A. I. Fedoseev

Trapped ions and the shielding of dust particles in low-density non-equilibrium plasma of glow discharge

A new model for the formation of trapped ions around a negatively charged dust particle immersed in low-density non-equilibrium plasma of gas discharge is presented. It is shown that the ionic coat leads to a shielding of the proper charge of the dust particle. In experiments it is only possible to detect the effective charge of a dust particle that is equal to the difference between the proper charge of the particle and the charge of trapped ions.

Acoustic properties of the disordered relaxor ferroelectric PbSc1/2Ta1/2O3

The behavior of acoustic phonons in crystals of a relaxor ferroelectric, namely, the lead scandium tantalate PbSc1/2Ta1/2O3 (PST), is studied in the vicinity of the diffuse phase transition. The behavior of longitudinal and transverse acoustic phonons in a PST single crystal is examined using Brillouin scattering. The phonon subsystem is found to behave anomalously in the vicinity of T = 297 K, which can probably be assigned to the existence of a phase transition. Analysis of the results obtained yields the values of the elastic moduli C11, C12, and C44 for the cubic phase of the crystal over a wide temperature range.

Anomalous behavior of the acoustic phonon velocity and elastic constants of relaxor ferroelectric PbMg1/3Ta2/3O3

The behavior of longitudinal acoustic (LA) and transverse acoustic (TA) phonons in a cubic relaxor PbMg1/3Ta2/3O3 (PMT) ferroelectric has been investigated by Brillouin light scattering. Analysis of the temperature dependences of phonon velocity in the temperature range from 50 to 870 K has revealed anomalies in the vicinity of the Burns temperature (TB≈570 K) for LA phonons and a wide frequency-dependent minimum of the velocities of LA and TA phonons in the same temperature region as the dielectric response anomaly. Using experimental data, temperature dependences of the C11 and C44 elastic constants have been calculated. The elastic constants have been found to be frequency-independent in the gigahertz range. The results obtained are discussed in the framework of modern ideas on the crystalline lattice dynamics of relaxor ferroelectrics.

Dynamical Properties of the Partially Disordered Crystals of Cs 5 H 3 (SO 4 ) 4 · x H 2 O

This paper presents studies of lattice dynamics of a crystal of Cs 5 H 3 (SO 4 ) 4 x H 2 O (PCHS) and its deuterated analog Cs 5 D 3 (SO 4 ) 4 x D 2 O (DPCHS) in wide temperature and frequency ranges by dielectric spectroscopy and modulated temperature differential scanning calorimetry. It is shown that deuteration of PCHS gives rise to an additional structural phase transition at T c3 , 250 K. The temperature of the isostrctural phase transition T c2 (372 K for PCHS and 370 K for DPCHS) has been determined more accurately. It has been found that deuteration does not suppress phase transition at T c2 . The temperature dependence of dielectric response is analyzed.

Isotope effect in Cs5H3(SO4)4·0.5H2O crystals

Abstract The sequence of phase transitions at temperatures from 290 to 420 K in the crystals of Cs5H3(SO4)4·0.5H2O (CTSM) and Cs5D3(SO4)4·0.5D2O (DCTSM) was studied by Brillouin light scattering and neutron diffraction. Anomalies in the temperature dependences of the velocity and damping of hypersonic acoustic phonons in the vicinity of 360 K were observed in the CTSM crystal. In contrast, no anomalies were found in the DCTSM crystal. Preliminary analysis of neutron diffraction data does not show symmetry changes in the temperature range studied. Thus it has been found that deuteration of the CTSM crystals leads to a change in the sequence of phase transitions.

Study of phase transformation dynamics in Na1/2Bi1/2TiO3 by Mandelstam-Brillouin scattering

The results of the study of Mandelstam-Brillouin light scattering in single-crystal Na1/2Bi1/2TiO3 within a broad temperature interval covering successively the cubic, tetragonal, and rhombohedral phases are reported. Measurements conducted within the range 820–300 K have revealed quasi-elastic scattering and distortion of the phonon spectra in shape, which suggests possible coupling between the low-frequency transverse optical and acoustic modes. These two observed phenomena are believed to be closely interrelated.

