Cation disorder in 3-layer Aurivillius phases: structural studies of Bi_2-xSr_xTi_1-xNb_2+xO₁₂ (0 <x <0.8) and Bi_4-xLa_xTi₃O₁₂ (x =1 and 2)

The solid solutions Bi2-xSr2+xTi1-xNb2+xO12 (0 < x < 0.6). (1) and Bi4-xLaxTi3O12 (x = 1 and 2) (2) have been analyzed in detail by a combination of powder X-ray and neutron diffraction, Both solid solutions adopt a tetragonal variant of the archetypal three-layer Aurivillius phase structure of the parent phase Bi4Ti3O12. The X-ray studies of (1) clearly show that, contrary to earlier assumptions, Sr2+ partially substitutes into the [Bi2O2] layers, even at the stoichiometric composition Bi2Sr2TiNb2O12, the maximum amount of substitution being about 30% at x = 0.6. The neutron study demonstrates that the Sr2+ in this site adopts a significantly more isotropic coordination environment than Bi3+, thus stabilizing this unexpected disorder. The disorder is shown to be driven by the requirement for size matching of the fluorite-like and perovskite-like layers. A similar level of disorder is found in Bi2La2Ti3O12, and this is shown to be due to the requirements for optimum local coordination environment at the La3+/Bi3+ sites. (C) 2000 Academic Press.