conclude that this phenomenon is spiral growth. Original electron diffraction patterns are obtained which show two kinds of polygonal spirals.     

Proprietes electriques et magnetiques de Mn0,25NbS2 et Mn0,33NbS2

Crystals of Mn_0.25NbS₂ and Mn_0.33NbS₂ phases were grown by iodine vapor transport. Their accurate composition has been determined and single phase specimens used for studies of magnetic and electrical properties. Both compounds show ferromagnetic interactions at low temperature. Curie paramagnetic temperatures are 120°K for Mn_0.25NbS₂ and 53°K for Mn_0.33NbS₂. The experimental magnetic moments indicate an intermediate valence state (between +2 and +3) for manganese in the compounds. Resistivity and Hall measurements show a metallic character and coincide with the intermediate valence state. Interpretations are given in terms of a simple band model, and an analysis of the magnetic interactions is proposed.     

Un oxyde de rhenium en relation de structure avec V2O5

The authors describe a rhenium oxide, Re₂O₅. It was revealed and investigated by means of transmission electron microscopy and single diffraction. The orthorhombic unic cell is related to the ReO₃ cubic cell in terms of: b=c=a_c= 3,747 A and a= 2 $\text{a}{}_{\mathrm{c}}\text{√2= 10,598}\text{A}\text{?}$. The crystal structure is explained from patterns, according to the structural scheme of Me₂O₅ and particularly to vanadium pentoxide structure. It is obvious that there exists a close relationship between them.     

Distribution du fluor et de l'oxygene dans l'oxyfluorotantalate de potassium K2Ta4O9F4

The crystal structure of potassium tantalum oxide fluoride K₂Ta₄O₉F₄ has been previously determined by X ray-diffraction, but this technique is unable to distinguish in the lattice between oxygen and fluorine atoms. From NMR measurements on ¹⁹F it is possible by a Van Vleck second moment approach to locate the F^? ions at the corners of the octahedra, forming infinite Ta-F-Ta … chains along the $\text{c}$ axis.     

Proprietes electriques et optiques de composes sulfures d'etain (II)

Ternary sulphides isolated from the SnS-BaS and SnS-Tl₂S systems have been prepared to examine some basic physical properties. Results on electrical conductivity and optical absorption measurements are reported. For Tl₄SnS₃ α, built up from sheets, electrical conductivity (σ// and σ⊥) on single crystal was carried out and shown a p type semiconductor with electronic gap of 1.13 eV.     

Etude par mesures electriques des correlations entre defauts de structure et potentiel chimique de l'oxygene dans WO3 non-stoechiometrique

The variation of the electrical resistivity of non-stoichiometric WO₃, as a function of oxygen pressure has been studied at 750°C and in the range of 1 atm ? P_O2 ? 10^?16.6 atm, corresponding to 0 ? x ? 0.04.The stair-like log₁₀ρ vs log₁₀ P_O2 curve shows evidence of the existence of seven Magneli type phases in which the conduction mechanism depends on the shear planes rather than on point defects.     

Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

The electrical conductivity and the thermoelectric power of trivalent iron oxide α-Fe₂O₃, studied in the crystallographic plane (001) and along the [001] axis, have shown typical anisotropic behaviour.In the same material, the existence of two electronic transitions at 2.14 and 2.50 eV respectively has been found after optical absorption measurements.At high temperature (T > 1020 K) the electrical conductivity has an essentially intrinsic origin, and is practically independent of the oxygen pressure, while the extrinsic contribution predominates below 720 K.According to the model developped for the interpretation of the above experimental results, the peculiar behaviour of the electrical conductivity results from variations in the mobility of the carriers, depending on the electronic levels they are likely to occupy. These levels were identified as the results of a split in the t_2g orbitals of iron under the influence of the rhombohedral. distortion of the crystal field.Furthermore, the anisotropy of mobility is strongly dependent on the magnetic order below Néel temperature.

