Lattice dynamics and central-mode phenomena in the dielectric response of ferroelectrics and related materials

In this review we describe and discuss recent results on the linear dielectric response of high-permittivity dielectric crystals and ceramics in the wide frequency range 10(2)-10(14) Hz. Our attention is paid to materials which exhibit some interesting low-frequency polar-phonon anomalies in combination with some additional dielectric dispersion below the polar phonon region. The following compounds are discussed: microwave ceramics Bi1.5Zn1.0Nb1.5O7, antiferroelectric PbZrO3 ceramics and AgNbO3 crystals, ferroelectric LiNaGe4O9, LaBGeO5, Cd2Nb2O7 and SrBi2 Ta2O9 crystals, incommensurate ferroelectric Sr2Nb2O7, Ba2NaNb5O15 and BCCD crystals, dipolar-glass crystal of Rb-1/2(ND4)(1/2)D2PO4, relaxor ferroelectric Pb(Sc1/2Ta1/2)O-3 and PLZT ceramics and PMN crystals, antiferroelectric PUT 2/95/5 ceramics and relaxor-based PMN-PT PZN-PT and BiScO3-PT crystals and ceramics. For these materials the polar phonon spectra are discussed together with their flow-frequency dielectric response and dispersion regions in between, in the 10-300 K or higher temperature range, depending on the existing phase transitions of interest. Some universal features of the disorder and anharmonicities are pointed out: central-mode phenomena near displacive phase transitions and polar nano-clusters which appear at rather high temperatures and remain present down to low temperatures if the material remains structurally disordered. The manifestation of the latter in the dielectric spectra consists in extreme broadening of the relaxation region on cooling which at low temperatures results in constant loss spectra (1/f noise).