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For the first time, the dielectric properties of the 9:1:7 TTB phases Ba9M2+Nb14O45 (M = Co, Mg, Zn) which form in Ba-deficient high-Q perovskite niobates Ba(M2+ 1/3Nb2/3)O3 have been measured in the radio-frequency and microwave range. The low-temperature dielectric relaxation found in Ba9M2+Nb14O45 was shown to give a strong contribution into their microwave dielectric loss. The measured data were used for a simple simulation of the Q-factor of a perovskite-TTB composite. Calculated parameters have been shown to correspond well to the measured data.

In this study, ceramic samples of lead zirconate titanate Pb(Zr0.42Ti0.58)O3 (PZT 42/58) doped with Fe (hard) and Nb (soft) were studied by Raman, infrared (IR) and THz spectroscopy in the temperature range from 900 to 20 K. From the evaluation of the IR and Raman phonons in undoped ceramics, we conclude that tetragonal PZT ceramics undergo a low-temperature phase transition to a tilted phase, revealed by anomalies in phonon parameters and the appearance of new modes. Doped samples also presented similar behaviour. The main differences between undoped and doped ceramics lie in the dielectric behaviour below phonon frequency range, where several mechanisms appear: a soft central mode (CM) located near 1 THz and two relaxations in the GHz range. Both types of doping raise the permittivity values below the phonon frequencies, not only increasing the dielectric contribution of the CM, but also modifying the dielectric response near the GHz range. Soft ceramics show higher permittivity with logarithmic increas...

Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

Broad-band dielectric responses (10 2 -10 14 Hz) of Sr 1-1.5x 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.

Dielectric response of two types of nominally pure hot-pressed PbZrO3 ceramics (PZ1 and PZ2) of similar density (97–98%) was studied in the 1 MHz–1 GHz and far infrared frequency ranges in a wide temperature interval. While the far infrared spectra of both ceramics did not significantly differ from that of the crystal and the permittivity of PZ1 appeared to be comparable to that of the crystal, the permittivity of PZ2 was much lower. This reduction was particularly drastic (of about four times) on approaching the antiferroelectric phase transition from the paraelectric phase. We succeeded to explain this effect using a brick-wall model assuming nano-cracks development along some of the grain boundaries in PZ2 in agreement with SEM observations and unusual mechanic fragility of this ceramics. Our estimates show that even small volume fraction (

Ferroelectric tantalates with the tetragonal tungsten bronze (TTB) type structure were studied over a wide frequency range from 10 2 to 10 9 Hz. Complementary microwave measurements at 36 GHz and micro-Raman experiments were performed in order to extend the dielectric study. A high frequency relaxation was observed above 10 8 Hz and fitted to the Cole–Cole formula. Different qualitative models are proposed to explain the understanding of the origin of the high frequency relaxation in ferroelectrics. Among these, polar clusters of ferroelectric active off-centred Ta 5+ ions are considered.

Dielectric properties of the Sr 1 m 1.5x Bi x TiO 3 ceramics (x = 0.002 - 0.167) were investigated in a wide frequency range 10 2 -10 14 Hz. Relaxational dispersion, characteristic for relaxor ferroelectrics, was observed for x S 0.0067. On heating it expands to higher frequencies, up to the submm range at T S 200 K. Two polarization processes, related to the hopping of Bi ions and to the polar cluster dynamics are considered to contribute to the relaxor behaviour.

Infrared reflectivity spectra of lead zirconate ceramics were measured in the frequency region of 20-3000 cm-1 and temperature range from 10 to 900 K. The data, extended to lower frequencies by coherent source submillimetre, time-domain terahertz transmission and microwave dielectric spectroscopies, were fitted with the factorized model of the dielectric function. Besides the polar optical phonons, one of which slightly softens in the vicinity of the antiferroelectric phase transition, a central-mode type dispersion was revealed in the 5-25 cm-1 (1011-1012 Hz) range. This central mode is found to be responsible for the strong dielectric anomaly in the paraelectric phase, whereas the lattice phonon contribution does not exceed e'~300 in the vicinity of Tc. We attribute the origin of the central mode to the lattice disorder caused by strongly anharmonic hopping of Pb ions.

