Polar phonons and specific features of the ferroelectric states in cadmium titanate

Polarized spectra of reflectance R(ν) and transmittance Tr(ν) of single-crystal CdTiO₃ samples have been obtained in the frequency range 7 < ν < 1000 cm^?1 and for temperatures from 5 to 300 K using IR Fourier spectroscopy and submillimeter-range techniques. Dispersion analysis was carried out in terms of the additive-oscillator model, and dielectric responses ε(ν) and ε″(ν) were calculated. The polar modes were assigned to particular symmetry types, and their oscillator parameters (dielectric contributions, normal frequencies, damping constants) were determined. The numerical values of the components of the static permittivity tensor, ε₁₁ and ε₃₃, are shown to be almost fully determined by the total dielectric contributions due to the B_3u and B_1u phonons, respectively. In the low-frequency domain, lines showing anomalous behavior of the oscillator parameters, which is characteristic of soft ferroelectric modes, were observed. It is shown that, in CdTiO₃ at cryogenic temperatures, there exist several different polar states with switching in the direction of the spontaneous polarization vector.