A. Brilingas

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.

Dielectric and ultrasonic investigation of phase transition in cuinp2s6 crystals

Investigation results of dielectric and ultrasonic properties of layered CuInP2S6 crystals are presented. At low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 7357.4 K (0.635 eV). The high-frequency part of the spectra is determined by relaxational soft mode. The critical slowing down and Debye-type dispersion show the order–disorder type of the phase transition. The temperature dependence of the relaxational soft mode and dielectric contribution show a quasi-one-dimensional behaviour. Ultrasonic velocity exhibits critical slowing down which is accompanied by attenuation peaks in the phase transition region. Layered CuInP2S6 crystals have extremely large elastic nonlinearity in the direction perpendicular to layers. The nonlinear elastic parameters substantially increases at the PT temperature.

Dielectric Relaxation in Ba2NaNb5(1-x)Ta5xO15 Single Crystrals.

This paper presents the results of dielectric dispersion in Ba 2 NaNb 5(1- x ) Ta 5 x O 15 (BNNT) single crystals over the frequency range 1 kHz to 4 GHz. The fundamental dielectric dispersion occurs above 10 7 Hz and is caused by a soft relaxational mode. This mode gives the main contribution to the high static permittivity which fits the Curie-Weiss law. The results indicate the anharmonic motion of the Nb(Ta) atoms and the order-disorder nature of the proper ferroelectric phase transition in BNNT.

Dipolar Glass Behaviour in Mixed CuInP2(S0.7Se0.3)6 Crystals

The dielectric spectrum of mixed CuInP2(S0.7Se0.3)6 crystals was investigated. The calculated distribution of relaxation times confirms glassy behaviour and the maxima of the distribution of the relaxation times follow Vogel-Fulcher law with parameters: activation energy E A = 9.4 · 10−2 eV, dipole freezing temperature T 0 = 21.9 K and attempt frequency υ0 = 1/(2π τ0) = 0.149 THz. From all presented results we see, that CuInP2(S0.7Se0.3)6 crystal shows typical dipolar glass dielectric dispersion and we may conclude that this crystal belongs to new family of materials that exhibits a dipolar glass behavior.

Pinning effect on microwave dielectric properties and soft mode in TlInS2 and TlGaSe2 ferroelectrics

Abstract This paper presents the results of an investigation of microwave dielectric dispersion in the proper semiconductive ferroelectrics TIInS2 and TIGaSe2 with an incommensurate structure modulation. In these crystals there is a strongly overdamped soft ferroelectric mode, whose frequency in the vicinity of the phase transitions drops to the millimetre wave region and causes dielectric microwave dispersion plus a high contribution to the static dielectric permittivity. Within the incommensurate phase crystal defects, such as impurities, cause pinning of the soft mode. Because of this pinning effect the phason frequency increases. Pinning also changes the dynamical dielectric properties and the contribution of the phason and amplitudon to the static permittivity.

Microwave and millimetre-wave dielectric response of Rb1-x(ND4)D2PO4 dipolar glass

The microwave and millimetre-wave dielectric response of Rb0.5(ND4)0.5D2PO4 dipolar glass has been studied around the frustration temperature Tf?120?K. It was found that above the frustration temperature Tf the soft relaxational deuteron mode is responsible for the whole dielectric dynamics below the phonon range. Its frequency decreases from that of submillimetre waves to that of microwaves following the classical law ?R = A(T-Tf), with A = 0.75?GHz?K-1, but in the vicinity of Tf the soft mode gradually shifts towards the glass-like dispersion predominantly related to the diffusion of Takagi defects according to the Vogel-Fulcher law. From the experimental results, the distribution of the relaxation times and of the local polarization at various temperatures is calculated. It is shown that the local polarization distribution function obtained from the dielectric response ?*(?,T) coincides well with that obtained from the NMR results.

Dielectric properties in the vicinity of the ferroelectric phase transition in a mixed crystal of deuterated BP0.01BPI0.99

The dielectric behaviour of ferroelectric hydrogen bonded deuterated betaine phosphate0.01 betaine phosphite0.99 is investigated in the region of the ferroelectric phase transition. Dielectric dispersion is investigated in the frequency range up to 12 GHz. The dielectric dynamics of a small admixture such as 1% BP is already able to change noticeably the character of the phase transition as well as the quasi-one-dimensional feature of deuterated betaine phosphite. The Debye type dispersion shows a critical slowing down. The frequency of the relaxational soft mode in the paraelectric phase varies according to the quasi-one-dimensional Ising model and decreases down to 0.55 GHz at the phase transition temperature (TC = 299 K). The activation energy for the deuteron flipping motion amounts to ΔU = 3.6kTC = 0.095 eV. The obtained results show the order-disorder character of the proper ferroelectric phase transition. However, the small admixture of 1% betaine phosphate changes noticeably the character of the phase transition as well as the quasi-one-dimensional feature in comparison to pure deuterated betaine phosphite.

Dielectric Dispersion and Distribution of the Relaxation Times of the Relaxor Ceramics BBT

From the dielectric measurements of the relaxor ceramics BaBi 2 Ta 2 O 9 (BBT) in a wide frequency range (10 1 –10 11 Hz) the real distribution function of the relaxation times f(τ) was calculated. The shape of the distribution function is asymmetrical in all measured temperature interval with the expressed peak of relaxation times about 10− 12 s in higher temperatures.

Dynamics of Phase Transition in 0.4NBT-0.4ST-0.2PT Solid Solution

In this paper we present dielectric spectroscopy results of NBT-(0.6-x)ST-xPT with x = 0.2 solid solution. Dielectric investigations clearly showed a relaxor—normal ferroelectric phase transition at TPT = 419 K and low temperature and low frequency dispersion similar to coexistence of dipolar glass and ferroelectric phase. The mean relaxation time above the phase transition follows Vogel—Fulcher law with following parameters: E A = 0.179 eV, τ 0 = 3.39·10−14 s, T VF = 223 K.

Dielectric response of antiferroelectric PLZT 2/95/5 ceramics in the range of 10 -1014 Hz and 10 -530K

Abstract Dielectric response of PLZT 2/95/5 ceramics was studied by classical bridge method, coaxial radio-frequency and waveguide microwave techniques, submillimetre transmission and infrared reflection spectroscopy in the temperature region 10 -550 K. Large dielectric anomaly appears near the phase transition from paraelectric to incommensurate phase (Ti=470 K) due to a critical relaxation in the GHz region whose frequency softens on cooling in the paraelectric phase and disappears in the antiferroelectric phase below 380 K. Many new phonon modes were resolved in the infrared reflectivity spectra at 10 K due to the Brillouin zone folding in the antiferroelectric phase and due to the low phonon damping. Both these facts show that the antiferroelectric phase is fully ordered