Browsing by Author "Lushnikov, Sergey G."

Now showing 1 - 9 of 9

Acoustic anomalies at phase transformation to quasi-2D proton glass state in Cs5H3(SO4)4.xH2O crystal
(2001) Gvasaliya, S.N.; Fedoseev, A. I.; Lushnikov, Sergey G.; Schmidt, V. Hugo; Tuthill, G. F.; Shuvalov, L. A.
The paper describes Brillouin light scattering studies of longitudinal acoustic (LA) phonons in Cs5H3(SO4)4×x H2O (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
Brillouin and Raman spectra anomalies in KNSBN with the tungsten bronze structure
(2000) Siny, I.G.; Lushnikov, Sergey G.; Siny, S. I.; Schmidt, V. Hugo; Savvinov, A. A.; Katiyar, R.S.
The Brillouin and Raman scattering from a complex single crystal from the tungsten-bronze family, (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 doped with Cu2+ (KNSBN:Cu), have been comparatively studied in a wide temperature range around the ferroelectric transition. Step-like anomalies in hypersonic velocity and damping confirm the first-order structural transition. These anomalies look like some perturbations on the high-temperature slopes of both a broad dip in sound velocity and a broad maximum in damping that develop in a wide temperature range. The acoustic behavior of KNSBN:Cu does not simply follow the Landau theory prediction valid for many ferroelectrics except relaxors, to which the KNSBN:Cu behavior is intrinsically analogous. The total intensity of the Raman spectra as well as the intensity of separate internal and external vibrations and their width correlate with acoustic anomalies, namely there are step-like drops at the same temperature of the first-order transition and a broad range where the intensity is drastically increased.
Brillouin light scattering anomalies and new phase transition in Cs5H3(SO4)4 crystals
(2000-02) Lushnikov, Sergey G.; Schmidt, V. Hugo; Shuvalov, L. A.; Dolbinina, V. V.
The behavior of hypersonic longitudinal acoustic phonons at temperatures ranging from 290 to 370 K in a crystal of Cs5H3(SO4)4·nH2O was studied by Brillouin light scattering. Anomalies in the temperature dependences of the frequency shift and spectral width of the Brillouin components in the vicinity of were observed. We found a “point of isotropization” near 360 K where We attribute it to an isostructural phase transition from P63/mmc⇔P63/mmc where the apparent acoustic symmetry changes from hexagonal to cubic. Possible models for the phase transition have been discussed.
Brillouin light scattering anomalies and new phase transition in Cs5H3(SO4)4 crystals
(2000) Lushnikov, Sergey G.; Shuvalov, L. A.; Dolbinina, V. V.; Schmidt, V. Hugo
The behavior of hypersonic longitudinal acoustic phonons at temperatures ranging from 290 to 370 K in a crystal of Cs5H3(SO4)4·nH2O was studied by Brillouin light scattering. Anomalies in the temperature dependences of the frequency shift and spectral width of the Brillouin components in the vicinity of T=360K were observed. We found a “point of isotropization” near 360 K where C11=C33=2.6X10 10 N/m2 We attribute it to an isostructural phase transition from P63/mmc⇔P63/mmc where the apparent acoustic symmetry changes from hexagonal to cubic. Possible models for the phase transition have been discussed.
Evidence for a quasi-two-dimensional proton glass state in Cs5H3(SO4)4.xH2O crystals
(2001) Lushnikov, Sergey G.; Gvasaliya, S. N.; Fedoseev, A. I.; Schmidt, V. Hugo; Tuthill, G. F.; Shuvalov, L. A.
We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs5H3(SO4)4 xH2O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (Tg = 260 K). Near Tg the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons. The damping of LA phonons propagating perpendicular to the basal plane can be fit by the Debye model and is due to the interaction between protons and LA phonons. This suggests that the proton glass state that is realized at T < Tg has a quasi-two-dimensional nature.
Isotope effect in Cs5H3(SO4)4.0.5H2O crystals
(1999-10) Lushnikov, Sergey G.; Belushkin, A.V.; Beskrovnyi, A.I.; Fedoseev, A. I.; Gvasaliya, S.N.; Shuvalov, L. A.; Schmidt, V. Hugo
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.
Light scattering from (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 with the tungsten bronze structure: An analogy with relaxor ferroelectrics
(2001) Siny, I.G.; Lushnikov, Sergey G.; Siny, S. I.; Schmidt, V. Hugo; Savvinov, A. A.; Katiyar, R.S.
The Brillouin and Raman scattering from a complex single crystal from the tungsten–bronze family, (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6(K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 doped with Cu2+Cu2+ (KNSBN:Cu), have been comparatively studied in a wide temperature range around the ferroelectric transition. Step-like anomalies in hypersonic velocity and damping confirm the first-order structural transition. These anomalies look like some perturbations on the high-temperature slopes of both a broad dip in sound velocity and a broad maximum in damping that develop in a wide temperature range. The acoustic behavior of KNSBN:Cu does not simply follow the Landau theory prediction valid for many ferroelectrics. Instead it resembles that of relaxors, to which the KNSBN:Cu behavior is analogous intrinsically. The total intensity of the Raman spectra as well as the intensity of separate internal and external vibrations and their width correlate with acoustic anomalies, namely there are step-like drops at the same temperature as the first-order transition and a broad range where the intensity is drastically increased. All these broad anomalies imply the existence of a wide preceding phase in respect to the relaxor ferroelectric state. Unusual properties of this preceding phase are discussed as well as the phase diagram relation to the dynamical evolution of other relaxors from the perovskite family, such as PbMg1/3Nb2/3O3PbMg1/3Nb2/3O3 and Na1/2Ba1/2TiO3.
Specific features of hypersonic damping in relaxor ferroelectrics
(1995) Siny, I.G.; Lushnikov, Sergey G.; Tu, Chi-Shun; Schmidt, V. Hugo
A comparative analysis of the hypersonic damping behavior in a wide temperature range has been carried out in a relaxor ferroelectric PbMg1/3Nb2/3O3-(PMN) and in a related crystal Na1/2Bi1/2TiO3- (NBT). The damping of longitudinal acoustic phonons was obtained from Brillouin scattering data. The main very broad maxima in hypersonic damping are found to be shifted to higher temperatures with respect to the corresponding main dielectric anomalies. Additional sharp peaks in the hypersonic damping are connected with an inner structural rearrangement in both crystals. This inner evolutionary process contributes to the hypersonic damping regardless of whether there is a real phase transition like in NBT or such a phase transition is frustrated like in PMN. Comparing NBT with PMN we suggest that the dynamics of these materials is determined by two coupled order parameters.
Temperature dependence of the generalized vibrational density of states of sodium bismuth titanate in the ferroelectric phase
(2000-08) Lushnikov, Sergey G.; Gvasaliya, S.N.; Siny, I.G.; Sashin, I.; Schmidt, V. Hugo; Uesu, T.
This paper describes studies of relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) by inelastic incoherent neutron scattering at temperatures from 50 to 300 K. Experimental time-of-flight spectra were recalculated into the generalized vibrational density of states (VDOS) function in terms of one-particle incoherent approximation and compared with the behavior of Raman spectra. It is shown that the VDOS of NBT in the ferroelectric phase has nontrivial temperature dependence. The relation between the temperature dependence of VDOS and the instability of the NBT crystalline lattice below room temperature is discussed.