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

Abstract

Three-layer Aurivillius ceramics Bi 2 SrCaNb 2 TiO 12 , Bi 2 Sr 1.5 Ca 0.5 Nb 2 TiO 12 , Bi 2 Sr 2 Nb 2 TiO 12 , Bi 2 Sr 1.5 Ba 0.5 Nb 2 TiO 12 , and Bi 2 SrBaNb 2 TiO 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 2 O 2 ) 2+ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi 2 SrBaNb 2 TiO 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 2 SrCaNb 2 TiO 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 2 O 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.