  Dielectric properties of BTZ-BCT ceramics were probed in the frequency range
from 10 Hz to 100 THz in a broad temperature range (10-900 K). Polar soft
phonon observed in infrared spectra softens with cooling, however below 500 K
its frequency becomes temperature independent. Simultaneously, a central mode
activates in terahertz and microwave spectra; and it actually drives the
ferroelectric phase transitions. Consequently, the phase transition strongly
resemble a crossover between the dispacive and order-disorder type. The central
mode vanishes below 200 K. The dielectric relaxation in the radiofrequency and
microwave range anomalously broadens on cooling below T$_{C1}$ resulting in the
nearly frequency independent dielectric loss bleow 200 K. This broadening comes
from a broad frequency distribution of ferroelectric domain wall vibrations.
Raman spectra reveal new phonons below 400 K, i.e. already 15 K above T$_{C1}$.
Several weak modes are detected in the paraelectric phase up to 500 K in the
Raman spectra. Activation of these modes is ascribed to the presence of polar
nanoclusters in the material.
