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.     

Kinetic study of the sulfurisation of cuprous halides in hydrogen sulfide. Cuprous bromide and iodide

The sulfurisation of cuprous bromides and iodides in hydrogen sulfide, at low pressure, leads to the formation of a porous chalcosine layer, as already investigated in the case of chlorides. The step which fixes the conversion rate is the diffusion of the gaseous reagents in the pores of the sulfide formed. Experimental activation energies are 15 and 34 kcal.Mole^?1 respectively, at every step of the reaction.     

Défauts et transitions de phases structurales

The paper reviews the main theoretical and experimental results on the influence of defects on the phenomena occurring in the vicinity of a crystalline phase transition, and also on the behaviour of incommensurate systems. The author examines first, the experimental data pertaining to the critical exponents of structural phase transitions. It is found that while these data are remarkably consistent with the theory for the transitions which are expected to possess a classicalbehaviour, there is a systematic, and important, disagreement with the theory for non-classicaltransitions. The trend of the deviations is the same as if fluctuations had an enhanced role in the transition. In relation with this experimental situation, he reviews theoretical works on the influence of defects on the critical behaviour. He emphasizes that the available theoretical results are qualitatively compatible with the assumption that the experimentally observed behaviour is in great part determined by defects of the asymmetric type.The specific situation of incommensurate systems is then considered (the correlation length associated with the phase of the modulation is infinite in the range of existence of the incommensurate phase). The properties of the puresystem are recalled and the crucial role of defects is justified. The main phenomena expected,on theoretical grounds, from the interaction between modulation and defects are enumerated. He notes that to a large extent, these phenomena are similar to the ones expected from the random pinning of the modulation by the underlying periodic lattice. Finally he reviews the experimental observations, in incommensurate systems, which have been related to the presence of defects.     

Mössbauer resonance studies on the Ca2Fe2O5-LaFeO3 system

Phases between Ca₂Fe₂O₅ and LaFeO₃ with La_1?2yCa_2yFe³⁺O_3?y formulation have been prepared in order to study the oxygen defects in perovskite-related ferrites. X-ray-diffraction analysis and magnetic measurements have confirmed the previous results i.e. the existence of a critical concentration of vacancies above which long range ordering appears. A detailed Mössbauer resonance study shows a continuous evolution of the iron environment, tetrahedra being formed even for low values of y. This result has been discussed and compared with previous ones for homologous CaTi_1?2yFe_2yO_?y phases.     

Influence de la pression partielle d'oxygene sur la nature des defauts ponctuels dans Ca2LaFe3O8+x

The type of point defects in non-stoichiometric Ca₂LaFe₃O_8+x ferrite has been studied by measuring the variation of electrical conductivity with oxygen partial pressure at various temperatures. Several ordering possibilities occur. The oxygen atoms inserted, mostly 1-charged, have a strong tendency to couple at sufficient oxygen pressure and at low temperature. Interstitial oxygen atoms in the lattice vacancies induce structural evolution as non-stoichiometry becomes significant enough.     

Scattering efficiency of aggregated clusters of spheres: dependence on configuration and composition

We study the orientation average scattering cross section of various isolated aggregates of identical spherical particles as functions of their size, optical properties, and spatial configurations. Two kinds of aggregates are studied: latex particles in water and rutile titanium dioxide pigments in a polymeric resin, with size parameters varying from 0.6 to 2.3. Calculations are performed by using a recursive centered T-matrix algorithm solution of the multiple scattering equation that we previously developed [J. Quant. Spectrosc. Radiat. Transfer 79-80, 533 (2003)]. We show that for a specific size of the constituent spheres, their respective couplings apparently vanish, regardless of the aggregate configuration, and that the scattering cross section of the entire cluster behaves as if its constituents were isolated. We found that the particular radius for which this phenomenon occurs is a function of the relative refractive index of the system. We also study the correlations between the strength of the coupling among the constituent spheres, and the pseudofractal dimension of the aggregate as it varies from 1 to 30.     

Variation thermique de la susceptibilite magnetique et de l'aimantation du ferrite bicalcique Ca2Fe2O5

An investigation of the magnetic susceptibility and the magnetization vs temperature has been carried out for Ca₂Fe₂O₅. The weak ferromagnetism behaviour of this compound was confirmed. The value of the ferromagnetic component was determined; an estimate of the various magnetic couplings was given and the importance of the thermal variation of the molecular field at high temperature pointed out.