Crystal structure refinements of the three-layer Aurivillius ceramics Bi2Sr2−xAxNb2TiO12 (A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Three-layer Aurivillius ceramics Bi₂SrCaNb₂TiO₁₂, Bi₂Sr_1.5Ca_0.5Nb₂TiO₁₂, Bi₂Sr₂Nb₂TiO₁₂, Bi₂Sr_1.5Ba_0.5Nb₂TiO₁₂, and Bi₂SrBaNb₂TiO₁₂ 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 (Bi₂O₂)²⁺ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi₂SrBaNb₂TiO₁₂ 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 Bi₂SrCaNb₂TiO₁₂ had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca²⁺ to remain on the A site while Ba²⁺ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi₂O₂)²⁺ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO₆ 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 TiO₆ octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of at 900°C.