Browsing by Author "Tuthill, G. F."

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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
Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics
(1996-10) Schmidt, V. Hugo; Tuthill, G. F.; Tu, Chi-Shun; Schogoleva, T.V.; Meschia, Steven C. L.
Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ. In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒfalls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 Å and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.
Damping and electromechanical energy losses in the piezoelectric polymer PVDF
(2004) Vinogradov, A. M.; Schmidt, V. Hugo; Tuthill, G. F.; Bohannan, Gary W.
Polyvinylidene fluoride (PVDF) is a piezoelectric polymer that has been used in many applications including microphones, transducers, sensors and actuators. The electromechanical properties of PVDF are commonly defined by the constitutive equations of piezoelectricity. This paper presents experimental evidence that the assumptions underlying the theory of piezoelectricity have certain limitations in terms of representing adequately the electromechanical properties of PVDF. It is shown that PVDF tends to demonstrate time-dependent behavior in the form of viscoelastic creep and dielectric relaxation, and measurable energy losses under cyclic loading conditions. Moreover, the response of PVDF strongly depends on temperature and cyclic frequencies.
Deuteration and pressure effects on landau free energy expansion coefficents in RbDXH2-xPO4
(1981-10) Schnackenberg, P. T.; Pipman, J.; Schmidt, V. Hugo; Tuthill, G. F.
Results are presented for undeuterated, 30% deuterated and 97% deuterated monocrystals of rubidium dihydrogen phosphate (RDP) for which the polarization was measured at various applied fields and accurately controlled temperatures within a degree above the ferroelectric transition temperature. The ferroelectric transition was found to be of second order at all deuterations. Some of these results were presented earlier.1–3
Domain wall freezing in KDP-type ferroelectrics
(2000-02) Schmidt, V. Hugo; Bohannan, Gary W.; Arbogast, Darin; Tuthill, G. F.
Hysteresis loops in KH2PO4 (KDP) and its ferroelectric (FE) isomorphs disappear some 60 K below Tc. This disappearance may result from an order–disorder transition of the domain wall. The lowest energy wall consists of a single layer of nonpolar H2PO4 groups of Slater energy ε0. Including only the Slater/Takagi interactions predicts that a domain wall can become wider by having small protrusions that then diffuse along the wall. Reducing temperature would decrease domain wall mobility without causing a freezing transition. However, if one includes the Ishibashi dipolar interaction, this dipolar energy is minimized for a zero-entropy smooth domain wall with a particular ordered H-bond arrangement. Accordingly, there could be an order–disorder transition within the wall, if the bias “field” favoring this H-bond ordering is not great enough to smear out the transition. We are applying this model to predict domain wall mobility temperature dependence, and simultaneously measuring FE hysteresis in KDP-ferroelectrics to determine the nature and sharpness of this proposed domain wall transition.
Electric-field effects of dielectric and optical properties in Pb(Mg1/3Nb2/3)0.65Ti0.35O3 crystal
(2005) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Tuthill, G. F.
Dielectric properties and domain structure have been measured as functions of temperature in a (001)-oriented Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal with and without a prior dc electric (E)-field poling. Without E-field poling, the dielectric loss exhibits a frequency-dependent maximum in the region of 120–180K, which can be described by a Vogel–Fulcher equation and fractal cluster model which implies structural irregularities within domains. With a prior poling a long-range monoclinic (tetragonal)→tetragonal (monoclinic) transition takes place near 212K upon heating. “Monoclinic (tetragonal)” represents that dominant monoclinic phase domains coexist with a small fraction of tetragonal phase domains. Optical transmission and birefringence were significantly enhanced by a prior E-field poling. The Cauchy equations for ordinary no and extraordinary nerefractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second-harmonic generation was not found.
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.
Random barrier height model for phase shifted conductivity in perovskites
(1997) Schmidt, V. Hugo; Tuthill, G. F.; Tu, Chi-Shun; Schogoleva, T.V.; Meschia, Steven C. L.
A large dielectric permittivity peak which occurs at low frequency and high temperature in many perovskite crystals and ceramics, and an associated difference in dc and ac conductivity, is attributed to phase shifted conductivity resulting from mobility barriers of different heights. A model is developed which has intrinsic barriers, and higher and more widely spaced extrinsic barriers, and special cases are examined. Model predictions show good agreement with experimental results of Stumpe, Wagner, and Bäuerle for an SrTiO3 single crystal 1.02 mm thick. The additional peaks seen by them at higher temperature for a crystal 0.24 mm thick are qualitatively accounted for by adding a third set of still higher barriers with spacing equal to the crystal thickness.