Cation disorder in Bi2Ln2Ti3O12 Aurivillius phases (Ln = La, Pr, Nd and Sm)

The synthesis and structure of triple layered Bi2Ln2Ti3O12 Aurivillius phases (Ln=La, Pr, Nd and Sm), prepared from K2Ln2Ti3O10 Ruddlesden-Popper precursors, has been investigated. These materials adopt a body centred tetragonal structure (space group I4/mmm, with unit cell parameters a∼3.8 Å and c∼33 Å) comprising a regular intergrowth of [Bi2O2]2+ fluorite-type and [Ln2Ti3O10]2− perovskite-type layers. A significant degree of cation disorder is present in the Bi2Ln2Ti3O12 system, involving the cross-substitution of Ln/Bi cations onto the Bi/Ln sites in the fluorite- and perovskite-type layers, respectively. As the size of the lanthanide cation is reduced, Bi/Ln disorder is significantly suppressed due to the effect of bond length mismatch in the perovskite-type layer in the crystal structure of Bi2Ln2Ti3O12. This offers a potential strategy for the chemical control of cation disorder in the Bi2Ln2Ti3O12 system.