Structure and Oxide Ion Conductivity Mechanism in Bi₂Al₄O₉ by Combined X-Ray and High-Resolution Neutron Powder Diffraction and ²⁷Al Solid State NMR

Author： Abrahams I. Bush A.J. Hawkes G.E. Nunes T.

Publisher： Academic Press

ISSN： 0022-4596

Source： Journal of Solid State Chemistry, Vol.147, Iss.2, 1999-11, pp. : 631-636

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Abstract

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) Å, and Z=2. The refinement terminated with R_wp=17.09% (X ray), 11.44% (neutron); R_ex=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.