Broad-band dielectric response of 0.5Ba(Ti_0.8Zr_0.2)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ piezoceramics: soft and central mode behaviour

Dielectric properties of 0.5Ba(Ti_0.8Zr_0.2)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ 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 transitions strongly resemble a crossover between the displacive 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 below 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 Raman spectra. Activation of these modes is ascribed to the presence of polar nanoclusters in the material.