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

The synthesis and structure of triple layered Bi{sub 2}Ln{sub 2}Ti{sub 3}O{sub 12} Aurivillius phases (Ln=La, Pr, Nd and Sm), prepared from K{sub 2}Ln{sub 2}Ti{sub 3}O{sub 10} Ruddlesden-Popper precursors, has been investigated. These materials adopt a body centred tetragonal structure (space group I4/mmm, with unit cell parameters a{approx}3.8 A and c{approx}33 A) comprising a regular intergrowth of [Bi{sub 2}O{sub 2}]{sup 2+} fluorite-type and [Ln{sub 2}Ti{sub 3}O{sub 10}]{sup 2-} perovskite-type layers. A significant degree of cation disorder is present in the Bi{sub 2}Ln{sub 2}Ti{sub 3}O{sub 12} 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 Bi{sub 2}Ln{sub 2}Ti{sub 3}O{sub 12}. This offers a potential strategy for the chemical control of cation disorder in the Bi{sub 2}Ln{sub 2}Ti{sub 3}O{sub 12} system.