Structural Evolution of Ferroelectric and Ferroelastic Barium Sodium Niobate Tungsten Bronze

The crystal structure of the ferroelastic and ferroelectric tungsten bronze Ba 2 NaNb 5 O 15 (BNN) has been debated. Here, we re-examine the crystal structure of BNN by ambient powder X-ray diffraction combined with density functional theory calculations. We demonstrate that the room temperature space group is Cmm 2 with significant cation disorder on the Ba and Na cation sublattices. Density functional theory calculations reveal a relatively flat energy landscape between structures of different symmetries, including the energetics of cation disorder. We also study the structural evolution and the ferroelectric and ferroelastic phase transitions by high-temperature X-ray diffraction and dilatometry. The ferroelectric phase transition at 570 °C is of first order and cause the cell to expand in the c direction, while the ferroelastic distortion starting at 270 °C takes place in the ab plane and does not affect the polarization. The phase transitions are not coupled, which means that BNN is a ferroic material with two primary and uncoupled order parameters.