Browsing by Author "Howell, Francis L."

Now showing 1 - 8 of 8

Complex permittivity of the deuterated and undeuterated proton glass Rb1-x(NH4)xH2AsO4
(1992-11-01) Howell, Francis L.; Pinto, Nicholas J.; Schmidt, V. Hugo
Dielectric measurements have been performed in both the deuterated and undeuterated proton glass rubidium ammonium dihydrogen arsenate Rb1−x(NH4)xH2AsO4 along the tetragonal a axis. Ammonium concentrations x of 0, 0.01, 0.05, and 0.10 were studied for the undeuterated sample, while ammonium concentrations of 0, 0.05, and 0.10 were studied for the deuterated counterpart. Glassy behavior is present for x=0.05 and 0.10 but could not be observed for lower ammonium concentrations. For x=0.05 and 0.10 we observed coexistence of paraelectric or proton-glass and ferroelectric order below the ‘‘glass transition’’ temperature Tg due to the random spatial fluctuations of the ammonium cation. Cole-Cole plots for the x=0.10 sample show that there is a distribution of relaxation times below Tg. This relaxation occurs in the proton-glass portion of the crystal. The activation energies corresponding to these relaxation processes are calculated.
Deuteron NMR study of dynamics and of coexistence of paraelectric and ferroelectric phases in Rb0.90(ND4)0.10D2AsO4
(1993-09-01) Pinto, Nicholas J.; Howell, Francis L.; Schmidt, V. Hugo
The deuteron glass Rb1−x(ND4)xD2AsO4 (DRADA) is a mixed crystal of RbD2AsO4 (DRDA) and ND4D2AsO4 (DADA). Deuteron nuclear magnetic resonance has been performed on the acid and ammonium deuterons. The crystal studied has an ammonium concentration (x=0.10) that puts it in the coexistence region of the phase diagram. Line-shape measurements of the ammonium deuterons show the coexistence of the ferroelectric (FE) and paraelectric (PE) phases as the temperature is lowered below the ferroelectric-phase-transition temperature Tc. The acid deuteron line shape on the other hand is found to broaden as the temperature is reduced but is unaffected by the ferroelectric transition. Spin-lattice-relaxation measurements have been performed and the activation energies for the relaxation processes have been computed. The relaxation-rate anomaly for acid deuterons in the ferroelectric-transition range indicates a short correlation length for the FE phase in the coexistence region of the phase diagram.
Deuteron NMR Study of Lithium Hydrazinium Sulfate
(1969-09) Howell, Francis L.; Schmidt, V. Hugo
The magnetic resonance spectrum of the deuterons in single crystals of LiN2D5SO4 has been studied between 78 and 458°K. The electric quadrupole coupling constants, the asymmetry parameters, and the orientations of the electric field gradient (efg) tensors relative to the crystal axes were obtained at 78°K, 193°K, 25°C, 65°C, and 95°C. The orientations of the efg tensors are considered in relation to nearby atomic sites. At 78°K the spectrum contains distinct lines due to the five deuterons of the N2D5 + ion. Between 110 and 150°K the lines due to the three deuterons of the ND3 group broaden and merge to a single pair which is still visible at 185°C. A small change with temperature in the coupling constant associated with this pair of lines is discussed. The lines due to the two deuterons of the ND2 group broaden and merge to a single pair between 0 and 50°C. These line mergers result from hindered rotation of the ND2 and ND3 groups. Estimates of the activation energies for these motions are made. These changes in the spectrum are related to the results of earlier NMR studies and to previous electrical conductivity and dielectric constant measurements.
Dielectric permittivity in paraelectric/ferroelectric coexistence region in several proton glasses
(1995) Howell, Francis L.; Fundaun, I. L.; Stadler, S.; Meschia, Steven C. L.; Tu, Chi-Shun; Schmidt, V. Hugo
We present results for the complex permittivities of several proton glasses of the form M/sub 1-x/(NZ/sub 4/)/sub x/Z/sub 2/AO/sub 4/, where M=Rb, Z=H or D, and A=As or P. All measurements were made perpendicular to the ferroelectric axis, so no effects of domain wall motion occurred. Phase coexistence was apparent in all species. The phosphate glasses exhibited a much narrower coexistence composition range than the arsenate glasses.
Dielectric Properties of Lithium Hydrazinium Sulfate.
(1971-12) Schmidt, V. Hugo; Drumheller, John E.; Howell, Francis L.
The dc conductivity and ac dielectric susceptibility of normal and deuterated lithium hydrazinium sulfate have been measured over a wide temperature range at frequencies up to 9.33 GHz. Over a very large temperature and frequency range the real and imaginary parts of the susceptibility are very large (up to ε′≃ε''≃106) and vary with frequency somewhat as f−12. This unusual behavior is shown to result form the nearly one-dimensional protonic conductivity and its extreme sensitivity to barriers caused by local structural defects. Etching studies indicate that the crystal is not ferroelectric, implying that the apparent hysteresis loops result from saturation of the ac conduction.
Phase Transitions in KH2PO4 and RbH2PO4 to 14 GPa Observed by Capacitance Change in a Diamond Anvil Cell
(1991) Bao, Z. X.; Schmidt, V. Hugo; Howell, Francis L.
Phase transitions in KH2PO4(KDP) and RbH2PO4 (RDP) at room temperature for pressures up to 14 GPa have been determined by means of capacitance measurements using a diamond anvil cell.Phase transitions occur in KDP near 2.5 and 7.0 GPa. In RDP, a transition was detected near 5.4 GPa.
Phase transitions in tris-sarcosine calcium chloride (TSCC) to 14 GPa
(1993) Bao, Z. X.; Schmidt, V. Hugo; Howell, Francis L.
The capacitance of samples of tris-sarcosine calcium chloride (TSCC) has been measured at room temperature at pressures up to 14 GPa. A diamond anvil cell was employed to control the pressure. Experimental results indicate that TSCC undergoes two phase transitions, near 1.3 GPa and 4.1 GPa.
Proton Glass Dielectric Susceptibility Compared with Monte Carlo and Bound Charge Semiconductor Model Predictions
(1990-06-01) Schmidt, V. Hugo; Trybula, Z.; He, Di; Drumheller, John E.; Stigers, C.; Li, Zhouning; Howell, Francis L.
Our latest results for dielectric permittivity and loss and protonic conductivity in RADP, RADA and DRADA proton glasses are presented. Improvements in our “bound charge semiconductor” model for dielectric behavior are discussed. Monte Carlo studies of the phase diagram and polarization decay are described. Bias order parameter vs. temperature plots from the simulation and from ND4 deuteron NMR lineshapes are compared.