Kamba, Stanislav and Simon, E. and Skoromets, V. and Bovtun, Viktor and Kempa, Michael and Pokorny, J. and Savinov, M. and Koruza, Jurij and Malič, Barbara (2016):
Broad-band dielectric response of 0.5Ba(Ti0.8Zr0.2)O3-0.5 (Ba0.7Ca0.3)TiO3 piezoceramics: soft and central mode behaviour.
In: Phase Transitions, 89 (7-8), Taylor&Francis, ISSN 0141-1594,
[Article]

Abstract

Dielectric properties of 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)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 TC1, 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 revel new phonons below 400 K, i.e. already 15 K above TC1. 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.

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