Browsing by Author "Schmidt, V. Hugo"

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133 Cs NMR Study of the Ferroelectric and Antiferroelectric Transitions in CsH 2 PO 4
(1991-05-01) Schuele, Paul J.; Schmidt, V. Hugo
Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric (FE) and antiferroelectric (AFE) phase transitions in cesium dihydrogen phosphate were studied using 133Cs nuclear magnetic resonance at 6.5 MHz. The spin-latticerelaxation time T1 was measured at temperatures down to the FE Curie point TC at pressures of 1 bar and at 1.5 and 3.0 kbar, down to the triple point Tt = 124.6 K at 3.3 kbar, and down to the AFE Neel point TN at 3.6 kbar. Far from the transition T1 decreases exponentially with decreasing temperature due to 1D fluctuations associated with the Jb interactions in disordered hydrogen-bonded chains running along the b axis. As temperature is decreased further, T1 decreases linearly as the JC interaction between hydrogen-bonded chains in b-c planes becomes important. From these results the pressure dependences of Jb, JC and the interplanar interaction Ja were calculated. At 3.3 kbar Ja changes sign, so the stacking of ordered planes becomes AFE rather than FE. At pressures above about 9 kbar, where the interaction JC extrapolates to zero, a new AFE phase is predicted in which each b-axis chain is oriented AFE with respect to nearest neighbors in both the a and c directions.
17O NQR Study of the Antiferroelectric Phase Transition in TlH2PO4
(1988) Seliger, J.; Zagar, V.; Blinc, Robert; Schmidt, V. Hugo
The temperature dependence of the 1 7O NQR spectra in TlH2PO4 has been measured using a proton‐1 7 O nuclear quadrupole double resonance technique. The results show that the protons in the short one‐dimensionally linked O1–H1‐‐O1 and O2–H2‐‐O2hydrogen bonds are moving between two equilibrium sites above T c and freeze into one of the off‐center sites below T c . The protons in the asymmetric O3–H3‐‐O4hydrogen bonds are static and ordered above and below T c .
75As nuclear quadrupole resonance in weakly substitutionally disordered Rb1‑x(NH4)xH2AsO4
(1994-07-11) Papantopoulos, G.; Papavassiliou, G.; Milia, F.; Schmidt, V. Hugo; Drumheller, John E.; Pinto, Nicholas J.; Blinc, Robert; Zalar, B.
A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi 1-x Sr x )FeO 3-δ multiferroic ceramics
(2015) Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chan, T.-S.; Chien, R. R.; Son, H.
Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi1−xSrx)FeO3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr2+ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi1−xSrx)FeO3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi1−xSrx)FeO3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.
A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics
(2015-03) Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chan, Ting-Shan; Chien, R.R.; Son, Hyungbin
Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi1−xSrx)FeO3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr2+ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of Fesingle bondOsingle bondFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi1−xSrx)FeO3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi1−xSrx)FeO3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.
AC Susceptibility of Biased One-Dimensional Stochastic Ising Model.
(1971-06) Schmidt, V. Hugo
The ac susceptibility for the one‐dimensional Ising model is obtained for arbitrary coupling strength in the presence of a dc bias field strong enough to align most of the dipoles in one direction. The dipole flip probability is assumed proportional to the Boltzmann factor corresponding to half the energy change resulting from the flip. The general expression for ac susceptibility is analyzed in three limiting cases: weak coupling with strong bias, strong coupling with strong bias, and strong coupling with weak bias. In the latter case, relatively long chains of anti‐aligned dipoles exist and give rise to large susceptibility.
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
Anisotropy in Anomalies of Hypersound Velocity and Attenuation in Ferroelectric TSCC
(1985-01) Hikita, T.; Wang, J. T.; Schnackenberg, P. T.; Schmidt, V. Hugo
From Brillouin shift and linewidth of longitudinal phonons propagating along the [100] and [001] directions of TSCC, the polarization relaxation time was calculated to be τ=3.1×10-12/(Tc-T) sec below the transition temperature Tc. The anomalies in the longitudinal phonons of the [010] propagation were carefully examined using an annealed crystal of excellent quality. No essential difference was observed between the velocities of a normal and high quality crystals. The relaxation time was deduced as a function of temperature from the observed anomalies in the velocity and linewidth. Spectra are observed for nearly forward scattering from the q\varparallel[010] phonons.
