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

Transmission electron microscopy observations on a new complex oxybromide with nominal composition Bi(4)Fe(1/3)W(2/3)O(8)Br, 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 Bi(4)Fe(1/2)W(1/2)O(8 - delta)Br and unit-cell parameters a = (1/gamma) 3.8, b = 3.8, c = 14.5 A (gamma = 0.10-0.15) in the superspace group Immm[(1 - gamma)00] no. 71.1.