Résumé

L'étude de la conductivité électrique et du pouvoir thermoélectrique de l'oxyde de fer Fe₂O₃α, effectuée selon la direction cristallographique [001] et dans le plan (001), a révélé une grande anisotropie de comportement. Sur le même matériau des mesures d'absorption optique ont montré l'existence de deux transitions électroniques, à 2,14 et 2,50 eV respectivement.Prenant en compte l'ensemble de ces résultats un modèle a été proposé pour expliquer les propriétés du matériau à diverses températures et sous différentes pressions partielles d'oxygène.A température élevée (T > 1020 K) la conductivité électrique, pratiquement indépendante de la pression d'oxygène, est à forte dominante intrinsèque. En revanche, à basse température (T < 720 K), la conductivité est d'origine extrinsèque.La diversité des mécanismes de conduction observés peut s'expliquer par l'existence de porteurs dont la mobilité varie suivant les niveaux d'énergie qu'ils occupent. Ces niveaux résultent de l'éclatement des orbitales t_2g du fer dû à la distorsion rhomboédrique du champ cristallin.Les mobilités anisotropes sont fortement affectées par l'ordre magnétique au-dessous de la température de Néel.     

Hydratation et texture fibreuse de V2O5 amorphe

Reversible hydration of amorphous V₂O₅ has been performed in order to study the effect of water upon the properties of the oxide. A drastic change of the ESR spectrum is observed, which turns from anisotropic to isotropic upon hydration. This has been interpreted in terms of a brownian motion which, by analogy with the V₂O₅ gels, is supposed to be due to the fibrous texture of the amorphous oxide.     

Le systeme V1−xFexO2: Proprietes structurales et magnetiques

The system V_1?xFe_xO₂ (x ? 0.020) was investigated by X-ray diffraction, DTA, magnetic susceptibility measurements and Mössbauer spectroscopy. The phase diagram and the magnetic properties are similar to those of the V_1?xCr_xO₂ and V_1?xAl_xO₂ systems previously studied. In oxygen deficient materials appears a Magnéli type shear phase, the Fe³⁺ ions occupying the shear planes.     

Intercalation d'ions alkylammonium et d'alkylamines a longues chaines dans les gels de V2O5

C_nH_2n+1N⁺(CH₃)₃ ions and C_nH_2n+1NH₂ amines with n ranging from 1 to 18 have been intercalated into the lamellar structure of V₂O₅ gels. Alkyl chain orientation in the interfoliar space has been deduced from the basal d-spacing evolution with the chain length : the chains are parallel, then tilted and finally perpendicular to the host lamellae as the chain length increases. Their monolayer disposition is also demonstrated. The large adaptability of the host structure proceeding from its gel state, leads to fast reaction at room temperature, especially with small polar heads such as -NH₂ (less than an hour). Contrary to usual intercalates, due to the gel state, only a mean composition can be defined. It has been found to correspond to 0.3–0.4 intercalated species per V₂O₅ : this is in good agreement with the upper limit (0.5) deduced from sterical considerations based upon a full filling of the interfoliar space. Thermal decomposition of alkyl chains restricts the stability of these intercalates to 160°C. More likely of the cationic exchange type with trimethylammonium ions, intercalation mechanism appears more complex with amines, involving probably also a host-guest charge transfer.     

Contribution a l'etude structurale des phases V1-xCrxO2

The authors have determined the phase diagram of the system V_1?xCr_xO₂ (0 ? x ? 0.15) by magnetic susceptibility, D. T. A. and X-ray diffraction measurements. At small chromium concentrations the existence of the intermediate phases M₂ and M₃, between M₁ and the rutile phase, has been confirmed. But it has been found that M₃ has actually a triclinic symmetry without destruction of the V-V pairs already existing in M₁. At higher concentrations (x > 0.07) there appears a monoclinic M₄ phase in which the V - V pairs are all destroyed.     

Polymorphisme de l'hexahydroaluminate trisodique Na3AlH6

Structural investigations on the cryolite-like compound Na₃AlH₆ have been carried out using high-temperature diffractometry. α-Na₃AlH₆ which crystallises in a monoclinic pseudo-cubic system under room-temperature conditions exhibits at 525 K a polymorphic transition into a f.c.c. form β-Na₃AlH₆. This result has been established by high-pressure-high-temperature studies. From structural point of view Na₃AlH₆-β is to be related to the high temperature modification of cryolite Na₃AlF₆     

Etude in situ par spectrométrie vibrationnelle de couches minces electrochromes d'oxyde de tungstène WO3 et d'oxyde de molybdene MoO3

Raman spectroscopy, internal and specular I.R. reflection spectroscopies have been applied to WO₃ and MoO₃ thin films (～ 5,000 A). These films are amorphous and transform to crystalline materials at 400°C. Structural similarities between WO₃ films and the new hexagonal form of WO₃ may explain the electrochromic properties of the former.