Negative evidence for a proton-tunneling mechanism in the phase transition ofKH2PO4-type crystals

1987 ◽  
Vol 36 (1) ◽  
pp. 874-876 ◽  
Author(s):  
Mizuhiko Ichikawa ◽  
Kiyosi Motida ◽  
Noboru Yamada
Keyword(s):  
Phase Transition ◽  
Negative Evidence ◽  
Proton Tunneling ◽  
Tunneling Mechanism

Spectroscopic and calorimetric studies of proton tunneling and deuteration-induced phase transition in crystalline iodo-hydroxyphenalenone and iodo-deuteroxyphenalenone

2001 ◽  
Vol 342 (1-2) ◽  
pp. 22-26 ◽  
Author(s):  
Takasuke Matsuo ◽  
Shipra Baluja ◽  
Yojiro Koike ◽  
Mitsuo Ohama ◽  
Tomoyuki Mochida ◽  
...  
Keyword(s):  
Phase Transition ◽  
Proton Tunneling ◽  
Calorimetric Studies

SOME CONSIDERATIONS ON THE ROLE OF PROTON TUNNELING IN CERTAIN CHARGE TRANSFER PROCESSES

1959 ◽  
Vol 37 (1) ◽  
pp. 178-189 ◽  
Author(s):  
B. E. Conway
Keyword(s):  
Isotopic Ratio ◽  
Proton Tunneling ◽  
Tunneling Mechanism ◽  
Symmetry Factor ◽  
Separation Factors ◽  
Favorable Case ◽  
High Overpotentials ◽  
Proton Discharge ◽  
New Criterion

Quantum mechanical tunneling of H atoms in certain reactions can have a rate comparable with that of the corresponding classical reaction. Proton tunneling appears to be the mechanism of proton transport in ice. Further studies of this mechanism have been made by determination of the a-c. and d-c. conductance of D2O (and H2O) ice under rigorous conditions of purification. Pre-electrolysis techniques have been applied to the ultrapurification of the D2O and H2O used for the conductance determinations. Isotopic ratios of conductance in solid H2O and D2O are obtained and discussed in terms of the mechanism of H+ or D+ transport in the solid and liquid substances. The theory of proton tunneling previously given is improved by using a quantal distribution function in the calculation of tunneling rates and better agreement with experiment is then obtained. The theoretical isotopic ratio of conductances by the tunneling mechanism in the ices is similar to that found experimentally and smaller than that predicted classically.Since the proton tunneling theory is quantitatively successful in the case of conductance of ice, its examination for other electrochemical processes involving H is necessary. A favorable case for investigation is the electrochemical hydrogen evolution reaction for which the barrier height for tunneling can be varied. Tunneling probabilities are calculated for proton and deuteron discharge at mercury from acid solutions using the theory of Eckart. At intermediate overpotentials the Tafel equation is still obeyed; at low overpotentials a linear current–potential relation is found as in the classical theory. H/D separation factors are calculated for the tunneling mechanism and it is shown that at intermediate and high overpotentials, tunneling leads to values of the separation factor comparable with those deduced classically. Only at low electrochemical rates of H or D production are high separation factors predicted. The tunneling mechanism, however, is distinguishable from the classical mechanism by a new criterion: the Tafel slopes for the tunneling process would be considerably larger (0.2–0.3) than those arising classically (0.12) for a simple discharge mechanism assuming a symmetry factor of 0.5. It is concluded that in certain cases proton tunneling may occur simultaneously with the classical reaction in electrochemical proton discharge and lead to anomalous Tafel 'b' values which are sometimes observed experimentally.

Role of the proton tunneling in the phase transition of K3D1?x H x (SO4)2

1995 ◽  
Vol 99 (1-2) ◽  
pp. 55-70 ◽  
Author(s):  
Y. Moritomo ◽  
Y. Tokura ◽  
N. Nagaosa ◽  
T. Suzuki ◽  
K. Kumagai
Keyword(s):  
Phase Transition ◽  
Proton Tunneling

