Structures and phase transitions in barium sodium niobate tungsten bronze (BNN)

Abstract

The room-temperature structure of the filled tetragonal tungsten bronze, Ba2NaNb5O15 (BNN), has been the subject of a number of studies, and these studies have given an almost corresponding number of different results. From a group theoretical examination of the different possibilities and a review of the published experimental results we conclude that the room-temperature structure is that proposed by Labbé et al. [J. Phys. Condens. Matter (1989), 2, 25–43] in the space group Bbm2 (Ama2 in standard setting) on a 2\sqrt{2}a × \sqrt{2}a × 2c cell. Upon heating, the structure remains ferroelectric but becomes tetragonal (space group P4bm) at 550 K, then paraelectric (space group P4/mbm) at and above 860 K.

Revisiting the structures and phase transitions of Ba2NaNb5O15

“…Considering the main question in this work, the roomtemperature structure of BNN, the excellent fits to the XRD and ND data obtained by Rietveld refinement make it clear that Ama2 is the correct space group below the ferroelastic transition. This is in agreement with recent group theory considerations (Whittle et al, 2021) and with earlier experimental work (Xiao-Qing et al, 1985;Labbe et al, 1990), although it was then only proposed as a good approximation to the true incommensurate structure. Whether Ama2 is only a good average approximation of a true incommensurate structure is not easy to determine from powder data, and is also somewhat of a philosophical question that depends on the point of reference.…”

“…Finally, group theory considerations make a strong argument that the most likely tilt systems for TTBs are those where the tilt angles are the same, or nearly the same, for all octahedra (Whittle et al, 2018(Whittle et al, , 2021. The symmetry mode approach used in this work gives experimental evidence in support of this.…”

“…Here we only mention that recent X-ray powder diffraction (XRD) data suggest Cmm2 (Aamlid et al, 2020), while group theory considerations and a thorough review of existing literature suggest Bbm2 (No. 40, with a 2 ffiffi ffi 2 p  ffiffi ffi 2 p  2 unit cell relative to the aristotype cell) to be the correct space group (Whittle et al, 2015(Whittle et al, , 2021. The Bbm2 space group is also reported experimentally as a good approximation to a true incommensurate structure (Xiao-Qing et al, 1985;Labbe et al, 1990).…”

“…36, with a ffiffi ffi 2 p  ffiffi ffi 2 p  2 unit cell relative to the aristotype cell) (Toledano, 1975). Since then, several space groups have been suggested, all of which are well summarized in the recent work by Whittle et al (2021). Here we only mention that recent X-ray powder diffraction (XRD) data suggest Cmm2 (Aamlid et al, 2020), while group theory considerations and a thorough review of existing literature suggest Bbm2 (No.…”

“…We start our analysis by determining the room-temperature structure of BNN. On the basis of existing literature, we focused our attention on potential space groups that have their ferroelastic strain along the [110] direction, relative to the parent structure, found to be the most probable according to group theory (Whittle et al, 2021) and experiments (Aamlid et al, 2020). An initial Pawley fit screening was performed with the following space groups: Cmm2, I4, Ama2, Ima2 and Cmc2 1 .…”