Alain Courtois

Sur l'existence des oxydes rhomboe driques A(III) B(II) B′(III)O4

Abstract The symmetry group of the rhombohedral structure of the mixed oxides A(III)B(II)B′(III)O4 is R3m: A is in an octahedral coordination, B and B′ occupy the same crystallographic sites and are in a pyramidal coordination with a triangular basis. The existence of this structural type with Ni2+ and Cr3+ ions is incompatible with this coordination close to tetrahedral. The geometric conditions necessary for adoption of the rhombohedral structure have been clearly defined by comparing the cation-oxygen and oxygen-oxygen distances determined on 4 monocrystals (InCuAlO4, InFe1, 75SiO, 25O4, YbFe2O4 and YbO,5EuO,5Fe2O4). The shortest cation-oxygen distance in the pyramidal site is designated by d′: it can be calculated through the relation: d ′ = 1 2 ( r B 2+ + r B ′ 3+ ) + r O 2− by using the ionic radii of ions in tetrahedral coordination, given by R.D. Shannon. The oxygen - A3+ cation distance of the octahedron, d, is close to the sum of the ionic radii of the ions in coordinence 6. Mixed oxides ABB′O4 can crystallize in the rhombohedral system if the ratio d′/d is between 0.842 and 0.888 and these two limiting values can be refined for each couple B, B′.

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.

Le systeme SbWO3 et les bronzes de tungstene — Antimoine

Abstract Evidence is obtained for a series of tungsten-antimony bronzes, through reaction of antimony with tungsten trioxide at 900°C. Five phases are characterized: the four first derive from WO 3 with the classical increase of symetry: monoclinic, orthorhombic, tetragonal and cubic; the last one is probably a superstructure of the perovskite.

La structure cristalline de Ca4Fe9O17: Des feuillets “hexagonaux” d'octaedres FeO6 et de bipyramides FeO5 lies par des tetraedres FeO4

The new calcium ferrite Ca4Fe9O17, belonging to the CaFe2+nO4+n family (n = 14), has not the same stacking process of “FeO” blocks in “CaFeO4” blocks, as the others terms of the series. It crystallizes in the monoclinic system, space group C2 with the parameters: a = 10,441 A, b = 6,025 A, c = 11,384 a and β = 98°80. Its structure is characterized by the presence of iron atoms in oxygen octahedra and trigonal based bipyramides stacking in hexagonal layers along c. These layers are linked by iron atoms on tetrahedral sites. Calcium atoms are hexagonaly located around each tetrahedron.

Distribution des atomes metalliques dans les structures apparentees des trois composes ternaires equiatomiques : RhMnSi, isotype de Co2P, RhMnGe, isotype de TiFeSi et PdMnGe, isotype de Fe2P

Abstract The distribution of metallic elements has been studied in three ternary equiatomic transition metal silicides and germanides : RhMnSi with TiNiSi (ordered anti-PbCl 2 ) structure, PdMnGe with Fe 2 P structure and RhMnGe with the closely related TiFeSi structure. The large influence of the relative electronegativity of transition metals upon the crystallographic order was shown.

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.

Sur l'existence de phases ternaires hydrogenees BeP2−xHy

Abstract The phosphidation of beryllium in the presence of traces of hydrogen leads to non stoichiometric ternary phases BeP2−xHy. The stoichiometric diphosphide BeP2 cannot be obtained. The hydrogenated phases are black and non hygroscopic. The structural study with X rays diffraction, neutrons diffraction, electronic microdiffraction, as well as the crystallographic data from single crystal, provide evidence for a quadratic cell with a = 7,08 A & c = 30,12 A . The stacking is diamond type when considering all the atoms : Be, P and H.