Viktor Bovtun

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

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.

Structure of the dielectric spectrum of relaxor ferroelectrics

Abstract 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.

Soft and central mode behaviour in PbMg1/3Nb2/3O3 relaxor ferroelectric

The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of broad-band dielectric and Fourier Transform Infrared (FTIR) transmission spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures between 20 and 900 K using PMN films on infrared transparent sapphire substrates. While thin film relaxors display reduced dielectric permittivity at low frequencies, their high frequency intrinsic or lattice response is shown to be the same as single crystal/ceramic specemins. It was observed that in contrast to the results of inelastic neutron scattering, the optic soft mode was underdamped at all temperatures. On heating, the TO1 soft phonon followed the Cochran law with an extrapolated critical temperature equal to the Burns temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode frequency leveled off and slightly increased above the Burns temperature. A central mode, describing the dynamics of polar nanoclusters appeared below the Burns temperature at frequencies near the optic soft mode and dramatically slowed down below 1 MHz on cooling below room temperature. It broadened on cooling, giving rise to frequency independent losses in microwave and lower frequency range below the freezing temperature of 200 K. In addition, a new heavily damped mode appeared in the FTIR spectra below the soft mode frequency at room temperature and below. The origin of this mode as well as the discrepancy between the soft mode damping in neutron and infrared spectra is discussed.

Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures

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.

Central-Peak Components and Polar Soft Mode in Relaxor PbMg1/3Nb2/3O3 Crystals

Dielectric response of relaxor PMN single crystals was investigated in a broad frequency range 100 Hz–100 THz by a combination of dielectric spectroscopy (100 Hz–1 GHz, 10–700 K), time-domain THz spectroscopy and infrared (IR) reflectivity (5–3000 cm−1, 10–300 K). Polar TO1 phonon (soft mode) is resolved in THz and IR spectra at all temperatures studied, follows the Cochran law and softens towards the Burns temperature on heating. The central mode (dielectric relaxation) splits below room temperature into two components. First one appears in the THz range and is assigned as IR activated phonon density of states due to the breaking of local symmetry as a consequence of polar nano-clusters. Second relaxation originates from cluster breathing and flipping. It anomalously broadens on cooling and leads to frequency-independent dielectric loss between 100 Hz–100 GHz below 200 K in the non-ergodic phase.

Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics

Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 μm) were measured from 102 to 1014 Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from ∼1 to ∼20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO3, an o...

Infrared and microwave dielectric response of the disordered antiferroelectric Ag(Ta,Nb)O3 system

Abstract Recent progress related to the investigation of infrared and microwave properties of the AgNbO3 and AgTaO3 single crystals and Ag(Ta,Nb)O3 ceramics is reviewed and discussed with the aid of new crystallographic considerations and symmetry approach. It is shown that optical phonons at any temperature contribute to the permittivity by about 100-200 only and the pronounced dielectric anomalies are due to a submillimetre overdamped excitation related to strong anharmonicities in Nb vibrations (Nb disorder). The best microwave properties near room temperature are achieved for the AgTa0.57Nb0.43O3 ceramic for which the permittivity e = 380 and the quality = 550 at 1.586 GHz with the permittivity temperature coefficient αe passing through zero at 20°C

Broadband dielectric spectroscopy of phonons and polar nanoclusters in PbMg 1 / 3 Nb 2 / 3 O 3 − 35 % PbTiO 3 ceramics: Grain size effects

Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grain size ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics of PbMg_(1/3)Nb_(2/3)O3-35%PbTiO3 were investigated at temperatures 10 - 900 K. e*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below Tc, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation. On the other hand, polar phonon frequencies and their temperature dependences are almost independent of the grain size, but the selection rules for the cubic symmetry are not obeyed and all phonons are split due to a locally broken symmetry by polar nanoregions and chemical disorder. The lowest-frequency polar phonon undergoes partial softening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but the dielectric anomaly is caused predominantly by flipping and breathing of polar nanoclusters. Due to contribution of both the soft phonon mode and dielectric relaxations into the dielectric constant, the ferroelectric phase transition, which corresponds to the percolation threshold of the polar nanoregions into macroscopic domains, can be considered as a special case of crossover between the displacive and order-disorder type.

Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions

Infrared reflectance, terahertz transmittance, and microwave resonance measurements show that SrTiO3 films, strained by ∼1% in biaxial tension by growing them on (110) DyScO3 substrates, undergo a pronounced phonon softening near 270 K. This in-plane soft-mode drives the ferroelectric transition. The appearance of two new low-frequency modes and splitting of the high-frequency TO4 mode provide evidence of an antiferrodistortive phase below ∼180 K.

Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22: Comparison of ceramics and single crystals

We prepared multiferroic Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22 ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at TC=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 TC. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D3d5 hexagonal structure. No evidence for a structural phase transition was found in the IR and Ram...