W0224

Crystal Structure Refinements of Three-Layer Aurivillius Ceramics. Michael S. Haluska and Scott T. Misture, New York State College of Ceramics at Alfred Univ., Alfred, NY 14802.

Combined x-ray and neutron diffraction Rietveld refinements were performed on the three layer Aurivillius ceramics Bi₂Sr_2-xA_xNb₂TiO₁₂ (A=Ca,Ba x=0.5,1). Lattice strain and site-mixing effects were observed as a function of systematic Ca or Ba substitution for Sr. Site-mixing was observed between the Bi and Sr sites and between the Nb and Ti sites. Both the degree of site-mixing between the Bi site and Sr sites and the a lattice parameter increased as the size of the substituting cation increased. Bond valence sum calculations show that the A-site goes from an underbonded to an overbonded state with increasing cation size substitution. The effects of lattice strain on ionic conductivity was also studied on oxygen-deficient Aurivillius ceramics based on the Bi₂Sr_2-xA_xNb₂TiO₁₂ composition in an attempt to correlate structure and ionic conductivity. The Sr-site in the oxygen deficient samples was substituted with Ca in an attempt to lower the strain on the A-site. Ionic conductivities obtained from AC impedance spectroscopy indicated a decreased ionic conductivity with substitution of Ca.