Insights into the polymorphism of Bi2W2O9: single crystal growth and a complete survey of the variable-temperature thermal and dielectric properties

Abstract

Bismuth layered Aurivillius oxides have attracted longstanding research interest due to their unique structural characteristics and outstanding performance in ferroelectric dielectrics. The complex phase evolution process as a function of temperature and pressure in this family also provides fascinating platforms for understanding the chemical origin of their versatility. Herein, high-quality single crystals of a bismuth layered Aurivillius oxide, Bi₂W₂O₉, with dimensions up to 29 × 23 × 11 mm³ were successfully grown via the top-seeded solution growth (TSSG) method using a mixture of WO₃ and Li₂B₄O₇ as a flux. The material crystallizes in an unexpected centrosymmetric orthorhombic Pbcn (no. 60) space group with unit cell parameters a = 23.705(2), b = 5.4148(6), and c = 5.4345(6) Å. A complete survey of the anisotropic and variable-temperature thermal and dielectric properties was carried out with the aim of gaining detailed insights into their polymorphism evolution process. The results demonstrate that, with increasing temperature, the compound undergoes complicated phase transitions and, by analogy, signatures of the transitions are summarized and compared with those in Bi₂WO₆ and Bi₂MoO₆. The phase sequences and transition mechanisms of Bi₂W₂O₉ are proposed by careful inspection and estimation.