Browsing by Author "Siny, I.G."
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Item 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.Item Brillouin scattering in a liquid crystal: frequency and temperature dependences(1994-02-25) Siny, I.G.; Tu, Chi-Shun; Pastalan, Gregory J.; Schmidt, V. HugoBrillouin scattering measurements have been carried out in the liquid crystal mixture Merck ZLI-2861 with a low viscosity and a broad nematic phase. Scattering from longitudinal acoustic phonons with different wave vectors was measured by changing the geometries from backscattreing to nearly forward scattering that cover the frequency range from 10 to 1 GHz. In the nematic phase the dispersion in both the hypersonic attenuation and sound velocity is describing by a single relaxation process with the characteristic frequency 3.2 GHz. Evident anomalies in attenuation and hypersonic velocity were found at lower temperatures in the vicinity of the phase transition from a nematic to, probably, a smectic state.Item Brillouin scattering in Na1/2Bi1/2TiO3(1994-02-01) Tu, Chi-Shun; Siny, I.G.; Schmidt, V. HugoMeasurements of Brillouin scattering in single crystal Na1/2Bi1/2TiO3 are reported from 50 to 900 K which covers a cubic-tetragonal-trigonal succession of phase transitions, ending in a ferroelectric state. An extended dip in the hypersonic sound velocity and an extended maximum in the hypersonic damping were found with the extreme in the internansition region. These anomalies are connected with a response of mixed fluctuations. The Brillouin spectra background confirms the existence of a central peak in light scattering whose intensity increases drastically in the same internansition region.Item Critical acoustic behavior of the relaxor ferroelectric Na1/2Bi1/2TiO3 in the intertransition region(1995-03-01) Siny, I.G.; Tu, Chi-Shun; Schmidt, V. HugoA cubic-tetragonal-trigonal sequence of phase transitions in the relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) has been studied by Brillouin scattering. Pronounced anomalies for both the hypersonic velocity and damping have been found that peak just between two phase transitions in contrast to the ordinary well-known behavior with anomalies in the vicinity of every transition point. The fact that these anomalies are centered in the intertransition region and extend beyond both transitions is attributed to the fluctuations of two coupled order parameters associated with disorder in the Na- and Bi-cation distribution in a manner characteristic of relaxor ferroelectrics. Such a phenomenon seems to be typical of relaxor materials and to be the basis of the diffuse phase-transition dynamics.Item High frequency relaxation in nematic liquid crystal mixtures by Brillouin scattering(1995) Siny, I.G.; Tu, Chi-Shun; Schmidt, V. HugoBrillouin scattering as a function of scattering angles from 180° to ∼10° has been measured in the nematic phases of two liquid crystal mixtures, E. Merck ZLI-2861 and E7, without any preferred orientation. Such measurements at room temperature cover the frequency range from ∼10 to ∼1 GHz. The behavior of both the frequency shifts and linewidths is explained by a single time internal relaxation process with the characteristic frequencies 3.2 (ZLI-2861) and 1.2 (E7) GHz respectively. The behavior of the hypersonic attenuation shows no correlation with the static bulk viscosity in the nematic phase.Item Hypersound anomalies and elastic constants in single-crystal PbMg1/3Nb2/3O3 by Brillouin scattering(1995-12-01) Tu, Chi-Shun; Schmidt, V. Hugo; Siny, I.G.The longitudinal (LA) and transverse (TA) Brillouin spectra along [001] phonon direction have been measured as a function of temperature (50–475 K) in single‐crystal PbMg1/3Nb2/3O3 with scattering angles θ s =180° and 32.5±0.2°. The Brillouin frequency shift with decreasing temperature shows a broad softening anomaly for both LA and TA phonon modes. For θ s =180°, a gradual growth in damping with maximum near 270 K is observed and is attributed to order parameter fluctuations. An additional Landau–Khalatnikov maximum is also observed at T c ∼212 K. This anomaly implies a rapid growth of ferroelectric ordering near T c and is consistent with the earlier linear birefringence results reported by Westphal et al. The elastic stiffness and compliance constants, C E 11, C E 44, and s E 44 are also determined between 200 and 370 K.Item 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.Item Sequence of dielectric anomalies and high-temperature relaxation behavior in Na1/2Bi1/2TiO3(1994-05-01) Tu, Chi-Shun; Siny, I.G.; Schmidt, V. HugoMeasurements of relative permittivity (ɛ=ɛ′-iɛ″) in the complex perovskite crystal Na1/2Bi1/2TiO3 were carried out as functions of frequency (0.02–300 kHz) and temperature (300–900 K) in both heating and cooling processes. We have found three anomalies separating four different temperature regions. In order of decreasing temperature, the first anomaly centered near 800 K is a smooth but rapid increase in low-frequency (<50 kHz) permittivity to very high values of order 1×105, associated with the cubic to tetragonal transition. This high permittivity shows considerable dispersion and decreases quite rapidly as temperature drops from 860 to 640 K. We attribute this large permittivity to superparaelectric clusters, and their slow relaxation to random electric fields from randomly placed Na+ and Bi3+ ions which redistribute and allow cluster reversal with the ρɛ0ɛ′dc time constant determined by the conductivity and permittivity. The other anomalies, seen previously by others, are a high-frequency permittivity (50 to 300 kHz) which shows no dispersion and increases slowly down to a peak of 3×103 near 640 K, where a trigonal phase exhibiting antiferroelectric and/or incommensurate characteristics is reported to set in. It then drops and develops weak low-frequency dispersion below 550 K which is the upper limit reported for spontaneous polarization decay.Item Specific features of hypersonic damping in relaxor ferroelectrics(1995) Siny, I.G.; Lushnikov, Sergey G.; Tu, Chi-Shun; Schmidt, V. HugoA 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.Item 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.