Structure and Oxide Ion Conductivity Mechanism in Bi2Al4O9 by Combined X-Ray and High-Resolution Neutron Powder Diffraction and 27Al 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 27Al MAS solid state NMR data confirms the presence of two Al sites, one octahedral and one tetrahedral, in an approximate 1:1 ratio. The structure of Bi2Al4O9 has been refined anisotropically in the orthorhombic space group Pbam, with a=7.7134(1), b=8.1139(2), c=5.6914(1) ?, and Z=2. The refinement terminated with Rwp= 17.09% (X ray), 11.44% (neutron); Rex=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 between the columns. The Bi 6s2 lone pairs point toward vacant sites in the structure. A one-dimensional conductivity mechanism along the c-axis vector is proposed involving these sites.