Abstract
The crystal structure of the ferroelastic and ferroelectric tungsten bronze Ba2NaNb5O15 (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 Cmm2 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.
Highlights
The tetragonal tungsten bronze (TTB) structure is comprised of corner-sharing octahedra that are linked in such a way that triangular, square, and pentagonal sites are formed
BNN is a uniaxial and proper ferroelectric material with a remarkably high Curie temperature (585 °C), which goes through a second ferroelastic phase transition at 260 °C.1−3 The Curie temperature of BNN is dependent on the thermal history, which was described already in 1969.4 This effect is well studied in another tungsten bronze, SrxBa1−xNb2O6 (SBN), and has been attributed to cation ordering from density functional theory (DFT) calculations as well as experiments.[5−8] The tetragonal tungsten bronze structure of BNN, first published by Jamieson,[1] has an aristotype phase with P4/mbm symmetry, as it is for all tetragonal tungsten bronzes
The BNN crystal structure is polar along the c direction, ferroelastic in the ab plane with a variety of possible tilt modulations, and significant cation disorder at room temperature is evident in BNN
Summary
The tetragonal tungsten bronze (TTB) structure is comprised of corner-sharing octahedra that are linked in such a way that triangular, square, and pentagonal sites are formed. The ferroelastic structure has been a matter of discussion; it was determined to be Cmm[2] by Jamieson and Niizeki et al.,[1,9] Ccm[21] by Toledano,[10] Bbm[2] by Labbe et al in 1990,11 and Ferriol concluded in 2001 that Pba[2] was the room temperature symmetry.[12] The difference between these structures is whether the ferroelastic distortion comes along the [1 1 0] or [1 0 0] directions of the aristotype tetragonal cell, and whether octahedral tilts cause a larger unit cell. The structure is affected by incommensurate modulations, a characteristic feature of a whole range of tungsten bronzes.[11,13]
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