Structure changes, compression-induced

A more direct proof of a pressure induced structural change results from X-ray scattering data described in the contribution of Als-Nielsen. Diffraction data taken below and above 7Cs show a drastic increase in the positional coherence length from about ten to more than 50 lattice spacings and a discontinuity of the lattice compressibility, indicative of a second order transition /29,30/. Reflectivity data indicate a drastic decrease in head group dimensions and increase in electron density above tCs /31/. This can be explained by a dehydration and ordering of head groups observed also for the low temperature phases of bilayer vesicles /32/. 

Lin et al. [17] studied the dynamics of copolymers adsorbed on an air-water interface. These measurements complemented the static measurements described above and in Fig. 4. The extent of the polymer films perpendicular to the surface is small compared to penetration distance and wavelength so that EWDLS is most sensitive to variation of composition in the plane of the interface. Figure 7 shows the measured normalized autocorrelation I (/) for different surface pressures. Frames a-d were taken during the first compression of the monolayer, and frames e-h were taken during the second compression. The difference between the two sets of measurements is an indication of structural changes induced by compression cycling. The frames e-g can be compared to the data in Fig. 4. The solid lines in the three frames are fits to a sum of two exponential functions, each with a characteristic decay time. The fast decay constant has a characteristic associated with diffusive motion of the disks. The slow decay constant ( several seconds) was ascribed to the dynamics of the associations of disks. 

Correspondingly, low-symmetry atomic or molecular arrangements comprising different types of chemical bonds normally exhibit pronounced anisotropy of the compressibility. Structural reorganizations due to pressure-induced phase transitions are associated with discontinuous volume decreases and normally increasing coordination numbers. These structural changes not only modify the coordination environment in the crystal structure but frequently also the electronic properties of the solid. 

Shock wave compression cannot only induce deformation in the form of high density of defects such as dislocations and twins but can also result in phase transition, structural changes and chemical reaction. These changes in the material are controlled by different components of stress, the mean stress and the deviatoric stress. The mean stress causes pressure-induced changes such as phase transformations while the deviators control the generation and motion of dislocations. 

The incommensurate, hexagonal monolayers are compressed compared to the bulk metal and they are rotated from the substrate by several degrees. From the results, the monolayer compressibility could be calculated. The restructuring (i.e. surface reconstruction) of top layers of single crystal metal surfaces as a function of solution composition and electrode potential has been studied [73]. The induced charge density was found to be the critical parameter [74]. Structural changes during... 

The pressure dependence of the compressibility is shown in Fig. 2.17. The HR system exhibits normal behavior, i.e. kt decreases monotonically with increasing pressure. However, for the water-like system the behavior is quite different. The compressibility increases with pressure, reaches a maximum value, and then decreases. The high value of the compressibility at low temperatures and the intermediate pressure of F 10 is a result of large fluctuations in the volume near the phase-transition-like pressure we saw in Fig. 2.15. This typical behavior is completely absent in the HR system. Note also that liquid water has a relatively small value of compressibility compared to other normal liquids. In the two-structure model, a negative contribution to the compressibility is obtained from structural rearrangement in the system induced by changes in the pressure (see Sec. 2.4). In the next section, we shall study the possible molecular mechanism that determines the value of the compressibility. 

Structural changes induced by the change in the temperature. It is clear that these minima also occur in the ranges of T and P at which we have observed the anomalous temperature dependence of the volume, namely 2 < T < 3 and 4 < P < 9. (See Sec. 2.5.3. Note, however, that in Sec. 2.5.3 we studied the primitive model, but as we have pointed out the cluster model exhibits the same behavior as the primitive model). Once we increase the pressure beyond P 10, the system is highly compressed and the quantity 9(Xhb)/9T becomes positive, i.e. increasing the temperature causes an expansion from the close-packed structure into the open, HBed structure. 

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