Browsing by Author "Katiyar, R.S."

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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.
Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals
(1999-01-01) Tu, Chi-Shun; Katiyar, R.S.; Schmidt, V. Hugo; Guo, R.; Bahalla, A.S.
The longitudinal (LA) Brillouin back-scattering spectra along the [001] phonon direction have been measured as a function of temperature for RbTiOAsO4 (RTA) and KTiOPO4 (KTP) single crystals. As temperature increases, the acoustic-phonon frequencies of RTA and KTP show a clear softening (which reaches a turning point at Tc∼800°C for RTA). We conclude that the transitions in RTA and KTP are either weakly first order or of second order. It seems likely that the alkali ions (K+ or Rb+) are strongly involved in the soft mode. A broad damping evolution (which attains a maximum at Tc∼800 °C in RTA) was observed for both crystals and can be attributed to the dynamic order-parameter fluctuations. The optical transmissions, refractive indices (nx,ny,nz) and the Cauchy equations (for RTA) were obtained as a function of wavelength. The LA[001] sound velocities VLA and elastic constants C33+(e233/ɛs33) were also calculated at room temperature for both crystals.
Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals
(1997-10-01) Tu, Chi-Shun; Yeh, Y.-L.; Schmidt, V. Hugo; Chien, R.-M.; Katiyar, R.S.; Guo, Ruqian; Guo, Ruyan; Bahalla, A.S.
The longitudinal Brillouin backscattering spectra along the [001] phonon direction have been measured as a function of temperature (24-896 °C) for both CsTiOAsO4 (CTA) and KTiOAsO4 (KTA). As the temperature approaches the ferroelectric transition temperature Tc, the acoustic phonon frequencies of both CTA and KTA show a strong softening (which reaches a minimum at Tc∼688°C for CTA). For both CTA and KTA, a broad damping peak was observed and can be attributed to the dynamic order-parameter fluctuations. This anomaly implies that an η2μ-type coupling is the dominant mechanism for the acoustic soft modes in CTA and KTA. Here η is the order parameter and μ is the strain. The refractive indices (nx,ny,nz) and Cauchy’s equations were obtained as a function of wavelength (0.4–1.7 μm) for KTA. The phase matching angles (θ,φ) of second harmonic generation for the fundamental wavelengths of 1.064 (type I) and 1.32 (types I and II) were also calculated.
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.