V. K. Malinovsky

Effect of nitrogen impurities on the Raman line width in diamonds

The dependence of the Raman line width in diamonds on the nitrogen impurity is experimentally studied. A linear relation between the nitrogen content and the width is found. It is demonstrated that the slope of the linear fit depends on the type of nitrogen defect. A relation between the Raman experiment and results from lattice parameter experiments is found and discussed.

The nature of the glass transition and the excess low-energy density of vibrational states in glasses

It is shown that the excess low-energy (2-10 meV) density of vibrational states (LEDOS) in glasses leads to an increase of the mean-square atomic displacements (R2) in comparison with the case of crystalline counterparts. The estimations show that due to the excess LEDOS (R2) at the glass temperature, Tg in glass is the same as in its crystalline counterpart at the melting point Tm. The relation between Tg and Tm is expressed through the parameters of the excess LEDOS and is shown to be in good agreement with the experimental rule Tm/Tg approximately=3/2. A criterion of glass-forming ability, based on the important role of the excess LEDOS, is formulated.

Investigation of amorphous states of SiO2 by Raman scattering spectroscopy

The vitreous SiO2 samples irradiated with fast neutrons at a dose of 5×1017−2.2×1020 per cm2 are investigated by the Raman scattering technique. It is demonstrated that the maximum of the low-frequency Raman spectrum (boson peak) shifts with an increase in the irradiation dose, and the medium-range order size decreases from 25 Å for the initial glass to 19 Å for the sample subjected to irradiation at a maximum dose. It is revealed that the fast relaxation intensity obtained from analysis of the low-frequency Raman spectra linearly correlates with the specific volume of the studied samples.

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.

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.

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.

Fast relaxation intensity versus silica glass density: existence of sharp peculiarity

Low-frequency Raman spectra of densified silica glasses, annealed at different temperatures and possessing various densities, are studied. The dependence of the fast relaxation intensity versus the density can be described by linear laws below and above the density 2.26 g cm−3, but the slope of the linear law below the critical density is ten times higher than the slope of the linear law above 2.26 g cm−3. The sharp peculiarity is interpreted as a consequence of an inhomogeneous glassy structure with local ordering similar to tridimite or β-cristobalite. The dependence of the silica sample density on annealing temperature is discussed in view of the temperature dependence of the liquid silica density. A hypothesis is proposed that the true dependence of the liquid silica density is a linear function of temperature.