Anode-pore tortuosity in solid oxide fuel cells found from gas and current flow rates
(2008) Schmidt, V. Hugo; Tsai, Chih-Long
The effect of solid oxide fuel cell (SOFC) anode thickness, porosity, pore size, and pore tortuosity on fuel and exhaust gas flow is calculated. Also determined is the concentration of these gases and of diluent gases as a function of position across the anode. The calculation is based on the dusty-gas model which includes a Knudsen (molecule–wall) collision term in the Stefan–Maxwell equation which is based on unlike-molecule collisions. Commonly made approximations are avoided in order to obtain more exact results. One such approximation is the assumption of uniform total gas pressure across the anode. Another such approximation is the assumption of zero fuel gas concentration at the anode–electrolyte interface under the anode saturation condition for which the SOFC output voltage goes to zero. Elimination of this approximation requires use of a model we developed (published elsewhere) for terminal voltage V as a function of electrolyte current density i. Key formulae from this model are presented. The formulae developed herein for gas flow and tortuosity are applied to the results of a series of careful experiments performed by another group, who used binary and ternary gas mixtures on the anode side of an SOFC. Our values for tortuosity are in a physically reasonable low range, from 1.7 to 3.3. They are in fair agreement with those obtained by the other group, once a difference in nomenclature is taken into account. This difference consists in their definition of tortuosity being what some call tortuosity factor, which is the square of what we and some others call tortuosity. The results emphasize the need for careful design of anode pore structures, especially in anode-supported SOFCs which require thicker anodes.
Anomalies of Hypersonic Velocity and Attenuation in Vinylidene Fluoride-Trifluoroethylene Copolymer from Brillouin Scattering
(1990) Liu, Zhimin; Schmidt, V. Hugo
The hypersonic behavior of a vinylidene fluoride and trifluoroethylene (70/30 mol %) copolymer has been investigated by Brillouin scattering without using index matching liquid. Both the glass and ferroelectric transitions are clearly seen in the acoustic behavior. The home‐prepared sample has excellent optical properties and let us obtain soundattenuation information in the temperature range of 23–123 °C, which another investigator did not discuss because of the bad optical properties of the sample surface. Anomalies of soundattenuation near the transition point have been observed.
Anomalies of hypersound velocity and attenuation in ferroelectric tris-sarcosine calcium chloride (TSCC) for small-angle and right-angle Brillouinscattering and Brillouin backscattering
(1986-07) Wang, J. T.; Schmidt, V. Hugo
The Brillouin spectra of ferroelectric tris-sarcosine calcium chloride have been observed using small-angle and right-angle scattering and also backscattering. For different-frequency phonons along the same direction, analogous anomalies in the sound velocity and the attenuation are seen. The smallest angle we have achieved is 7.48°. The temperature and frequency dependences of the sound velocity are discussed. The fact that the linewidth maximum for [001] phonons occurs somewhat below Tc seems to indicate that the anomalies are due to piezoelectric coupling induced by spontaneous polarization below Tc. For [010] phonons the elementary relaxation times which relate to the energy are estimated as τE0=5.25×10−13 sec above Tc and τE0=3.32×10−12 sec below Tc. The phonon attenuations are also estimated and compared with the observed ones. For the [001] phonons the elementary relaxation time is estimated as τ0=5.25×10−14 sec, in good agreement with the value obtained from right-angle Brillouin scattering.
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.
Brillouin scattering in a liquid crystal: frequency and temperature dependences
(1994-02-25) Siny, I.G.; Tu, Chi-Shun; Pastalan, Gregory J.; Schmidt, V. Hugo
Brillouin 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.
Brillouin scattering in Na1/2Bi1/2TiO3
(1994-02-01) Tu, Chi-Shun; Siny, I.G.; Schmidt, V. Hugo
Measurements 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.
