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On the basis of existing experiments, the grain-size effect on the permittivity and dielectric function in the case of ferroelectric and relaxor ferroelectric dense single-phase ceramics is reviewed and critically discussed. Relevant data exist for BaTiO3 (BTO), SrTiO3 (STO), Pb(Zr,Ti)O3 (PZT), Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). In the case of displacive ferroelectrics, the grain-size effect (decrease of the permittivity with decreasing grain size) can be attributed to a narrow low-permittivity grain-boundary layer (dead layer), whose effect can be modeled as a core-shell composite using an effective-medium approach. The strongest size effect was observed for STO, for which a model of two dead layers with a microscopic meaning was suggested. In the case of relaxor ferroelectrics it appears that rather thick layers near grain boundaries are influenced in such a way that the slow relaxation dispersion is there missing, which could be interpreted as pinning of the polar nano-regions by the grain boundaries, which prevents their flipping. In the ferroelectric phases, the grain-size effect is dominated by the domain structure (domain-wall contribution), which is processing and grain-size dependent and is present even in nano-grain ceramics. [ABSTRACT FROM AUTHOR]

This paper reviews the characteristic dielectric dispersion of relaxor ferroelectrics PbMg1/3Nb2/3O3, and (Pb1-xLax)(ZryTi1-y)1-x/4O3 and ferroelectric BiScO3-PbTiO3 solid solutions in the frequency range of lO0 Hz-2 THz. In relaxors the main dielectric dispersion at high temperatures appears in the microwave and mm-wave region, which softens and broadens on cooling. Distribution of relaxation times below room temperature is very broad and below freezing temperature it results in nearly frequency independent dielectric loss in the entire available frequency range. The broad distribution of relaxation times stems from structural inhomogeneities which cause random fields and have influence on the distribution of activation energies for hopping of disordered ions. Landau-type thermodynamic model was proposed for explanation of the distribution of relaxation times. Besides the frequency independent dielectric loss, additional dielectric relaxation of not thermally activated origin was observed in BiScO3-64%PbTiO3 below 1 GHz. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue. Articles published within the issue were taken from the XVII Czech -- Polish Seminar held in Znojmo, Czech Republic on May 22-26, 2006. The topics covered include the physics of the ferroelectrics and relaxor ferroelectrics with perovskite structure as well as phase transitions, aspects of ferroelectricity and structural phase transitions.

Abstract: Terahertz (THz) transmissivity and infrared (IR) reflectivity spectra of orthorhombic microwave (MW) ceramics Bi(Nb1−x V x )O4 (0.002< x <0.032) were measured between 4 and 3000cm−1 (0.09–90THz) at room temperature. A well underdamped mode, presumably the ferroelectric soft mode, was observed at 25cm−1. Complex permittivity spectra obtained from the fits to our data were extrapolated down to the MW range and compared with the dielectric data near 5GHz. The linear extrapolation of dielectric losses from THz down to the MW range is in agreement with the experimental MW losses. Addition of 3.2% of vanadium reduces the sintering temperature to 850°C and the dielectric properties (ɛ′=42.2, Q·f =14,000GHz, τ f =+10ppm/°C) remain at a level satisfactory for MW applications. Somewhat lower MW losses were observed in a sample sintered in the N2 atmosphere. [Copyright &y& Elsevier]

Broad-band dielectric responses (102–1014 Hz) of Sr1-1.5xBixTiO3 (x=0.0067÷0.133) ceramics and K1-xLixTaO3 (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. [Copyright &y& Elsevier]