J. M. Moreau

Structural characterization of PbWO4 and related new phase Pb7W8O(32−x)

Abstract Single crystals of PbWO 4 have been obtained by the Czochralski method. Atomic structure has been determined by X-ray diffraction (scheelite type, tetragonal, space group I 4 1 / a , a = 5.456(2), c = 12.020(2) A). With PbO deficiency in the melt, crystallization occurred with a new atomic arrangement determined by X-ray diffraction (tetragonal, space group P 4/ nnc , a = 7.719(2), c = 12.018(2) A). Unit cell parameter a related to the scheelite type by a factor of √2. Oxygen and lead sites not fully occupied gave a formula Pb 7 W 8 O (32− x ) with x = 3.2(8).

A new structural model for Pb-deficient PbWO4

Abstract A previously reported structure proposal for Pb-deficient PbWO4 (Pb7W8O32−x, space group P4/nnc) could be rejected based on neutron powder diffraction data. A new model in agreement with X-ray and neutron diffraction data is proposed (tetragonal, tP48, space group P-4-h10g2dcba, a=7.7208(1), c=12.0417(2) A). It is a superstructure of the scheelite type with partial vacancies on a one-fold lead site, leading to the composition Pb7.5W8O32.

A new phase in the TlAgTe system: crystal structure of Tl4Ag18Te11

Abstract A new stable phase in the ternary TlAgTe system has been identified. X-ray diffraction examination of single crystals was used to establish the crystal structure of Tl4Ag18Te11. The cubic space group is F 4 3m and the lattice parameter is a = 1.8717(8) nm, Z = 8. The relationship to the cubic BaHg11-type structure and the In2Te3-type structure are established. The structure is described as a three-dimensional array of large channels of silver and tellurium atoms hosting linear chains of thallium atoms. These independent channels are built up with cubes and square antiprisms stacked along the fourfold axis and they are connected by smaller channels built up with smaller square antiprisms and cubes hosting tellurium atoms.

A new phase in the TlAgTe system: crystal structure of Tl4Ag24−xTe15−y

Abstract A new stable phase Tl 4 Ag 24− x Te 15− y in the ternary TlAgTe system has been identified by X-ray diffraction on single crystals. The space group is P 6 and the lattice parameters are a = 1.1418(4) nm and c = 0.9220(1) nm with one formula per unit cell ( Z = 1). The structure is described as an arrangement of two types of channels parallel to the [001] direction. Two-thirds of these channels are hexagonal and built up with silver and tellurium atoms, hosting thallium atoms. One-third are octahedral chains of silver atoms with centres statistically occupied by tellurium atoms. In addition, the refinement indicates partial occupation of some silver positions.

Structure and electrical resistivity of the new yellow oxygenated molybdenum bronze GdMo16O44

Abstract The crystalline structure and electrical resistivity of the new yellow molybdenum bronze GdMo16O44 are presented. Single crystal X-ray diffraction analysis indicate a rhombohedral space group R 3 with lattice parameters a = 10.780(2) A and c = 27.493(4) A . The outstanding features are; i) all polyhedra share corners with their neighbours; ii) the structure is built up from clusters of eight MoO6 octahedra separated from other clusters and from GdO8 cubes by MoO4 tetrahedra; iii) six-fold and four-fold coordinated molybdenum atoms have a valence state close to +5 and +6 respectively. As expected by the structural results the compound is found semiconducting.

A Search for New Permanent Magnet Materials: Crystallographic and Magnetic Properties of New Compounds

Permanent magnet materials are characterized by high Curie temperature, large spontaneous magnetization and large magnetic anisotropy. The best natural elements to produce these properties are rare earth and transition elements (Fe and Co). New materials could be obtained by addition of others elements such as B, Si or C which stabilized the corresponding atomic structures. New compounds have been synthetized and their crystallographic and magnetic properties have been studied : Nd(CoxMn1-8)12, Nd(N1xMn1-x)12, RFe10SiC0.5 (R = Ce, Pr, Nd, Sm), DyFe2SiC, DyFe2Si2C, R(FexCo1-x)10SiC0.5. Curie temperatures of 950 K for NdCo10SiC0.5 and 670 K for SmCo10SiC0.5 were measured.