Microstructure of laser-irradiated, conducting Kapton polyimide

1995 ◽  
Vol 53 ◽  
pp. 502-503
Author(s):  
D. L. Callahan ◽  
Z. Ball ◽  
H. M. Phillips ◽  
R. Sauerbrey
Keyword(s):  
Electron Microscopy ◽  
Phase Transition ◽  
Transmission Electron Microscopy ◽  
Cross Section ◽  
Film Surface ◽  
Cross Sectional ◽  
General Utility ◽  
Transmission Electron ◽  
Considerable Debate ◽  
Potential Applications

Ultraviolet laser-irradiation can be used to induce an insulator-to-conductor phase transition on the surface of Kapton polyimide. Such structures have potential applications as resistors or conductors for VLSI applications as well as general utility electrodes. Although the percolative nature of the phase transformation has been well-established, there has been little definitive work on the mechanism or extent of transformation. In particular, there has been considerable debate about whether or not the transition is primarily photothermal in nature, as we propose, or photochemical. In this study, cross-sectional optical microscopy and transmission electron microscopy are utilized to characterize the nature of microstructural changes associated with the laser-induced pyrolysis of polyimide.Laser-modified polyimide samples initially 12 μm thick were prepared in cross-section by standard ultramicrotomy. Resulting contraction in parallel to the film surface has led to distortions in apparent magnification. The scale bars shown are calibrated for the direction normal to the film surface only.

The structure of membranes and membrane proteins embedded in amorphous ice

1992 ◽  
Vol 50 (1) ◽  
pp. 432-433
Author(s):  
Uwe Lücken ◽  
Joachim Jäger
Keyword(s):  
Phase Transition ◽  
Transition Temperature ◽  
Membrane Proteins ◽  
Phase Transition Temperature ◽  
Lipid Vesicles ◽  
Freezing Stress ◽  
Amorphous Ice ◽  
Different Temperatures ◽  
Band Pass ◽  
Lipid Polymorphism

TEM imaging of frozen-hydrated lipid vesicles has been done by several groups Thermotrophic and lyotrophic polymorphism has been reported. By using image processing, computer simulation and tilt experiments, we tried to learn about the influence of freezing-stress and defocus artifacts on the lipid polymorphism and fine structure of the bilayer profile. We show integrated membrane proteins do modulate the bilayer structure and the morphology of the vesicles.Phase transitions of DMPC vesicles were visualized after freezing under equilibrium conditions at different temperatures in a controlled-environment vitrification system. Below the main phase transition temperature of 24°C (Fig. 1), vesicles show a facetted appearance due to the quasicrystalline areas. A gradual increase in temperature leads to melting processes with different morphology in the bilayer profile. Far above the phase transition temperature the bilayer profile is still present. In the band-pass-filtered images (Fig. 2) no significant change in the width of the bilayer profile is visible.

Electron Microscope study of commensurate-incommensurate phase transition in BaZnGeO4

1990 ◽  
Vol 48 (4) ◽  
pp. 172-173
Author(s):  
Naoki Yamamoto ◽  
Makoto Kikuchi ◽  
Tooru Atake ◽  
Akihiro Hamano ◽  
Yasutoshi Saito
Keyword(s):  
Phase Transition ◽  
Electron Microscope Study ◽  
Room Temperature ◽  
Hexagonal Lattice ◽  
Ferroelectric Materials ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
X Ray ◽  
Periodic Arrays ◽  
Modulation Wave Vector

BaZnGeO4 undergoes many phase transitions from I to V phase. The highest temperature phase I has a BaAl2O4 type structure with a hexagonal lattice. Recent X-ray diffraction study showed that the incommensurate (IC) lattice modulation appears along the c axis in the III and IV phases with a period of about 4c, and a commensurate (C) phase with a modulated period of 4c exists between the III and IV phases in the narrow temperature region (—58°C to —47°C on cooling), called the III' phase. The modulations in the IC phases are considered displacive type, but the detailed structures have not been studied. It is also not clear whether the modulation changes into periodic arrays of discommensurations (DC’s) near the III-III' and IV-V phase transition temperature as found in the ferroelectric materials such as Rb2ZnCl4.At room temperature (III phase) satellite reflections were seen around the fundamental reflections in a diffraction pattern (Fig.1) and they aligned along a certain direction deviated from the c* direction, which indicates that the modulation wave vector q tilts from the c* axis. The tilt angle is about 2 degree at room temperature and depends on temperature.

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