Novel defect pyrochlores ABi2B5O16 (A = Cs, Rb; B = Ta, Nb)

The crystal structures of three oxides with compositions CsBi2Nb5O16, CsBi2Ta5O16, and RbBi2Ta5O16 have been determined from powder neutron diffraction data. A common structure of the defect pyrochlore type has been found. Atomic positions in space group Fd3m are 3.2 O and 3.2 Cs(Rb) in 8b, 6.4 Bi in 16d, 16 Ta(Nb) in 16c, and 48 O in 48f. There is some evidence that the Rb atoms actually occupy 32e sites. On electrostatic grounds it is highly …