Brillouin scattering in single crystals of Rb1-x(ND4)xD2AsO4
(1995) Tu, Chi-Shun; Schmidt, V. Hugo
The LA[100] (a-axis) Brillouin backscattering phonon spectra have been measured as a function of temperature (20-370 K) in the mixed deuterated ferroelectric (FE)-antiferroelectric (AFE) system Rb/sub 1-x/(ND/sub 4/)/sub x/D/sub 2/AsO/sub 4/ (DRADA-x) for ammonium concentrations x=0. 0.10 and 0.28. The Brillouin frequency shift with decreasing temperature shows hardening (positive coupling) whose steepness decreases with higher ND/sub 4/ content. For RbD/sub 2/AsO/sub 4/ (DRDA), a Landau-Khalatnikov-like maximum (which persists in weaker form for x=0.10) was observed and the polarization relaxation time is estimated to be /spl tau/-3.8/spl times/10/sup -12//(T/sub c/-T) sec, where T/sub c/ is the ferroelectric transition temperature. For both x=0.10 and 0.28, a broad damping peak anomaly which is stronger in x=0.28 was observed and can be connected with the dynamic order parameter fluctuations. Taking into account earlier NMR and dielectric results, we conclude that the local structure competition between FE ordering and AFE ordering is the origin of these broad damping anomalies.
Brillouin scattering near the ferroelectric phase transition in TSCC
(1985-06) Hikita, T.; Schnackenberg, P. T.; Schmidt, V. Hugo
Brillouin spectra from longitudinal phonons in ferroelectric tris-sarcosine calcium chloride (TSCC) propagating along [100], [010] and [001] have been measured as functions of temperature. Large anomalies were found in the Brillouin shift and linewidth in the [100] and [001] phonons. These anomalies are interpreted as arising from the linear coupling of the polarization and phonons. From the the temperature where the linewidth is maximum, the relaxation time of the polarization fluctuations is estimated to be τ = 3.1×10−12/(Tc−T) sec, where Tc is the ferroelectric transition temperature. We also observed anomalies in Brillouin shift and linewidth of the [010] phonons which propagate along the ferroelectric axis. These anomalies are interpreted as coming from electrostrictive coupling. The energy relaxation time was estimated to be τE=2.5×10−10/(T−Tc) sec in the paraelectric (PE) phase and τE=1.0x10−9/(T−Tc) sec in the ferroelectric (FE) phase, by comparing our Brillouin results with those f the ultrasonic measurements.
Brillouin scattering study of the ferroelectric phase transition in tris-sarcosine calcium chloride
(1985-01) Hikita, T.; Schnackenberg, P. T.; Schmidt, V. Hugo
Brillouin spectra from longitudinal phonons in ferroelectric tris-sarcosine calcium chloride propagating along [100], [010], and [001] have been measured as functions of temperature. Large anomalies were found in the Brillouin shift and linewidth in the [100] and [001] phonons. These anomalies are interpreted as arising from the linear coupling of the polarization and the phonons. From the temperature where the linewidth is maximum, the relaxation time of the polarization fluctuations is estimated to be τ=3.1×10−12/(Tc-T) sec, where Tc is the ferroelectric transition temperature. We also observed anomalies in Brillouin shift and linewidth of the [010] phonons which propagate along the ferroelectric b axis. These anomalies are interpreted as coming from electro- strictive coupling. The energy-relaxation time was estimated to be τE=2.5×10−10/(T-Tc) sec in the paraelectric phase and τE=1.0×10−9/(Tc-T) sec in the ferroelectric phase, by comparing our Brillouin results with those of the ultrasonic measurements.
Brillouin Spectroscopic Studies of VF2/TFE Copolymers
(1990-12-01) Liu, Zhimin; Schmidt, V. Hugo
The temperature dependence of sound velocity was studied in thin sheets of isotropic and oriented copolymers of vinylidene fluoride (81 mol%) and tetrafluoroethylene (19 mol%) by Brillouin scattering. For oriented samples, we studied the differences between the sound velocities along the x-axis (chain direction) and y-axis (perpendicular to x in sample plane). The ferroelectric-paraelectric transition points were not observed because this Tc is very close to the melting point of these copolymers. A partial thermal hysteresis loop was observed for all samples. The transitions from paraelectric phase to ferroelectric phase are completed near 85°C for the isotropic sample and the oriented sample with sound propagating along y. For propagation along x-axis, this transiton is completed at about 100°C which is very close to results from other methods. The behaviors of temperature dependence of sound velocity along x and y axes are quite different, as we observed also in VF2/TrFE copolymers. It allows generalization of our conclusions concerning copolymers of vinylidene fluoride and trifluoroethylene.