[en] Dielectric properties of Eu_0.5Ba_0.5TiO₃ ceramics were investigated between 10 and 300 K in the frequency range of 1 MHz-100 THz. Permittivity exhibits a strong peak near the ferroelectric phase transition at 215 K. This is mainly due to softening of the lowest frequency polar phonon revealed in THz and infrared spectra. Dielectric relaxation was observed also below the ferroelectric soft mode frequency in the whole investigated temperature region, but it is probably caused by some defects such as Eu³⁺ cations or oxygen vacancies. This implies that the ferroelectric phase transition has predominantly a displacive character. Raman scattering spectra revealed a lowering of crystal symmetry in the ferroelectric phase and XRD analysis indicated orthorhombic A2mm symmetry below 215 K. The magnetic measurements performed at various frequencies in the field cooled and field heating regime after cooling in zero magnetic fields excluded spin glass behavior and proved an antiferromagnetic order below 1.9 K in Eu_0.5Ba_0.5TiO₃.

[en] Pb_1.83Mg_0.29Nb_1.71O_6.39 crystallizing in a cubic pyrochlore structure exhibits a typical feature of quantum paraelectrics--its permittivity continuously increases on cooling and levels off below ∼30 K without any signature of a structural phase transition. Broadband dielectric spectra do not show any dielectric dispersion in the real part of permittivity up to 8.8 GHz. Terahertz and infrared spectra reveal a soft polar optic mode, which is responsible for the temperature dependence of the permittivity. The leveling off of the permittivity at low temperatures obeys the Barrett formula, and the fitted vibrational zero-point energy (1/2)k_BT₁ corresponds to the measured soft-mode frequency. The number of observed infrared phonons exceeds that predicted from the factor-group analysis, which indicates that the structure is at least locally noncubic

[en] Composites of multiwalled carbon nanotubes with poly(ethylene terephthalate) (PET–MWCNT) with up to 3 vol% MWCNTs were prepared and characterized by broad-band AC conductivity and dielectric spectroscopy up to the infrared range using several techniques. A very low electrical percolation threshold of 0.07 vol% MWCNTs was revealed from the low-frequency conductivity plateau as well as from DC conductivity, whose values show the same critical power dependence on MWCNT concentration with the exponent t = 4.3. Above the plateau, the AC conductivity increases with frequency up to the THz range, where it becomes overlapped with the absorption of vibrational modes. The temperature dependence down to ∼5 K has shown semiconductor behaviour with a concentration-independent but weakly temperature-dependent small activation energy of ∼3 meV. The behaviour is compatible with the previously suggested fluctuation-induced tunnelling conductivity model through a thin (∼1 nm) polymer contact layer among the adjacent MWCNTs within percolated clusters. At higher frequencies, deviations from the simple universal conductivity behaviour are observed, indicating some distribution of energy barriers for an electron hopping mechanism. (paper)

[en] Ceramic Ba_0.6Sr_0.4TiO₃ (BST-0.6) samples were studied in the broad spectral range of 10⁶-10¹⁴ Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectric dispersion mechanism in the paraelectric phase was shown to be of strongly anharmonic soft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectric transition was shown to consist of two coupled overdamped THz excitations, which show classical features of a coupled soft and central mode, known from many ferroelectric crystals with a dynamics near the displacive and order-disorder crossover. Similar behaviour has been recently revealed and theoretically simulated in pure BaTiO₃ (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101 167402). Also for the BST system, this feature was confirmed by the theory based on molecular dynamics simulations with an effective first-principles Hamiltonian. In all the ferroelectric phases, additional relaxation dispersion appeared in the GHz range, assigned to ferroelectric domain-wall dynamics. The microwave losses were analysed from the point of view of applications. The paraelectric losses above 1 GHz are comparable with those in single crystals and appear to be of intrinsic multi-phonon origin. The ceramic BST system is therefore well suited for applications in the whole microwave range.