Specific Features of Brillouin Spectra at a High-Temperature Phase Transition in Cs5H3(SO4)4xnH2O Crystals

This paper presents detailed Brillouin light scattering studies of the acoustic response of a Cs5H3(SO4)4xnH2O (PCHS) crystal in the vicinity of a superionic (superprotonic) structural phase transition of the first order. Just above the phase transition, splitting of the Brillouin doublet is observed. The ‘two-mode’ behavior of the longitudinal acoustic phonon can be explained by coexistence of phases at a structural phase transition of the first order. Above the phase transition, in the superionic phase, an additional doublet forbidden by the selection rules appears in a narrow temperature interval. It is concluded that an anomalous behavior of Brillouin light scattering can be attributed to the influence of dynamically disordered protons on the phonon subsystem.

Elastic Properties of Disordered Lead Scandotantalate Crystal

Abstract Results of studies of elastic properties of relaxor ferroelectric lead scandotantalate crystal in the vicinity of a diffuse phase transition are presented. Brillouin scatttering from longitudinal and transverse acoustic phonons was investigated. Analaysis of data yielded elastic C11, C12, and C44 moduli for the cubic phase of the crystal in a wide temperature range.

Acoustic anomalies at phase transformation to quasi-2D proton glass state in Cs5H3(SO4)4 × xH2O crystal

The paper describes Brillouin light scattering studies of longitudinal acoustic (LA) phonons in Cs5H3(SO4)4 × xH2O (PCHS) crystals at temperatures from 100 K to 360 K. The acoustic response of the crystal at different frequencies is analysed in detail. It is shown that both the velocity and damping of sound exhibit a strong dispersion caused by relaxation processes in the region of transformation into the glass-like phase (Tg ≈ 260 K). A strong anisotropy in the acoustic response, attributable to the quasi-two dimensional (quasi-2D) structure of PCHS, is revealed. The LA phonon damping is calculated in the framework of a number of relaxation models. It is shown that, in the vicinity of Tg, anomalies in ultrasonic damping of the LA phonons propagating in the basal plane reflect the cooperative effect of freezing of acid protons. At the same time, the anomaly in damping of the LA phonon propagating perpendicular to the basal plane is described in terms of the Debye model and is due to the interaction between protons on hydrogen bonds and LA phonons. This suggests that the proton glass state realized at T< T g is of quasi-2D nature.

Multicomponent Quasi-Elastic Light Scattering in Na1/2Bi1/2TiO3 as Studied by Broadband Brillouin Scattering

Brillouin light scattering studies of the low-frequency region of vibrational spectra of a partially disordered ferroelectric crystal Na1/2Bi1/2TiO3 (NBT) from 300 to 850 K are presented. It is shown that the light scattering spectra contain a multicomponent quasi-elastic scattering (QELS) component observed in a broad frequency range, from 800 to 0.7 GHz. Reconstruction of the inelastic light scattering spectra in NBT in the frequency region from 0.7 to 800 GHz did not reveal relaxation processes (α and β relaxations) typical of disordered compounds (glasses, supercooled liquids, etc.). The fractal approach also proved to be inapplicable to the description of QELS in NBT. Detailed analysis of the Brillouin spectra with different free spectral ranges (“frequency windows”) showed that different contributions to the quasi-elastic light scattering manifest themselves in different frequency windows. These contributions are associated with the structural phase transitions and other processes responsible for the emergence of QELS (domain structure evolution, heterophase fluctuations, etc.). Thus, we suggest a new approach to the analysis of the temperature behavior of quasi-elastic scattering. It allows one to study the critical dynamics of the crystal lattice and the nonphonon contributions into vibrational spectra of partially disordered crystals.