Browsing by Author "Gvasaliya, S.N."

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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
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.
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.