[en] Dielectric response of perovskite Sr_1_−_xBa_xMnO_3 (x  =  0.43 and 0.45) ceramics was investigated using microwave, THz and infrared spectroscopic techniques in order to study the ferroelectric and antiferromagnetic phase transitions with critical temperatures T _C  ≈  350 K and T _N  ≈  200 K, respectively. The two lowest-frequency polar phonons are overdamped above T _N and they exhibit pronounced softening on heating towards T _C. Nevertheless, permittivity ε′ in the THz range shows only a small anomaly at T _C because the phonon contribution to ε′ is rather small. The phonons are coupled with a central mode which provides the main contribution to the dielectric anomaly at T _C. Thus, the ferroelectric phase transition has characteristics of a crossover from displacive to order–disorder type. At the same time, the intrinsic THz central peak is partially screened by conductivity and related Maxwell–Wagner relaxation, which dominates the microwave and lower-frequency spectra. Below T _N, the ferroelectric distortion markedly decreases, which has an influence on the frequencies of both the central and soft modes. Therefore, ε′ in the THz range increases at T _N on cooling. In spite of the strong spin–phonon coupling near T _N, surprisingly no magnetodielectric effect was observed in the THz spectra upon applying magnetic field of up to 7 T, which is in contradiction with the theoretically expected huge magnetoelectric coupling. We explain this fact as due to the insensitivity of T _N to magnetic field. (paper)

[en] We prepared multiferroic Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at T_C=312 K, similar as in single crystal. Ferroelectric (FE) phase can be induced by external magnetic field in all investigated samples and the phase diagram in ceramics qualitatively resembles that of the single crystal. The range of magnetic fields, where the FE phase is induced, broadens after annealing of single crystal. Ceramics quenched after sintering exhibit several orders of magnitude lower conductivity than the single crystal. Heavily damped magnetic resonance was discovered in terahertz spectra at 10 K and its frequency softens below 5 GHz near T_C. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D_3d⁵ hexagonal structure. No evidence for a structural phase transition was found in the IR and Raman spectra on cooling (in zero magnetic field) or in the room-temperature IR spectra with external static magnetic field up to 0.3 T.

[en] The relaxor ferroelectric PbMg_1/3Nb_2/3O₃ (PMN) is investigated by means of dielectric and Fourier transform far infrared transmission spectroscopy in the frequency range from 10 kHz to 15 THz at temperatures between 20 and 900 K using mostly thin films on infrared transparent sapphire substrates. While the thin film relaxors display reduced dielectric permittivity at low frequencies, their high frequency lattice response is shown to be the same as for single-crystal/ceramic specimens. In contrast to the results of inelastic neutron scattering, the optic soft mode is found to be underdamped at all temperatures. On heating, the TO1 soft phonon follows the Cochran law with an extrapolated critical temperature of 670 K near to the Burns temperature. Above 450 K the soft mode frequency levels off near 50 cm^-1 and above the Burns temperature it slightly hardens. Central-mode-type dispersion assigned to the dynamics of polar nanoclusters appears below the Burns temperature at frequencies near to but below the soft mode and slows down and broadens dramatically on cooling, finally, below the freezing temperature of 200 K, giving rise to frequency independent losses from the microwave range down. A new explanation of the phonon 'waterfall' effect in inelastic neutron scattering spectra is proposed

[en] Dielectric properties of the relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 were studied in the broad frequency range of 100 Hz-1 THz at low temperatures below the freezing temperature. Nearly frequency-independent dielectric losses were observed up to 1 GHz on cooling down to 10 K. Their magnitude decreases exponentially with temperature, but remains remarkable high down to 10 K. A Landau-type thermodynamic model based on the perovskite structure near the morphotropic phase boundary is proposed for calculating the energy barriers for polarization reversal near the polar cluster boundaries and explaining the broad distribution function of relaxation times, which fits the observed frequency dependences of permittivity and losses below 1 GHz. High dielectric losses in the submillimetre region were explained by shear wave emission of vibrating polar cluster walls in an ac electric field and by piezoelectric resonances on polar clusters