Dynamics of the phase transitions in Bi-layered ferroelectrics with Aurivillius structure: Dielectric response in the terahertz spectral range

We have investigated the dielectric response in the terahertz (THz) spectral range of a series of Bi-layered ferroelectrics with Aurivillius structure involving ferroelectric compounds (Bi4 Ti3 O12, Sr Bi2 Ta2 O9, Sr Bi2 Nb2 O9), a relaxor ferroelectric (Ba Bi2 Nb2 O9), and intermediate-type compound (Sr0.5 Ba0.5 Bi2 Ta2 O9). The lowest-frequency polar phonons were studied by means of the time-domain THz transmission spectroscopy in the frequency range 0.1-2 THz at temperatures 10-950 K. Although previous structural studies suggested a displacive character of the structural phase transitions, no soft-mode anomalies were observed in our THz spectra near Tc in any of the investigated compounds. A gradual and only partial softening of the lowest frequency polar phonon was revealed during heating. Dielectric anomalies near Tc in all the compounds should be caused by slowing down of relaxations, directly observed in some cases below the polar phonon range. The ferroelectric transitions are therefore not classically displacive. In analogy to other relaxor ferroelectrics, existence of dynamic polar clusters is suggested to be in origin of such relaxations. Ferroelectric transitions in such cases are connected with an abrupt freezing and rise of these clusters into domains and the classical division of phase transitions into displacive and order-disorder is no more sufficient.