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.     

Crystal structure determination and stability of copper (II) hydroxymethylphosphonate dihydrate

Copper(II) hydroxymethylphosphonate dihydrate HOCH₂PO₃Cu, 2 H₂O is triclinic : a = 10.870(3), b = 5.130(2), $\text{c}\text{= 5.628(2)}\text{A}\text{?}$, α = 96.6(1)β = 114.6(1), γ = 97.2(1)°, S.G. Pl?, Z = 2. The structure was solved by the heavy atom method and full matrix least-squares refinement to a final R value = 0.035. The two Cu atoms in the cell are structurally different : Cu(1) is octahedrally surrounded by four phosphonate oxygen atoms and two hydroxyl groups. Cu(1)O₆ octahedra chains extend along [010]. Cu(2) is coordinated to two phosphonate oxygen atoms and four water molecules. Cu(2)O₆ octahedra link the chains to form infinite layers parallel to [101]. These layers are held together by means of hydrogen bonds. The right formula must therefore be written [Cu(HOCH₂PO₃)₂] Cu, 4 H₂O. The greater stability of the octahedral configuration versus the t.b.p. one and the occurence of bonding between copper atoms and hydroxyl groups can explain the increase of stability observed from methylphosphonate to hydroxymethylphosphonate copper complexes.     

New methods for the determination of hydrogen content of aluminum and its alloys: Part I. Improvements in the vacuum extraction method

The vacuum hot extraction method, first proposed by Ransley for the determination of hydrogen content of aluminum and its alloys, was systematically investigated to check if it could be considered as a reference procedure for the development of other methods directly applicable in routine work. For that, the use of an improved apparatus involving a mass spectrometer as gas detector and specimens intentionally charged with deuterium —a tracer whose properties are similar to those of hydrogen—permits the following conclusions: 1) The method is correct for any alloy, even for all the alloys with a high magnesium or zinc content, 2) The “getter effect” which could lead to inaccurate results does not occur, provided the pumping speed of the transfer pump exceeds the rate of outgassing from the sample, 3) The extraction from the solid phase seems to be complete, since results obtained in this condition are not significantly different from those obtained from melted samples. 4) The time required for an analytical determination (several hours) due essentially to the preliminary operations, cannot be practically reduced by extraction at higher temperature in the liquid state. The Ransley method can thus be considered as a reference method for checking other more practical methods.     

Estimation of SiC JFET temperature during short-circuit operations

This paper presents results showing the robustness of different SiC JFET transistors from SiCED in current limitation regime or short-circuit operation. Crystal temperature during failure was estimated after different electrical characterizations and using appropriate models of saturation current which is used as a thermal indicator. This work shows the exceptional robustness of SiC JFET transistors in current limitation mode compared to Si devices (MOSFETS and IGBTs).     

Inhibitory effects of trans-resveratrol analogs molecules on the proliferation and the cell cycle progression of human colon tumoral cells

Resveratrol may function as a cancer chemopreventive agent. However, few data are available on the antitumoral activities of its dimer, epsilon-viniferin, also present in human diet. So, the effects of resveratrol, epsilon-viniferin, of their acetylated forms (resveratrol triacetate, epsilon-viniferin pentaacetate) and of vineatrol (a wine grape extract) were compared on human adenocarcinoma colon cells. Resveratrol and resveratrol triacetate inhibit cell proliferation and arrest cell cycle. epsilon-Viniferin and epsilon-viniferin pentaacetate slightly reduce cell proliferation. Vineatrol inhibits cell proliferation and favors an accumulation in the S phase of the cell cycle. Consequently, resveratrol triacetate and vineatrol could constitute new putative anticancer agents on colon carcinoma.