The dielectric dispersion of the transparent relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 was determined in a wide frequency range including the microwave and infrared range. The number of observed polar phonons in infrared spectra gives evidence about the locally broken cubic symmetry and the presence of polar nanoclusters in the whole investigated temperature range up to 530 K. A single broad and symmetric dispersion that occurs below the polar phonon frequencies was fitted with the Cole-Cole formula and a uniform distribution of Debye relaxations. On decreasing temperature, the distribution of relaxation times becomes extremely broad which indicates increasing correlation among the clusters. The mean relaxation time diverges according to the Vogel-Fulcher law with the same freezing temperature 230±5 K for both ceramics, but different activation energies 1370 K and 1040 K for the 8/65/35 and 9.5/65/35 sample, respectively. The shortest relaxation time is about 10-12 s and remains almost temperature independent. Below room temperature, the loss spectra become essentially frequency independent and the permittivity increases linearly with decreasing logarithm of frequency. The slope of this dependence is proportional to T  4 in the investigated temperature range (above 210 K) which indicates appreciable anharmonicity of the potential for polarization fluctuations.

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Dielectric relaxation phenomena are studied in detail at the 107 K structural phase transition in SrTiO 3 . These loss phenomena are explained in terms of the dynamics of domain walls that occur at the cubic-totetragonal phase transition and exist as precursor phenomena ͑locally ordered regions͒ in the high-temperature phase. We speculate that the same dynamics is responsible for the occurrence of a central peak in neutronscattering experiments and we predict a relaxation rate of 1ϫ10 6 sec Ϫ1 at the structural phase transition. Finally we compare our results with predictions of ⌽ 4 lattice models. ͓S0163-1829͑97͒05742-1͔ Three decades ago SrTiO 3 was studied intensively experimentally, as well as theoretically. It was characterized as an incipient ferroelectric, with a Curie temperature close to 40 K and it has been shown conclusively that quantum fluctuations suppress long-range ferroelectric order. 1 Utilizing inelastic neutron-scattering techniques, Cowley 2 has demonstrated that the Lyddane-Sachs-Teller relation holds in this material. At T S ϭ107 K, SrTiO 3 undergoes a structural phase transition from a cubic high-temperature phase into a tetragonal ground state. This phase transition is close to second order and is driven by the softening of a transverse optical phonon at the zone-boundary R point of the Brillouin zone. The soft modes are well defined and underdamped and can be followed as far as close to T S . 3 Recently, SrTiO 3 attracted renewed interest: Müller and co-workers 4 detected indications of a low-temperature phase transition in electronparamagnetic resonance ͑EPR͒ experiments. This observation was interpreted as a transition into a coherent quantum state.Above the 107 K structural phase transition, in addition to the soft mode a central peak ͑cp͒ has been detected at the 1 2 ͑113͒ reciprocal lattice point. 5,6 Later on cp phenomena have been found to be ubiquitous at structural phase transitions but still they are theoretically not well understood. In neutron-scattering studies the cp in SrTiO 3 has been observed some 50 K above the structural phase transition temperature with a weight that grows relative to that of the phonon sidebands and diverges for T→T S . The most puzzling phenomenon is the extreme narrowness of the central peak: Even in high-resolution neutron-scattering studies it has been shown that the width of the cp close to T S was limited by the experimental resolution and the width was estimated to be smaller than 0.08 mev (ϳ2ϫ10 7 sec Ϫ1 ). 7 From EPR experiments 8 a relaxation rate of 6ϫ10 7 sec Ϫ1 has been deduced at TϭT S ϩ2 K. And finally, from Mössbauer experiments using 14.4 keV ␥ rays, 9 it has been concluded that the energy width at T S must be smaller than 8 ϫ10 8 eV, corresponding to an upper limit of the relaxation rate of 2ϫ10 7 sec Ϫ1 .Presently, no unique and fully accepted theoretical description exists. The cp in SrTiO 3 has been described by Schwabl 10 using a phenomenological ansatz, where the phonons are coupled to an external mode or by Siberglitt 11 and b...

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Dielectric response of PLZT 2/95/5 ceramics was studied by classical bridge method. coaxial radio-frequency and waveguide microwave techniques, submillimetre transmission and infrared reflection spectroscopy in the temperature region 10 -550 K. Large dielectric anomaly appears near the phase transition from paraelectric to incommensurate phase (Ti=470 K) due to a critical relaxation in the GHz region whose frequency softens on cooling in the paraelectric phase and disappears in the antiferroelectric phase below 380 K. Many new phonon modes were resolved in the infrared reflectivity spectra at 10 K due to the Brillouin zone folding in the antiferroelectric phase and due to the low phonon damping. Both these facts show that the antiferroelectric phase is fully ordered.