The crystal chemistry of brownmillerite- and N = 3 Aurivillius-type ceramic conductors for fuel cell applications

The crystal structures of n=3 Aurivillius-type phases: Bi4Ti 3O12, Bi2Sr2Nb2TiO 12, Bi2Sr2Nb2Ti0.25Al 0.75O11.625, Bi2SrBaNb2TiO12, and Bi2La2Ti3O12, were characterized using X-ray diffraction. The crystal structures for the latter four phases at room temperature and for Bi4Ti3O12 above 675°C are tetragonal unit cells with I4/mmm space group symmetry and lattice parameters ranging from 3.829 ≤ a ≤ 3.918 A and 32.937 ≤ c ≤ 33.618 A.; For n=3 Aurivillius-type phases, Bi2A2+ 2Nb2BO12-d, the hypothetical crystal structure assumes that Bi3+ completely occupies the M site and alkaline earth cations, A2+, occupy the A site. However, XRD and computer simulation evince that 12-22% of the Bi3+ and A2+ cations are disordered between the A and M sites. Rietveld refinement indicates that the crystal structure distorts so that A2+ cations on the M site are 8 to 12 coordinated, rather than being 4 coordinated like Bi 3+ on the M site.; The electrical conductivity of the solid solution Bi2Sr 2Nb2Ti1-xAlxO12-x/2 was studied. The solubility limits were 0 ≤ x ≤ 0.75. As x was increased, the vacancy concentration in the oxygen sublattice increased and the activation energy for conduction decreased. However, the overall conductivity did not increase.; Many compositions produced n=3 Aurivillius-type phases mixed with secondary phases, demonstrating that the compositions exceed the solubility limits for one or more cations. These observations were used to derive tolerance factor limits for phase stability. These tolerance factors suggest that the relationship between M and B sites restricts the maximum size of the B site cation; and that the relationship between A and B sites constrains the minimum size of the B site cation and the maximum size of the A site cation. The relationship between A and M site cations was correlated to orthorhombic distortion of the crystal structure.; These tolerance factor limits can be used to identify those compositions most likely to produce n=3 Aurivillius-type phases.