The structural ferroelectric-paraelectric transition has been definitely observed by electron diffraction tomography in the tetragonal tungsten-bronze (TTB) Sr0.35Ba0.61Nb2O6.04 (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).
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 νTHz), 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 TC near 1 GHz and hardens on cooling, consistent with the oscillations of the ferroelectric domain walls.