Unusual High‐Temperature Structural Behaviour in Ferroelectric Bi 2 WO 6

The crystal structure of Aurivillius phase ferroelectric Bi 2 WO 6 has been studied in detail as a function of temperature by using high‐resolution powder neutron diffraction. In agreement with an earlier study, a transition from space group P 2 1 ab to B 2 cb occurs at about 660 °C. This transition corresponds to the loss of one octahedral tilt mode within the perovskite‐like WO 4 layer of the structure. A second, reconstructive, phase transition occurs around 960 °C, corresponding to the ferroelectric Curie point; in contrast to previous suggestions, the structure of this high‐temperature phase contains layers of stoichiometry WO 4 , with WO 6 octahedra sharing edges and corners, and with the fluorite‐like Bi 2 O 2 layers remaining essentially unchanged. This structure is closely related to that of the ambient temperature phase of lanthanide‐doped derivatives, for example, Bi 0.7 Yb 1.3 WO 6 recently reported. This phase‐transition behaviour is in stark contrast to that of other members of the Aurivillius family, such as SrBi 2 Ta 2 O 9 and Bi 4 Ti 3 O 12 , which retain the archetypal Aurivillius connectivity at all temperatures.