Broad-band dielectric response of doped incipient ferroelectrics

Broad-band dielectric responses (10 2–10 14 Hz) of Sr 1-1.5 x Bi x TiO 3 ( x=0.0067÷0.133) ceramics and K 1- x Li x TaO 3 ( x=0.050÷14) single crystals are analysed and compared. The common features are emphasized: softening of the polar phonon is suppressed by the doping, dielectric dispersion takes place in a wide frequency range below polar phonon modes, multiple relaxation regions and distribution of relaxation times occurs. Mechanisms of the main contributions to the dielectric permittivity and temperature evolution of their characteristic frequencies in doped incipient ferroelectrics are discussed. Dynamics of the soft phonon, off-centred ions and polar clusters, induced by the off-centred ions interacting via crystal lattice, is considered to be responsible for the observed dielectric response. Dielectric response of doped incipient ferroelectrics is compared with that of model relaxor ferroelectrics.