Order, disorder and structural modulations in Bi–Fe–W–O–Br Sillén–Aurivillius intergrowths

Abstract

Transmission electron microscopy observations on a new complex oxybromide with nominal composition Bi4Fe1/3W2/3O8Br, heated at high temperature, reveal the transformation of its basic structure yielding two types of crystals. The first crystal type shows ordered and disordered extended defects leading to a new family of intergrowths between one Sillén block and n Aurivillius blocks and occasionally between one Aurivillius block and n Sillén blocks. The second type presents a compositionally modulated structure, determined by electron diffraction, with an average composition Bi4Fe1/2W1/2O8 − δBr and unit-cell parameters a = (1/γ) 3.8, b = 3.8, c = 14.5 Å (γ = 0.10–0.15) in the superspace group Immm[(1 − γ)00] no. 71.1.