Wide range dielectric response (10–10 14 Hz) of three relaxor ferroelectrics: PMN, PLZT and PST is analyzed. The common features are: low phonon contribution to the high static permittivity; dielectric relaxation in a wide frequency range; widening of the relaxation times distribution with decreasing temperature down to the freezing temperature T f , where extrapolated divergence of the mean relaxation time takes place; change of dielectric behaviour at the Burns temperature T B . Dynamics of polar nanoclusters, which appear below T B , is considered to be responsible for the relaxor behaviour. It enables us to define the general structure of the dielectric spectrum of relaxor ferroelectrics. Only above T B the fundamental dielectric contribution is due to polar phonons. Below T B the contributions caused by polar clusters dynamics prevail: both dipole reversal of polar cluster and fluctuations of their boundaries contribute to the dielectric response.

Dielectric properties of the AgNbO3 (AN) and Ag1−xLixNbO3 ceramics with x = 0.02, 0.04, 0.06 were investigated at microwaves (MW, 36 GHz) in the temperature range of 100 ÷ 570 K. including the phase transitions in AN at 340K (weak ferroelectric - antiferroelectric) and at 530K (antiferroelectric-antiferroelectric). The results of MW measurements were compared with radio frequency and IR dielectric properties. No significant dielectric dispersion was observed. The anomalies of e′(T) and e″(T), corresponding to the phase transitions, were observed in AN and in Ag1−xLixNbO3. The Li additives for x ≤0.04 do not result in significant change of the MW dielectric properties. The ceramic with x = 0.06 shows a much lower MW e′ below 250 K and a sharp step-like anomaly of e′(T) at ≈ 425 K. This indicates a significant change of the structure which is also confirmed by the pronounced change in the IR reflectivity.

Microwave dielectric properties at ∼35 GHz of B-site ordered (PST-O) and disordered with Pb-vacancies (PST-DV) relaxor ceramics Pb(Sc½Ta½)O3 were investigated in the temperature range 100 ÷ 550 K. The results were compared and fitted together with the new and earlier data in the 100 Hz ÷ 1 GHz range and recent data on infrared and submillimetre response (3·1011 ÷ 1014 Hz). The dielectric response below polar phonon frequencies comprises two separate dispersion regions in both types of samples from which only the lower frequency one (108 ÷ 109Hz) shows partial critical slowing down. The basic role of the Pb disorder and ordered nano-clusters in the paraelectric phase is emphasized.

Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to Tc1≃770 K, which supports the ferroelasticity and Amam symmetry of the intermediate phase between ferroelectric and paraelectric phases. Far-infrared spectra of SBT ceramics, single crystal and thin films show a well underdamped optical soft mode at 28 cm−1, which partially softens to 21 cm−1 near the ferroelectric transition temperature (Tc2≅600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near Tc2. On the basis of high-frequency measurements, which do not show a significant dispersion, central-mode-type dispersion in the 10–100 GHz range is proposed as an explanation. So, the phase transition at Tc2 apparently shows a crossover behavior between the displacive and the order–disorder type.

Lattice dynamics and dielectric relaxation phenomena of $({\mathrm{Sr}}_{1\ensuremath{-}1.5\mathrm{x}}{\mathrm{Bi}}_{\mathrm{x}}){\mathrm{TiO}}_{3}$ ceramics with $x=0.0067--0.133$ were studied in the temperature range of 10--400 K. The polar ${\mathrm{TO}}_{1}$ soft mode in the IR spectra is strongly stiffened by Bi doping and its softening on cooling is gradually stopped. Increasing degree of local polar distortion induced by doping was detected by progressive activation of ${\mathrm{TO}}_{2}$ and ${\mathrm{TO}}_{4}$ polar modes in the Raman spectra. Wide range ${(10}^{2}--1.8\ifmmode\times\else\texttimes\fi{}{10}^{9};$ ${10}^{11}--2\ifmmode\times\else\texttimes\fi{}{10}^{13}\mathrm{Hz})$ dielectric spectra were determined and carefully fitted with an appropriate model of relaxation time distribution. At least four relaxation regions were revealed and the dominant thermally activated region was assigned to nanocluster polarization dynamics. The enhanced absorption in the submillimeter range was assigned to anharmonic quasi-Debye losses.

