College of Letters & Science
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Item Short-range interaction explanation of ferroelectric, antiferroelectric and proton glass behavior in pure RDP, ADP, and mixed RDP-ADP crystals respectively(1985-01) Schmidt, V. Hugo; Wang, J. T.; Schnackenberg, P. T.A unified model is presented for Rb1-x(NH4)xH2PO4 crystals over the whole range x=0 (RDP) to x=1 (ADP). Two short-range interactions are postulated. One is the Slater energy ε0 which is kept at the value for RDP for all x. The other is an interaction εa between two hydrogens in O-H...O bonds across an NH4+ ion from each other. It is responsible for the off-center positions observed for ammonium ions in the antiferroelectric phase of ADP. Its strength is assumed proportional to x. By minimizing the free energy, one finds a range of x for which no transition occurs, but instead proton glass behavior sets in. Fox x near 0 and 1 respectively, first-order ferroelectric and antiferroelectric transitions are predicted. Both phase boundaries are close to those observed experimentally.Item Thermal Properties of the Frustrated Rb0.52(ND4)0.48D2PO4 System at Low Temperatures(1985-01) Lawless, W. N.; Schmidt, V. HugoSpecific heat and thermal conductivity data, 1.5–35 K, are reported on the mixed (frustrated) ferroelectric-antiferroelectric crystal Rb0.52(ND4)0.48D2PO4. A glasslike linear term in the specific heat is resolved below 5 K and ascribed to random electric fields caused by the random distribution of Rb ions, in good agreement with the theory of McWhan et al. A maximum in C/T3 at 13.5 K is attributed to dispersionless motions of the deuterium ions, by analogy with KDP. In contrast to the specific heat, the thermal conductivity is not glasslike, displaying a maximum at 12.5 K of 37 mW cm-1K-1. At lower temperatures, the behavior of the thermal conductivity is tentatively ascribed to resonant phonon scattering from the Rb–ND4 system. The Debye temperature for the crystal is 303 K, in reasonably good agreement with extrapolations based on θD for KDP.Item Anisotropy in Anomalies of Hypersound Velocity and Attenuation in Ferroelectric TSCC(1985-01) Hikita, T.; Wang, J. T.; Schnackenberg, P. T.; Schmidt, V. HugoFrom 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.Item Magnetic pendulum apparatus for analog demonstration of first-order and second-order phase transitions and tricritical points(1984) Schmidt, V. Hugo; Childers, B. R.A simple apparatus is described in which the deflection of a pendulum with an electromagnet mounted at its tip is the analog of the magnitude of the order parameter in a phase transition. Its deflection is governed by the current through this electromagnet and currents through fixed electromagnets just below it. By suitable variation of these currents, the pendulum can be made to undergo gradual (second‐order transition analog) or sudden (first‐order transition analog) displacements from the vertical, as well as the intermediate situation corresponding to a tricritical point. Results for critical and tricritical exponents obtained with this apparatus are presented.Item Review of order-disorder models for KDP-family crystals(1987-03) Schmidt, V. HugoThe various order-disorder-type models for crystals in the KDP (potassium dihydrogen phosphate) family are reviewed, beginning with the Slater-Takagi-Senko model, continuing with tunneling additions by Blinc and Svetina, and analyzing the pressure-induced tricritical point in the Slater-Takagi-Senko model. The possibility of a deuteration-induced tricritical point at atmospheric pressure in RDP (rubidium dihydrogen phosphate) is explored. The Ishibashi model for ADP (ammonium dihydrogen phosphate) is reviewed, and finally various theories and computer simulations for the mixed RDP-ADP crystal system are discussed in terms of their prediction of the observed ferroelectric and antiferroelectric transitions and proton glass behavior.Item N.m.r. study of the ferroelectric phase transition in a 70/30 mol% copolymer of vinylidene fluoride (VF2) and trifluoroethylene (TrFE)(1985-07) Legrand, J. F.; Schuele, Paul J.; Schmidt, V. HugoNuclear magnetic resonance (n.m.r.) studies of 19F nuclei in a 70/30 mol% random copolymer of vinylidene fluoride and trifluoroethylene were performed at 9.14 MHz and 20.0 MHz. The free induction decays (FIDs) were analysed in terms of two T2 components attributed to the amorphous and crystalline portions of the polymer. The changes in crystallinity as well as the effects of the ferroelectric transition were observed during cycles of heating and cooling between 20°C and 140°C. The crystalline component of the FID lengthens by a factor of 2 at 100°C on heating and decreases by this factor at 60°C on cooling, thus exhibiting the thermal hysteresis of this ferroelectric transition. The spin-lattice relaxation was also investigated. From measurements at 9.14 MHz the observed longitudinal relaxation time T1 appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well defined minima of T1 were observed, which are associated with the ferroelectric transition (especially after repeated annealing of the samples). Results are discussed in terms of the crystalline phase structure, which appears dynamically disordered above the ferroelectric phase transition. An analogy is considered with the plastic phase transitions encountered in molecular crystals.Item Brillouin scattering near the ferroelectric phase transition in TSCC(1985-06) Hikita, T.; Schnackenberg, P. T.; Schmidt, V. HugoBrillouin 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.Item Comment on “dielectric study of the ferroelectric transition of KH2PO4”(1976-08) Western, Arthur B.; Schmidt, V. HugoWe reanalyze a.c. dielectric data taken by Eberhard and Horn for KH2PO4 near Tc, and obtain a much lower value of the critical bias field, near 300 V/cm in accord with results found by other workers.Item Soft modes and proton tunneling in PbHPO4, squaric acid and KH2PO4, type ferroelectrics(1984-01) Blinc, Robert; Schmidt, V. HugoIt is pointed out that the huge observed differences in the soft mode frequencies of KH2PO4, PbHPO4, CsH2PO4 and squaric acid do not necessarily reflect a large difference in the proton 0—H———O tunneling frequencies but rather result from a difference in the short range correlations renormalizing the single proton O——H—O tunneling frequency.Item Slater-senko model for CsH2PO4(1984-04) Schmidt, V. HugoThis model for CsH2PO4 considers all six Slater H2PO4 configurations and employs the Senko long-range interaction to obtain a ferroelectric transition above 0 K. Model predictions are compared with experimental results for susceptibility, spontaneous polarization, and correlation length.
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