Bulk and surface structures of the Aurivillius phases: Bi4-xLaxTi3O12 (0 ≤ x ≤ 2.00)
A convergent beam electron diffraction (CBED) study of Bi2La2Ti3O12 was performed. As in the case of Bi3LaTi3O12, it reveals a lowering of symmetry with respect to the idealized structure (space group I4/mmm) of Aurivillius phases. The structure of Bi4-xLaxTi3O12 (x = 1 and 2) compounds has been refined from powder X-ray diffraction data, showing that the interlayer mismatch between fluorite-like and perovskite-like layers is relieved by the conjuction of cation disorder and lowering of symmetry. HREM studies performed on Bi4-xLaxTi3O12 (x = 0, 0.75, and 2) compounds show that the La-containing compounds exhibit intergrowth defects within a thickness of ∼5 nm from the crystal surface to bulk. Taking into account both the enrichment of the surface with La and Bi, and the preference of La3+ for a 12-fold coordination, a model is proposed for the formation of such defects. They correspond to the growth of A-site cation-deficient perovskite compounds, (Bi,La)3.33Bi2Ti2O12 and (Bi,La)3.66Bi3TiO12.