Unusual High‐Temperature Structural Behaviour in Ferroelectric Bi2WO6

Abstract

The crystal structure of Aurivillius phase ferroelectric Bi₂WO₆ 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 P2₁ab to B2cb occurs at about 660 °C. This transition corresponds to the loss of one octahedral tilt mode within the perovskite‐like WO₄ 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 WO₄, with WO₆ octahedra sharing edges and corners, and with the fluorite‐like Bi₂O₂ layers remaining essentially unchanged. This structure is closely related to that of the ambient temperature phase of lanthanide‐doped derivatives, for example, Bi_0.7Yb_1.3WO₆ recently reported. This phase‐transition behaviour is in stark contrast to that of other members of the Aurivillius family, such as SrBi₂Ta₂O₉ and Bi₄Ti₃O₁₂, which retain the archetypal Aurivillius connectivity at all temperatures.