Crystal structure refinements of the three-layer Aurivillius ceramics Bi 2Sr 2− xAxNb 2TiO 12 ( A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Abstract

Three-layer Aurivillius ceramics Bi 2SrCaNb 2TiO 12, Bi 2Sr 1.5Ca 0.5Nb 2TiO 12, Bi 2Sr 2Nb 2TiO 12, Bi 2Sr 1.5Ba 0.5Nb 2TiO 12, and Bi 2SrBaNb 2TiO 12 were formed via solid-state synthesis and their structures characterized by combined Rietveld analysis of powder X-ray and neutron diffraction data. Static disorder was observed in the form of mixed cation occupancies between the Bi and the Sr, Ca, or Ba on the A sites in the perovskite block, as well as between the Nb and Ti sites. The degree of site mixing between the Bi site in the (Bi 2O 2) 2+ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi 2SrBaNb 2TiO 12 displayed the greatest degree of Bi–A site static disorder. Bond valence sum (BVS) calculations showed an increase in A site BVS with average A site cation radius. All compositions except Bi 2SrCaNb 2TiO 12 had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca 2+ to remain on the A site while Ba 2+ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi 2O 2) 2+ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO 6 octahedra were observed in structural models that included full oxygen occupancy. However, excellent structure models and more reasonable BVS values were obtained by assuming oxygen vacancies in the TiO 6 octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of 10 −4 S/ cm at 900°C.