A POWDER NEUTRON-DIFFRACTION INVESTIGATION OF STRUCTURE AND CATION ORDERING IN BA2+XBI2-XO6-Y

The structures and cation ordering of several barium bismuth oxide phases, Ba2+xBi2-xO6-y (x = 0.22, 0.28, 0.40, 0.67), have been determined at ambient temperature from constant wavelength and time-of-flight neutron powder diffraction data. The end member of the series, Ba2Bi(III)Bi(V)O6, has a monoclinic distortion of the simple cubic perovskite structure with an ordered arrangement of the Bi3+ and Bi5+ cations on the B sites. When additional barium atoms are introduced into the structure, substitution of Ba for Bi(III) occurs to give phases with the composition Ba2[(Bi1-xBax)Bi]O6 when x < 0.6. The monoclinic structure at x = 0 changes first to a rhombohedral structure and then to cubic as x is increased. At x = 0.67, the structure remains cubic but the cation distribution changes to give an unusual arrangement, represented by Ba5/3Bi1/3[BaBi(V)]O5.67, with Bi3+ cations on the perovskite A sites and complete Ba2+/Bi5+ ordering on the B sites. © 1993 Academic Press, Inc.