Jean-François Brice

Preparation de la solution solide hydrurofluoree CaF2−xHx (0 < x ⩽ 1,24) etude structurale par diffraction des rayons X et par diffraction des neutrons

By studying the CaH2CaF2 system at 700°C, we associate F− and H− anions in the same ternary combination. The existence of solid solutions of composition CaF2−xHx (0 < x ⩽ 1.24) is established, crystallizing in CaF2-type cubic symmetry. The solid solution parameter is considered constant as the composition varies, since there is a variation of 1100 A between pure CaF2 (a = 5.463 A) and the limit phase CaF0.76H1.24 (a = 5.452(5) A). Thus the two anions H− and F− are approximately the same size in the mixed hydride fluorides of calcium. X-ray and neutron diffraction studies indicate a structure in which there is a statistical substitution of fluoride ions by hydride ions on the 8c sites of the CaF2 unit cell.

Etude structurale de l'hydruro-oxyde LaHO par diffraction des rayons X et par diffraction des neutrons

Abstract The hydride-oxide LaHO crystallizes with a fluorite superstructure. The unit cell is tetragonal, space group P4 nmm , a = 8,074(1) A, c = 5,739(1) A, and Z = 8. The structure has been determined by two complementary techniques: X-ray and neutron diffraction. The La3+ ions are situated in the center of cubes at the corners of which four O2− and four H− are placed, there exist three different types of environment according to the distribution of the two anion species (the anions of the same type form either a tetrahedron or occupy a face or a diagonal plane of the cube). The LaO(H) distances observed are discussed by taking into account the relationship which exists between bond strength and bond length. The average value of d 〈OH〉 is 2,85 A, which is rather unusual. It confirms that we are indeed dealing with a hydride-oxide simultaneously associating the O2− and H− ions in a ternary combination. Through energy considerations (electrostatic site potential and lattice potential), the structural arrangement of LaHO is compared to that encountered in the case of the homologous lanthanum oxifluorides.

Caracterisation de nouveaux ferrites d'indium: In2Fe4O9 et InFeO3

Abstract In the system Fe 2 O 3 -In 2 O 3 , two new ferrites are characterized: In 2 Fe 4 O 9 at 400°C, and InFeO 3 at 700°C; the first is isotypic of Ln 2 Fe 4 O 9 with iron coordinated by four, five and six oxygen atoms, the second is isotypic of YAlO 3 with iron in oxygen trigonal based FeO 5 bipyramids only.

Contribution a l'etude de la conductivite ionique de l'orthophosphate Na3PO4

Abstract The present work is concerned with the ionic conductivity of pure trisodium orthophosphate Na3PO4, devoid of any trace of hydroxide NaOH. At the allotropic transition (330°C), we observe a jump of the ionic conductivity and a slight decrease in the activation energy (ΔE = 0,70 ± 0,02 eV for the quadratic variety and ΔE = 0,60 ± 0,04 eV for cubic γ-Na3PO4). Na3PO4 can be considered to be an electrolytic solid with medium conductivity ( σ = 1.10 −4 Ω −1 cm −1 at 370°C).

Etude de la conducivité ionique des hydrurofluorures CaF2−xHx

Abstract The present work is concerned with the electrical conductivity of the calcium hydride fluorides CaF2−xHx between 20 and 200°C. The study was made using the complementary methods of complex admittance conductivity measurements and dielectric loss measurements. The influence of quenching and doping by Na+ has permitted us to identify the two conductivity domains observed. The low-temperature domain, which is characterized by a slight thermal increase in conductivity, corresponds to the reorientation of [V·(F,H)&.z.sbnd;Na′Ca]∗ dipoles due to impurities. The second domain corresponds to the first stage of ionic conduction which results from the migration of V·(F,H) vacancies formed by the thermal dissociation of [V·(F,H)&.z.sbnd;Na′Ca]∗ complexes. The H − F − substitution increases the mobility of charge carriers and the conductivity increases by a factor of 103 between the compositions CaF2 and CaFH. Thus calcium hydride fluorides can be considered to be electrolytic solids with medium conductivity (CaF1.06H0.94: σ = 10−6 ω−1 cm−1 at 130°C).

Structure cristalline du ferrite hemicalcique CaFe4O7

The hemicalcic ferrite CaFe4O7 crystallizes in the monoclinic system, space group C2 with the parameters: a=10.409 A, b=6.005 A, c=31.640 A, β=96°30. Its crystalline structure is related to that of the hexagonal ferrites. It is constituted by an alternating stacking process along c axis of two structural blocks with the following characteristics: • - a plane of trigonal based FeO5 bipyramids surrounded by two mixed FeCa Layers • - a triple layer of iron atoms, formed by a plane of mixed tetrahedral and octahedral polyhedra surrounded by two octahedral polyhedra; this ordering is also encountered in the spinel structure (mixed and “kagome” system).

Conductivite ionique des oxydes Li8MO6 (M = Ce, Hf)

Abstract The electrical conductivity of the oxides Li8CeO6 and Li8HfO6, which are thermodynamically stable against Li (1), has been studied under oxygen atmosphere in the temperature range between room temperature and 200°C. As for Li8ZrO6 and Li8SnO6 (2), the materials can be considered as solid electrolytes with medium conductivity. The interpretation of the results, considering the Ca2+ ions as impurities, are in agreement with those based on the diffusion mechanisms of the Li+ ions recently established by 7Li NMR in isostructural oxides by Simpson et al. (3).

Recherche de matériauxa`conductivitéionique ame´liore´e de´rivant des hydrurofluorures de calcium CaF2−xHx: conductivite´ ionique des phases Ca1−yNay(F2−xHx)1−y/2

Abstract The doping of CaF 2− x H x hydridefluorides by aliovalent ions is studied, the aim being the preparation of materials of improved ionic conductivity. It is shown that doping by monovalent Na + ions is possible. Three hydrogen-rich phases, formulated Na y Ca 1− y (F 2− x H x ) 1− y /2 have been studied. Their conductivity is mainly ionic, but, compared with the same doping in CaF 2 , the conductivity enhancement is low. This result is interpreted from energetic and structural considerations.

Preparation et etude structurale d'un triarseniure de calcium: CaAs3

CaAs 3 is prepared through reaction of gaseous arsenic with calcium metal at 800–900°C in a sealed silica tube. X-ray powder diffraction is indexed by analogy with CaP 3 . The triclinic cell of CaAs 3 is refined by applying the least-squares method: a =5.854(5) ; b =5.832(5) ; c =5.901(5) ; α =70.2(1) o , β =80.3(1) o , γ =75.7(1) o . The compound is isotypic with CaP 3 (space group P ī). Its structure is refined with X-ray powder intensities. The CaAs 3 structure can be interpreted as formed of layers of arsenic, between which we can find the calcium atoms. The chains of As atoms are reminiscent of those existing in the orthorhombic As structure.

Preparation et proprietes du diphosphure de beryllium: BeP2

The compound BeP2 is obtained by two methods : • - direct action of phosphor on beryllium metal at 800–1000°C • - removal of arsenic with phosphor in the diarsenide BeAs2. BeP2 is a non hygroscopic brown-red powder. The X rays diffraction provides evidence for a quadratic cell with a = 7,08 A & c = 15,06 A. The atomic stacking is diamond type.