Multiple polarization mechanisms across the ferroelectric phase transition of the tetragonal tungsten-bronze Sr0.35Ba0.69Nb2O6.04

The broadband dielectric response of the uniaxial ferroelectric strontium barium niobate with 35% Sr has been studied from 1 kHz to 30 THz along the polar $c$ axis using high-frequency and low-frequency dielectric spectroscopies, time-domain terahertz spectroscopy, and far-infrared reflectivity, in the wide temperature interval 20--600 K. The mechanisms that contribute to the ferroelectric phase transition and to the dielectric anomaly in this material are the softening of an anharmonic excitation in the THz range related to cation hopping and the slowing down of a relaxation in the GHz range, which saturates below ${T}_{\mathrm{C}}$ near 1 MHz. In the ferroelectric phase, a relaxation, related to domain-wall dynamics, appears in the sub-GHz range and hardens on cooling up to 10 GHz. The ferroelectric-paraelectric transition, investigated by electron diffraction, has been assessed to the appearance of a supplementary mirror plane perpendicular to the polar axis.