A Powder Neutron Diffraction Investigation of Structure and Cation Ordering in Ba 2+ xBi 2- xO 6- y

Abstract

The structures and cation ordering of several barium bismuth oxide phases, Ba 2+ x Bi 2- x O 6- 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, Ba 2Bi(III)Bi(V)O 6, has a monoclinic distortion of the simple cubic perovskite structure with an ordered arrangement of the Bi 3+ and Bi 5+ 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 Ba 2[(Bi 1- x Ba x )Bi]O 6 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 Ba 5/3Bi 1/3[BaBi(V)]O 5.67, with Bi 3+ cations on the perovskite A sites and complete Ba 2+/Bi 5+ ordering on the B sites.