Synthesis and structural mechanisms of the 2201-type ferrites and polytypes: Fe2(Sr2 − xAx)FeO6.5 − δ/2 (A = Ba,La, Tl,Pb and Bi)

The Fe₂(Sr_2 ? xA_x)FeO_6.5 ? δ/2 systems have been investigated, by doping the iron rich 2201-type parent structure with Ba²⁺, La³⁺ and 5d¹⁰ post-transition cations. The syntheses have been carried out up to the limit of the 2201-type solid solutions, in order to test the role of the double iron layer Fe₂O_2.5 ? δ/2. The localisation of the charge carriers in these compounds is consistent with their strong antiferro-magnetism. The investigation was then carried out in the transition part of the diagram up to the formation of stable phases. The study of structural mechanisms was carried using high resolution electron microscopy (transmission and scanning transmission), electron diffraction and energy dispersive spectroscopy. Different non-stoichiometry mechanisms are observed, depending on the electronic structure and chemical properties of the doping elements. The specific behavior of the modulated double iron layer is discussed.     

Benchmarking and improvements of measurement techniques for local-time-averaged two-phase flow parameters

Accurate measurements of local time-averaged two-phase flow parameters is of fundamental importance for the evaluation of two-phase flow models as well as for the development of closure relations used in the two-fluid model. For cross-calibration purposes, we compared the measurements of local-time-averaged two-phase flow parameters using a four-sensor conductivity probe developed by the TRSL (Thermal-hydraulics and Reactor Safety Laboratory) at Purdue University, and a four-sensor optical probe developed by the CEA (French Atomic Energy Commission). A system coupling methodology was also performed in order to isolate the effects of each instrumentation's components, i.e., probe, acquisition, thresholding, and processing effects. Experimental results have shown a reasonable agreement in the local void fraction, local bubble velocity, and local interfacial area concentration measurements comparisons. However, it is shown that particular attention must be given to the sensor probe design in order to get accurate measurements.     

Evolution of charge ordering in manganites

Charge ordering phenomena in the manganites Ca_1-xSm_xMnO₃ have been studied for , using electron diffraction and lattice imaging, completed by magnetic and transport measurements. Three domains can be distinguished, depending on the nature of the structural transitions with temperature. For , the structural transition from a pseudo-tetragonal to a monoclinic form, with decreasing temperature, coincides with the competition between ferromagnetism and antiferromagnetism that is characterized by the temperature T_peak on the M ( T ) curves; short-range charge ordering is observed for manganites. For the second domain, , a structural transition from an orthorhombic to a long-range charge ordered state is clearly observed with decreasing temperature. The corresponding temperature T_CO coincides with the temperature T_peak deduced from magnetic measurements. This long range charge ordering, which appears along a, is either commensurate or incommensurate depending on the x value, with a modulation vector, q being close to x. These modulated superstructures correspond to a stacking of single Mn³⁺ stripes with multiple Mn⁴⁺ stripes along a, either in a commensurate or in an incommensurate manner. The third domain , is characterized by a transition to a charge ordered state with commensurate superstructure at low temperature. The latter can be described as a “partially” charge ordered state in which single “Mn³⁺” stripes alternate with mixed “Mn³⁺/Mn⁴⁺” stripes. Received 17 June 1998     

Charge ordering-disordering in the Th-doped CaMnO3

The structural transitions that appear in the manganites Ca_1-xTh_xMnO₃ versus temperature are studied in connection with their magnetic and transport properties, and compared to those of the Ca_1-xLn_xMnO₃ manganites. An orthorhombic to monoclinic transition is observed for low x values (;this structural distortion, also observed for Ln-doped oxides, is related to the magnetoresistance properties. For higher x values (), modulated commensurate and incommensurate phases are obtained at low temperature, with , b =2 a p and , which are related to Mn³⁺/Mn⁴⁺ charge ordering (CO) phenomena. T values, determined from electron diffraction, are in agreement with those determined from the M ( T ) curves. The low temperature electron microscopy shows that the CO in those oxides is more complex than in Ln-doped manganites. In particular, the destabilisation of CO and consequently of the antiferromagnetic interactions is evidenced as the thorium content increases which may explain the appearance of a spin-glass like behavior for higher x values not seen for Ca_1-xSm_xMnO₃ phases . Received 2 November 1998     

Enzyme-catalyzed syntheses of poly(1,6-hexanediyl maleate) and poly(1,6-hexanediyl fumarate) in organic medium

The lipase-catalyzed preparation of poly(1,6-hexanediyl maleate) by transesterification of 1,6-hexanediol and dimethyl maleate is described. A configurationally pure poly(1,6-hexanediyl maleate) exhibiting exclusively cis structure was obtained. During the reaction, a substantial amount of macrolactones was formed. They were isolated, and the cyclic oligomer with y = 2 was found to predominate. Cycles are semi-crystalline, while no melting point was detected for the linear poly(1,6-hexanediyl maleate). We assume that the linear unsaturated polyester is completely amorphous owing to its cis configuration.     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Problèmes linéaires dans le Compendy de la praticque des nombres de Barthélemy de Romans et Mathieu Préhoude (1471): Une approche nouvelle basée sur des sources proches du Liber abbaci de Léonard de Pise

The Compendy de la praticque des nombres (1471) is one of a number of commercial arithmetics produced in southern France during the medieval period. Its interest and originality rest in its treatment of problem-solving. The author of the text limited his treatment to an in-depth analysis of only a few types of problems, not treating particular cases but rather emphasizing general methods. The sources from which he drew were very close to the Liber abbaci of Leonardo Fibonacci and many of them were new to the southern French arithmetic tradition. Thus, the Compendy sheds new light on the transmission of arithmetic thought into Europe. Copyright 2000 Academic Press.Le Compendy de la praticque des nombres (1471) est un traité qui appartient au groupe des arithmétiques commerciales du Sud de la France. L'intérêt et l'originalité du texte résident dans la partie consacrée à la résolution de problèmes. L'auteur sélectionne quelques types de problèmes seulement, auxquels il consacre une longue étude, délaissant les cas particuliers pour privilégier les méthodes. Ce faisant, il utilise de nouvelles sources, proches du “Liber abbaci” de Léonard de Pise, étrangères aux autres arithmétiques françaises de la même famille. Le Compendy nous apporte ainsi un éclairage nouveau sur la transmission de l'algorisme. Copyright 2000 Academic Press.MSC subject classifications: 01A35; 01A40.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.