Jean-Claude Boivin

Le diagramme des phases solides du systeme Bi2O3CaO

The phase diagram Bi 2 O 3 CaO has been investigated in the solid state by high temperature X-ray analysis and D.T.A. methods. Four compounds (Bi 14 Ca 5 O 26 , Bi 2 CaO 4 , Bi 10 Ca 7 O 22 , and Bi 6 Ca 7 O 16 ), which melt incongruently, and four solid solutions (fcc, bcc, and two rhombohedral) have been characterized. Quenchings of the fcc solid solution lead to several metastable fluorite-type ss. These, as well as the quenched forms of the rhombohedral and bcc solid solutions, have defect oxygen lattice consistent with the formula Bi (2−2 n ) Ca (2 n −1) O (2− n ) □ n .

Etude structurale du conducteur anionique Bi0,765Sr0,235O1,383

Abstract The structure of an O2− conductor, the rhombohedral (a = 9.75A, α = 23.49°) low-temperature form of the solid solution Bi 1−x Sr x O 1,5− x 2 (x = 0.235, Z = 3) has been solved in space group R 3 m by means of Fourier synthesis and least-squares refinements. Intensity data were corrected for absorption. The final R value is 0.030 for 302 independent planes. Most of the bismuth atoms are located on a first set with a distorted tetrahedral coordination while the remaining bismuth and the strontium atoms are distributed on a single position with an eightfold (6 + 2) oxygen coordination. Two types of anionic sites are detected within the tetrahedral voids of the cation sheets stacked along the [III] axis. This results in a layered structure, with loosely bound oxide ions, providing a basis for the interpretation of the ionic conductivity of the phase.

Solid-state reaction pathways of Sillenite-phase formation studied by high-temperature X-ray diffractometry and differential thermal analysis

Abstract This paper reports on the use of high-temperature X-ray diffractometry, in conjunction with differential thermal analysis, to study the solid-state reactions and phase transformations involved in and leading to the synthesis of various (bismuth oxide-based) Sillenite-type compounds. These, which have γ-body centered cubic (bcc) structures related to γ-Bi 2 O 3 , are generally represented by the formula Bi 12 MO 20 ; here, the second cation M=Ge, Si, Ti, Zn, Al, as well as the two-cation combination {Si 0.5 Ge 0.5 }. In each case, using the two techniques, measurements were performed on starting powder-mixtures of bismuth and cation M oxides, formulated with desired proportions to favor the Sillenite (γ-)phase formation. Henceforth, the proceeding of solid-phase reactions and occurring of phase transitions, with rising temperature, were monitored and elucidated by the evolution of the X-ray diffractometry patterns and the appearance of thermal events in the differential thermal analysis curve, as both sets of results were also correlated.

Détermination de la structure cristalline du ferrite de baryum Ba2Fe6O11

Abstract The crystal structure of dibarium triferrite Ba 2 Fe 6 O 11 has been solved by direct methods, using intensity data collected by means of an automated diffractometer (MoKα radiation) and corrected for absorption. It crystallizes in the orthorhombic space group Pnnm : a = 23.024(10)A, b = 5.181(3) A, c = 8.900(4) A, Z = 4. Program MULTAN was successfully used for locating Ba 2+ and most of the Fe 3+ ions. The structure was further refined by conventional Fourier and least-squares methods (full-matrix program) to a final R value of 0.045 for 1448 observed reflections. Fe 3+ ions occur in both octahedral (Feue5f8O mean distance: 2.02 A) and tetrahedral (Feue5f8O mean distance: 1.865 A) coordination. Two types of Ba 2+ ions are found, with six and seven neighboring oxygen atoms. The structure consists of sheets of edge-shared FeO 6 octahedra which are connected by means of corner-shared tetrahedra.

A new anionic conductive vitreous phase based on bismuth, lead, cadmium, oxygen and fluorine: electrical properties and structural study of the recrystallized phase

Investigations in the Bi2O3–CdO–PbF2 system using silica tubes as synthesis containers indicate the existence of a large vitreous domain. Structural determination, using powder diffraction and Rietveld methods, has shown that the recrystallized phase is isotypic to the trigonal phase belonging to the Bi2O3–PbO–PbF2 system. Raman studies evidence boson and large peaks, typical of a vitreous phase containing lead in its formulation. Conductivity measurements display σ values close to 10−3xa0Sxa0cm−1 at 250u2006°C with an activation energy of 0.6xa0eV. These values are among the best obtained for an anionic conductive glass.

Phases stability and conduction properties in the Bi2O3-PbOPbF2 system

Abstract The Bi2O3-PbOue5f8PbF2 system was investigated by means of X-ray diffraction at temperatures between 770 and 950 K. Two solid solution domains have been evidenced. The first one exhibits a tetragonal cell (a = 7.74(1), c = 5.78(1) A for Bi(0.857)Pb(0.143)O(1.357)F(0.143)). Conductivity measurements performed by complex impedance method show that the value of σ remains lower than 10−7 (Ω.cm)−1 at 470 K. The second domain, isostructural with A La203 (hexagonal cell, a = 4.101(2), c = 6.087(2) A for Bi(0.5)Pb(0.5)0 F(0.5)) is more attractive: σ values close to 3×10 −4 (Ω.cm) −1 are reached at 470 K. The conductivity is strongly influenced by both oxygen and fluorine contents.