Viktor Porokhonskyy

Infrared, Raman and high-frequency dielectric spectroscopy and the phase transitions in Na1/2Bi1/2TiO3

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.

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.

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.

Broadband dielectric spectroscopy of (1-x)BiScO3-xPbTiO3 piezoelectrics

Dielectric spectra of high-temperature piezoelectric (1−x)BiScO3–xPbTiO3 of composition near the morphotropic phase boundary (x≅0.64) were investigated in the frequency range of 100 Hz–1 THz at temperatures between 10 and 900 K. Below the ferroelectric phase transition Tc≅700 K, in addition to polar phonons, two other polarization mechanisms were detected: one, centered in the 100 MHz–1 GHz range probably caused by sound generation due to ferroelectric-ferroelastic domain wall motion; another evidences dynamic disorder of some ions. The former process, characterized by the temperature-independent mean relaxation time, vanishes below ∼250 K. The latter mechanism, caused by compositional disorder, results in the nearly frequency-independent losses (1/f noise) at low temperatures.

Microwave dielectric properties of the ordered and disordered Pb(Sc½Ta½)o3 ceramics

Abstract 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.

Broad-band dielectric response of doped incipient 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.

Microwave dielectric properties of the Ag1−xLixNbO3 (x = 0 ÷ 0.06) ceramics

Abstract 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 ε′(T) and ε″(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 ε′ below 250 K and a sharp step-like anomaly of ε′(T) at ≈ 425 K. This indicates a significant change of the structure which is also confirmed by the pronounced change in the IR reflectivity.

Influence of porosity on the dielectric response and central-mode dynamics in PbZrO3 ceramics

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 (<1%) of the air gaps in the sample may drastically reduce its low-frequency permittivity and cause pronounced stiffening of the central-mode-type dispersion in PbZrO3.

Wide-Frequency Range Dielectric Relaxations in Sr 1−1.5x Bi x TiO 3 Ceramics

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.