Synchrotron X-ray study of orthorhombic Rb(3)Ta(5)O(14) with a modified pyrochlore structure.
The structure of a new orthorhombic trirubidium pentatantalum oxide (Rb(3)Ta(5)O(14)) phase with Pnma symmetry was identified from a half sphere of synchrotron X-ray data measured at a wavelength of 0.85 A. This notionally linked TaO(6) octahedral structure broadly consists of three different modifications of the pyrochlore ring motif with layer stacking normal to (205) planes. Successive pyrochlore layers do not simply stack normal to these planes but are offset along the [100] axis. An unusual aspect of this structure is the occurrence of TaO(5) trigonal bipyramids in structurally complex regions where the modified pyrochlore rings connect.
Synchrotron X-ray study of ortho­rhombic Rb3Ta5O14 with a modified pyrochlore structure
The structure of a new orthorhombic trirubidium pentatantalum oxide (Rb3Ta5O14) phase with Pnma symmetry was identified from a half sphere of synchrotron X-ray data measured at a wavelength of 0.85 A. This notionally linked TaO6 octahedral structure broadly consists of three different modifications of the pyrochlore ring motif with layer stacking normal to (205) planes. Successive pyrochlore layers do not simply stack normal to these planes but are offset along the [100] axis. An unusual aspect of this structure is the occurrence of TaO5 trigonal bipyramids in structurally complex regions where the modified pyrochlore rings connect.
Un niobate de rubidium d'un type structural nouveau: Rb4Nb6O17·3H2O
Resume Un nouvel oxyde double: Rb4Nb6O17·3H2O a ete synthethise en monocristaux. Il cristallise dans le systeme orthorhombique, groupe spatial Pmnb avec a = 7,83(1), b = 39,06(5), c = 6,57(1)A, Z = 4. La structure a ete resolue par diffraction des rayons X a partir de 811 reflexions mesurees sur films, par etude des series de Patterson et de Fourier a trois dimensions. L'affinement par moindres carres conduit a un facteur R = 0,124 pour les 581 reflexions telles que F> F max 20 . L'edifice cristallin se compose de feuillets constitues par deux plans d'octaedres NbO6 relies par la moitie des alcalins. Les deux autres atomes de rubidium, situes en surface et entre les neuillets, sont entoures de molecules d'eau qui assurent seules la cohesion de l'ensemble au moyen de liaisons RbH2ORb, et par l'intermediaire de deux liaisons hydrogene avec les atomes d'oxygene des feuillets. Cet arrangement explique le clivage et l'aspect des cristaux qui rappelle celui des micas.