A. M. Pugachev

Bias Field Effect on the Dielectric and Pyroelectric Response of Single Crystal of Uniaxial Relaxor Sr0.75Ba0.25Nb2O6

Dielectric and pyroelectric studies of lead-free uniaxial relaxor Sr0.75Ba0.25Nb2O6 (SBN75) single crystals have been carried out under different bias field values. E,T-phase diagram has been constructed using these data. Dependences of both the temperature and the height of dielectric permittivity and pyroelectric coefficient maxima on the bias field are similar to those observed previously for PbMg1/3Nb2/3O3 - PbTiO3 crystals and support the presence of the critical endpoint in the E,T-phase diagram of SBN75 and a critical behavior of field-induced pyroelectric response in uniaxial relaxors.

Low-frequency Raman scattering study of the ferroelectric phase transition in the DKDP crystal

The low-frequency Raman scattering spectra of the DKDP ferroelectric crystal are studied in the temperature range 30–393 K. At temperatures above 150 K, the Raman spectra exhibit a central peak which reflects the lattice relaxation susceptibility. The width and integrated intensity of the central peak are derived from the experimental spectra. The critical slowing down of the relaxation response predicted by the Ginzburg-Landau-Devonshire theory is observed throughout the temperature range in which the central peak persists. Its integrated intensity does not, however, follow the predictions of the theory and reveals a strong temperature dependence in the ferroelectric phase and a weaker dependence in the paraphase. It is shown that the thermal activation law describes well the temperature dependence of the intensity of the central peak. An interpretation is proposed according to which the intensity of order parameter fluctuations is related to the activation barrier whose height is proportional to the deviation from the phase transition temperature.

Second harmonic generation in the paraelectric phase in powders and ceramics of BaTiO3

A second harmonic signal has been clearly observed in powders and ceramics of BaTiO3 in the high-temperature phase. The effect manifests itself at temperatures 500 K above the temperature of the phase transition from the tetragonal phase to the cubic phase, which is also significantly higher than the Burns temperature of BaTiO3. The temperature dependence of the second harmonic generation signal I2ω in the paraelectric phase becomes a straight line when plotted in the Arrhenius coordinates lnI2ω(1/T). The experimental results obtained indicate the presence of noncentrosymmetric local regions in the paraelectric phase over a wide range of temperatures. The contribution from these polar regions to the second harmonic generation increases as the phase transition temperature is approached. The temperature dependence of the second harmonic generation in barium titanate ceramics is significantly affected by the technique used for pressing and annealing of the samples.

The nature of low-frequency Raman scattering in congruent melting crystals of lithium niobate

Raman spectra of congruent melting crystals of lithium niobate are studied. It is shown that a band near ∼100 cm−1 corresponds to first-order Raman scattering and is due to nonstoichiometry of the crystal. The adequacy of various models for description of this band is discussed.

Low-frequency Raman scattering in As2S3 glass former around the liquid?glass transition

Low-frequency light scattering spectra of As2S3 glass former have been investigated over a temperature range of 300?712?K. Using the model of a damped oscillator, the parameters of the intensities of the boson peak and of the fast relaxation as well as the boson peak position and the relaxational time are extracted as a function of temperature. The temperature increase of the fast relaxation becomes more sharp above Tg and stabilizes at T = 600?K. By analogy with other glass formers it is suggested that the critical temperature Tc within the framework of the mode-coupling theory should be near 600?K for As2S3. It was found that the damping parameter at Tc is the same for As2S3 and B2O3; an explanation of this coincidence is suggested. The boson peak is found to be present up to 712?K with almost the same strength as at low temperatures.

Low Frequency Raman Scattering in BaTiO3 Crystal

The central peak (CP) in Raman scattering in BTO crystals was investigated in wide spectral and temperature ranges. The identical CP properties both in the VV and the VH scattering geometries, best fitting of low-frequency Raman spectra by the Lorentz function, and obtaining the typical temperature dependences for the CP amplitude and width support the relaxation nature of CP in the BTO crystals.

Study of the central peak in Raman spectra of SBN crystals

The Raman spectra of SBN crystals were recorded over wide spectral and temperature ranges. It is shown that a central peak representative of relaxation processes is observable in different spectral configurations of Raman measurements and cannot be described in the single relaxation time approximation. In the vicinity of the temperature corresponding to the onset of nucleation of polar regions (the Burns temperature), no specific features in the behavior of the central peak were observed.

Salient properties of raman central peak in LiNbO3 and LiTaO3 crystals

Results of recent investigations of Raman scattering in LiNbO 3 and LiTaO 3 are reviewed. It is shown that the temperature dependences of the central peak and soft-mode frequency are very similar for these two crystals if the temperature is scaled by the Curie temperature T c . Below 0.9T c the ferroelectric dynamics is adequately described by the vibrational soft-mode and by a relaxational contribution from local fluctuations of the order parameter, providing the central peak. Within (0.9–1.1)T c the soft-mode is overdamped, leading to intensive contribution of the collective relaxation response. In this temperature range the ferroelectric dynamics is described by order-disorder scenario.

Dielectric susceptibility of a deuterated KDP crystal from experiment on Raman scattering and in the cluster approximation

The temperature dependence of the static dielectric susceptibility obtained from the experiment on Raman scattering in a deuterated KDP (DKDP) crystal has been described using the cluster approximation in terms of the microscopic mean-field theory. The optimum values obtained for parameters of the cluster approximation are in good agreement with experimental data over a wide temperature range. These values are found to be comparable to the estimates previously obtained in describing experimental data on the DKDP crystal.

Central Peak in Raman Spectra of Ferroelectric KDP and DKDP Crystals

The ferroelectric phase transition in K(H1 − x D x )2PO4 crystals was investigated by Raman scattering. It was found that the temperature dependence of the central peak in typical order—disorder crystals K(H 1 − x D x )2PO4 consists of three temperature ranges: in the nearest vicinity of the phase transition temperature the width and the amplitude of the central peak follow the theoretical predictions; in the second temperature range the “critical slowing down” is manifested whereas the behavior of the amplitude is strongly nonlinear; and outside these temperature ranges the central peak does not follow the theoretical predictions.