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.