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.
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