The complex dielectric response of ${\mathrm{Bi}}_{1.5}{\mathrm{Zn}}_{1.0}{\mathrm{Nb}}_{1.5}{\mathrm{O}}_{7}$ cubic pyrochlore ceramics was investigated between 100 Hz and 100 THz by a combination of low-frequency capacitance bridges, a high-frequency coaxial technique, time domain transmission THz spectroscopy, and infrared spectroscopy. The data obtained between 10 K and 400 K revealed glasslike dielectric behavior: dielectric relaxation is observed over a wide frequency and temperature range, and the dielectric permittivity and loss maxima shift to higher temperature values by almost 200 K with increasing measuring frequency. The distribution of relaxation frequencies broadens on cooling and can be described by a uniform distribution. The high-frequency end of the distribution at $\ensuremath{\sim}{10}^{11} \mathrm{Hz}$ is almost temperature independent and its low-frequency end obeys the Arrhenius Law with an activation energy of $\ensuremath{\sim}0.2 \mathrm{eV}.$ The relaxation is assigned to the local hopping of atoms in the A and O' positions of the pyrochlore structure among several local potential minima. The barrier height for hopping is distributed between 0 and 0.2 eV. Such an anomalously broad distribution may have its origin in the inhomogeneous distribution of ${\mathrm{Zn}}^{2+}$ atoms and vacancies on ${\mathrm{Bi}}^{3+}$ sites, which gives rise to random fields and nonperiodic interatomic potential. Frequency independent dielectric losses $(1/f$ noise) are observed at low temperatures, which seems to be a universal behavior of disordered systems at low temperatures.

Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_{3}$ ceramics in the 10--300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\ensuremath{\omega}}_{\mathrm{TO}1}^{\ensuremath{-}\ensuremath{\alpha}}(\ensuremath{\alpha}\ensuremath{\approx}1.6)$ where ${\ensuremath{\omega}}_{\mathrm{TO}1}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the ${E}_{g}$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_{6}$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller ${A}_{1g}\ensuremath{-}{E}_{g}$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.

Dielectric, depolarized light scattering (LS) and optical Kerr effect (OKE) data are critically discussed in an attempt to achieve a common interpretation of the evolution of dynamic susceptibility in molecular glass formers at temperatures down to the glass transition T g . The so-called intermediate power-law, observed in OKE data below a certain temperature T x , is identified with the excess wing, long since known from dielectric spectroscopy, with a temperature-independent exponent. This is in contrast with several recent analyses that concluded a considerable temperature dependence of spectral shapes. We introduce a new approach to disentangle α-peak and excess wing contributions in the dielectric spectra, which allows for frequency-temperature superposition (FTS) of the α-process at all temperatures above Tg. From the LS spectra we conclude, in particular, that FTS holds even at temperatures well above the melting point, i.e. in normal equilibrium liquids. Attempting to correlate the fragility and stretching, our conclusions are opposite to those made previously. Specifically, we observe that a high fragility is associated with a less stretched relaxation function.

The dielectric properties of undoped, Nb-, and Fe-doped Pb(Zr,Ti)O3 ceramics with composition near morphotropic phase boundary were investigated in the frequency range from 1 MHz to 20.2 GHz at room temperature. Temperature dependences of dielectric permittivity e′ and loss e″ are measured at 100 kHz from 50 to 300 K and around 13.4 GHz from 100 to 300 K. These measurements permit estimation of the upper limit of the intrinsic permittivity and lower limit of the extrinsic contributions to the permittivity as a function of temperature. The extrinsic contributions account for more than 50% of the quasistatic dielectric permittivity in studied samples.