The ferroelectric phase transition of the tetragonal tungsten-bronze SBN-35 unveiled

The structural ferroelectric-paraelectric transition has been definitely observed by electron diffraction tomography in the tetragonal tungsten-bronze (TTB) Sr_0.35Ba_0.61Nb₂O_6.04 (SBN-35) from the paraelectric group P4/mbm to the ferroelectric Pmbm. At 625 K, the refined structure shows that the average structure of SBN-35 is tetragonal with an almost negligible orthorhombic distortion [Fig.1].

Fig. 1: Refined structure of SBN-35 at 625 K.

The combination of structural and broad-band dielectric studies in SBN-35 suggests that ferroelectricity in TTBs in caused by a more complex mechanism than in perovskites. Several excitations were identified related to the multiple mechanisms responsible for the ferroelectric phase transition [Fig.2]:

• Phonons, related to cation displacements along the polar axis,
• An anharmonic excitation located in the THz range (the CM ν_THz), caused by the dynamic disorder of Sr and Ba atoms located at the A2 sites in the pentagonal channels, as supported by the high anisotropic displacements found in the electron diffraction experiment.
• A relaxation in the GHz range, ν₀₁, which slows down to several MHz on cooling and related probably to Nb atoms dynamics.
• A relaxation which appears in the spectra below T_C near 1 GHz and hardens on cooling, consistent with the oscillations of the ferroelectric domain walls.

Results were published in [1].

Fig. 2: Temperature dependences of the frequencies of the different excitations found in SBN-35 and their dielectric contributions.

References
[1] E. Buixaderas, M. Kempa, V. Bovtun, C. Kadlec, M. Savinov, F. Borodavka, P. Vaněk, G. Steciuk, L. Palatinus, and J. Dec, Multiple polarization mechanisms across the ferroelectric phase transition of the tetragonal tungsten-bronze Sr_0.35Ba_0.61Nb₂O_6.04 , Phys. Rev. Materials 2, 124402 (2018).