Browsing by Author "Schuele, Paul J."

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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.
Cs 133 Nuclear Magnetic Resonance Study of One-Dimensional Fluctuations in CsH 2 PO 4 and Its Ferroelectric and Antiferroelectric Transitions at High Pressure
(1989-02) Schuele, Paul J.; Schmidt, V. Hugo
Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric and antiferroelectric phase transitions in cesium dihydrogen phosphate were studied by means of the Cs133 nuclear magnetic resonance (NMR) spin-lattice relaxation time T1. We measured T1 at 6.5 MHz at temperatures down to the ferroelectric (FE) Curie point TC at 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 antiferroelectric (AFE) Néel point TN at 3.6 kbar. With decreasing temperature, T1 first decreases exponentially due to 1D fluctuations associated with the Jb interactions in disordered hydrogen-bonded chains running along b. As the temperature falls further, T1 then decreases linearly as the Jc interaction between these chains in hydrogen-bonded planes comes into play. From these results and the known pressure derivatives of TC and TN, we calculated pressure dependences for Jb, Jc, and for the interplanar interaction Ja. At 3.3 kbar Ja changes sign, so the plane stacking becomes AFE instead of FE. Above 8.9 kbar, where Jc extrapolates to zero, a new AFE phase with a checkerboard arrangement of FE b chains is predicted.
High-pressure–low-temperature apparatus for NMR study of phase transitions
(1982-07) Schuele, Paul J.; Schmidt, V. Hugo
An apparatus has been developed for nuclear‐magnetic‐resonance (NMR)measurement of relaxation times in solids at hydrostaticpressures to 7 kbar and temperatures down to 77 K. Sample temperature can be controlled accurately with ±2 mK stability allowing measurement of dynamic phenomena very near phase transitions. The high‐pressure vessel is equipped with additional electrical feedthroughs so that dielectricmeasurements can be carried out concurrently, providing additional information on ferroelectric phase transitions.
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. Hugo
Nuclear 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.