Structure and oxide ion conductivity mechanism in Bi2Al4O9 by combined X-ray and high-resolution neutron powder diffraction and Al-27 solid state NMR

The structure of Bi2Al4O9 has been refined using a combination of X-ray and high-resolution neutron powder diffraction. Iterative simulation of the Al -27 MAS solid state NMR data confirms the presence of two Al sites, one oct ahedral and one tetrahedral, in an approximate 1:1 ratio. The structure of Bi2Al4O9 has been refined anisotropically in the orthorhombic space group P bam, with a = 7.7134(1), b = 8.1139(2), c = 5.6914(1) Angstrom, and Z = 2. The refinement terminated with R-wp = 17.09% (X ray), 11.44% (neutron); R-e x = 4.52% (X ray), 3.22% (neutron). The structure consists of edge-sharing columns of AlO6 octahedra running parallel to the c-axis. These columns are linked by pairs of apex sharing AlO4 tetrahedra, with Bi atoms located bet ween the columns. The Bi 6s(2) lone pairs point toward vacant sites in the structure. A one-dimensional conductivity mechanism along the c-axis vector is proposed involving these sites. (C) 1999 Academic Press.