Abstract
Low-frequency (100 Hz–1 MHz) dielectric permittivity and infrared reflectivity studies were performed for barium-calcium-titanate single crystals of the congruently melting composition Ba0.77Ca0.23TiO3, nominally pure (BCT77/23) and doped by Cr (70 ppm, BCT77/23:Cr). Both the materials reveal a sharp ϵ′(T) maximum at 375 K (cooling run) with a pronounced temperature hysteresis, evidencing a first-order ferroelectric phase transition (PT) O h → C 4v. The high-temperature wing of the ϵ′(T) maximum reveals a weak dispersion of a non-relaxation nature. The low-temperature wing contains a reproducible step in the ϵ′(T) dependence. Another low-temperature PT was found at ∼41 K. Cr doping strongly influences this PT shifting the position of the ϵ′(T) maximum to ∼20 K. Polarised IR reflectivity shows a huge A 1 − E splitting of TO 1 polar phonons below the ferroelectric PT as in the pure BaTiO3. IR spectra indicate a displacive and diffuse character for the low temperature PT, but leave open the question about the predominance of displacive or order-disorder character of the high temperature PT.
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