Structural changes in the myosin filament and cross-bridges during active force development in single intact frog muscle fibres: stiffness and X-ray diffraction measurements

The structure of d(ecagonal)-Al-Cu-Rh has been studied as a function of temperature by in-situ single-crystal X-ray diffraction in order to contribute to the discussion on energy or entropy stabilization of quasicrystals (QC) [1]. The experiments were performed at 293 K, 1223 K, 1153 K, 1083 K, and 1013 K. A common subset of 1460 unique reflections was used for the comparative structure refinements at each temperature. The results obtained for the HT structure refinements of d-Al-Cu-Rh QC seem to contradict a pure phasonic-entropy-based stabilization mechanism [2] for this QC. The trends observed for the ln func(I(T1 )/I(T2 )) vs.|k⊥ |^2 plots indicate that the best on-average quasiperiodic order exists between 1083 K and 1153 K, however, what that actually means is unclear. It could indicate towards a small phasonic contribution to entropy, but such contribution is not seen in the structure refinements. A rough estimation of the hypothetic phason instability temperature shows that it would be kinetically inaccessible and thus the phase transition to a 12 Å low T structure (at ~800 K) is most likely not phason-driven. Except for the obvious increase in the amplitude of the thermal motion, no other significant structural changes, in particular no sources of additional phason-related configurational entropy, were found. All structures are refined to very similar R-values, which proves that the quality of the refinement at each temperature is the same. This suggests, that concerning the stability factors, some QCs could be similar to other HT complex intermetallic phases. The experimental results clearly show that at least the ~4 Å structure of d-Al-Cu-Rh is a HT phase therefore entropy plays an important role in its stabilisation mechanism lowering the free energy. However, the main source of this entropy is probably not related to phason flips, but rather to lattice vibrations, occupational disorder unrelated to phason flips like split positions along the periodic axis.

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Investigations of Actinide Metals and Compounds under Pressure Provide Important Insights into Bonding and Chemistry

MRS Proceedings ◽

10.1557/proc-802-dd1.5 ◽

2003 ◽

Vol 802 ◽

Cited By ~ 3

Author(s):

R. G. Haire ◽

S. Heathman ◽

T. Le Bihan ◽

A. Lindbaum ◽

M. Iridi

Keyword(s):

Structural Changes ◽

Chemical Properties ◽

X Ray Diffraction ◽

Interatomic Distances ◽

X Ray ◽

Effect Of Pressure ◽

Electronic Configurations ◽

5F Electrons ◽

Actinide Series ◽

Alloys And Compounds

ABSTRACTOne effect of pressure on elements and compounds is to decease their interatomic distances, which can bring about dramatic perturbations in their electronic nature and bonding, which can be reflected in changes in physical and/or chemical properties. One important issue in the actinide series of elements is the effect of pressure on the 5f-electrons. We have probed changes in electronic behavior with pressure by monitoring structure by X-ray diffraction, and have studied several actinide metals and compounds from thorium through einsteinium. These studies have employed angle dispersive diffraction using synchrotron radiation, and energy dispersive techniques via conventional X-ray sources. The 5f-electrons of actinide metals and their alloys are often affected significantly by pressure, while with compounds, the structural changes are often not linked to the involvement of 5 f-electron. We shall present some of our more recent findings from studies of selected actinide metals, alloys and compounds under pressure. A discussion of the results in terms of the changes in electronic configurations and bonding with regard to the element's position in the series is also addressed.

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Structural changes in titanium dioxide nanocrystals during plastic deformation

Journal of Applied Crystallography ◽

10.1107/s0021889805020418 ◽

2005 ◽

Vol 38 (5) ◽

pp. 749-756 ◽

Cited By ~ 5

Author(s):

Ulrich Gesenhues

Keyword(s):

Structural Changes ◽

Crystal Size ◽

Lower Layer ◽

High Energy ◽

Primary Crystal ◽

X Ray Diffraction ◽

X Ray ◽

Transmission Electron ◽

Hall Method ◽

Means Of Transmission

The polygonization of 200 nm rutile crystals during dry ball-milling at 10gwas monitored in detail by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD). The TEM results showed how to modify the Williamson–Hall method for a successful evaluation of crystal size and microstrain from XRD profiles. Macrostrain development was determined from the minute shift of the most intense reflection. In addition, changes in pycnometrical density were monitored. Accordingly, the primary crystal is disintegrated during milling into a mosaic of 12–35 nm pieces where the grain boundaries induce up to 1.2% microstrain in a lower layer of 6 nm thickness. Macrostrain in the interior of the crystals rises to 0.03%. The pycnometrical density, reflecting the packing density of atoms in the grain boundary, decreases steadily by 1.1%. The results bear relevance to our understanding of plastic flow and the mechanism of phase transitions of metal oxides during high-energy milling.

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