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 unlikely that the Cs(Rb) and O atoms are randomly distributed in 8b. Evidence for short-range order is apparent in the X-ray powder diffraction data. Cell constants are 10.528(1) Å (Cs, Nb), 10.504(1) Å (Cs, Ta), and 10.510(2) Å (Rb, Ta), respectively, with 48f x parameters 0.3139(2) (Cs, Nb), 0.3152(2) (Cs, Ta), and 0.3153(2) (Rb, Ta). Isotropic temperature factors for the 8b and 16d site atoms ranged from 3 to 6 (Å)2. These large values suggest anharmonic behavior or high mobility for some of the ions. Evidence for this is found in the presence of significant dielectric loss effects in all of the compounds studied.