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Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

Infrared reflectivity spectra of lead zirconate ceramics were measured in the frequency region of 20-3000 cm-1 and temperature range from 10 to 900 K. The data, extended to lower frequencies by coherent source submillimetre, time-domain terahertz transmission and microwave dielectric spectroscopies, were fitted with the factorized model of the dielectric function. Besides the polar optical phonons, one of which slightly softens in the vicinity of the antiferroelectric phase transition, a central-mode type dispersion was revealed in the 5-25 cm-1 (1011-1012 Hz) range. This central mode is found to be responsible for the strong dielectric anomaly in the paraelectric phase, whereas the lattice phonon contribution does not exceed e'~300 in the vicinity of Tc. We attribute the origin of the central mode to the lattice disorder caused by strongly anharmonic hopping of Pb ions.

Dielectric, depolarized light scattering (LS) and optical Kerr effect (OKE) data are critically discussed in an attempt to achieve a common interpretation of the evolution of dynamic susceptibility in molecular glass formers at temperatures down to the glass transition T g . The so-called intermediate power-law, observed in OKE data below a certain temperature T x , is identified with the excess wing, long since known from dielectric spectroscopy, with a temperature-independent exponent. This is in contrast with several recent analyses that concluded a considerable temperature dependence of spectral shapes. We introduce a new approach to disentangle α-peak and excess wing contributions in the dielectric spectra, which allows for frequency-temperature superposition (FTS) of the α-process at all temperatures above Tg. From the LS spectra we conclude, in particular, that FTS holds even at temperatures well above the melting point, i.e. in normal equilibrium liquids. Attempting to correlate the fragility and stretching, our conclusions are opposite to those made previously. Specifically, we observe that a high fragility is associated with a less stretched relaxation function.