Phase transition high pressure

BF Variano, NE Schlotter, DM Hwang, CJ Sandroff. Investigation of finite size effects in a 1st order phase-transition—High-pressure Raman-study of CdS microcrystallites. J Chem Phys 88 2848-2850, 1988. 

Key words Neutron scattering -microemuision - lamellar structure -first-order phase transition - high pressure... 

Figure 8 Schematic illustration for the formation of SAMs of alkanethiolates on gold [73], (A) Alkanethiols adopt the highly mobile lattice-gas phase (B) above a critical value of coverage, striped-phase islands are formed (C) surface reaches saturation coverage of striped phase (D) surface undergoes lateral-pressure-induced solid phase transition high-density islands nucleate and grow at domain boundaries (E) high-density islands grow at the expense of the striped phase until the surface reaches saturation...

Saxena S. K. and Dubrovinsky L. S. (2000) Iron phases at high pressures and temperatures phase transition and melting. Am. Mineral. 85, 372-375. 

It can be seen from Fig.3 that chromium films differ from molybdenum films in the mechanical behavior essentially. So, unloading curve for chromium at depth about 330 nm shows displacement discontinuity that, as is known, testifies to phase transition in silicon under loading (a metal phase of high pressure Si II [6]). On unloading curve for molybdenum the phase transition in silicon is not fixed. Besides average contact pressure in molybdenum film is lower, than in a chromium film more than in 2 times and this distinction increases with reduction of depth of contact. 

In consequence, the predominant emphasis of high-pressure research on polymers continues to be on phase transitions. Typically, pressures up to only — 5-10 kbar (0.5-1.0 GPa) are required, because the melting points have then reached values of — 300°C, a temperature where spontaneous cross-linking occurs, and the integrity of the constituent macromolecules is no longer retained. 

For an ionic surfactant such as AOT, increasing salinity leads to a 2-3-2 transition. The AOT-brine-propane system conforms to this classical behavior in several ways. For example, increases in the salinity of the aqueous phase increase the proportion of AOT in the propane phase. At high pressures, the size of the reverse micelles (as reflected by JVo) decreases as salinity increases. The pressure at which the 2-3 transition occurs decreases as salinity increases, an indication of increasing surfactant affinity for the propane phase. One interesting aspect of the AOT-brine-propane system is that the amount of NaCI required to effect phase changes appears to be higher than is the case in conventional liquid solvents. For propane at 310 bar and 37°C, about 1.1 wt% salt is required to drive AOT into the propane phase. For the heptane-brine-AOT system at the same concentration, only 0.5 wt% salt is required to achieve the same effect [36]. This result suggests that propane is a weaker solvent for AOT than heptane. 

The nature of the 4f electrons in lanthanides and their compounds may be broadly characterized as being either localized or itinerant and is held responsible for a wide range of physical and chemical properties of both the elements and compounds. The localized states are marked by tightly bound shells or narrow bands of highly correlated electrons near the Fermi level and are observed at ambient conditions for all of the lanthanide elements. The pressure variable has a dramatic effect on the electronic structure of lanthanides, which in turn, drives a sequence of structural phase transitions under pressure. A very important manifestation of this change is the pressure-induced s to d electron transfer that is known to give rise to the... 

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. 

The first insulating valence fluctuations were studied by Jayaraman et al. (1970) for SmS. An insulating black phase of SmS at ambient pressure changes into a metallic golden phase at high pressures. In other words, divalence of Sm + changes into the intermediate valence between 2+ and 3+. The application of hydrostatic pressure is associated with a smaller volume, which introduces flie 4f valence transition and consequently delocalization of 4f electrons. 

Both above-mentioned particularities of actinides with itinerant electrons, high density and low-symmetry structures, can be induced in actinides and lanthanides with localized f electrons by the action of pressure. Compression leads to hybridization and overlapping of f electron orbitals and, thus, generates also in these regular metals states which are similar to those found in U, Np, and Pu at ambient pressure. Phase transitions under pressure to low-symmetry structures have up to now been reported in Am, Cm, Bk, and Cffor the actinides, and inC Pr, Nd, and Sm for the lanthanides. 

An example of a pressure-gap system is CO oxidation on rutheninm. Rntheninm does not exhibit any activity for CO oxidation at low pressnres however, at pressures in the several Torr regime, Ru has the highest activity of all relevant transition metals (i.e., Pt, Pd, Or, Rh, Ru) [16]. In 2000, a surface science study by Over et al. revealed that the active phase at high pressures is not the metal Ru(OOOl) surface, instead the catalytically highly active phase for CO oxidation is a RUO2 film,... 

All phase transitions involving blue phases (BPj, BPji, BPjji) of S-(-i-)-4 -(2-methyl-butyl)phenyl-4-n-decyloxy and -dodecyl-oxy benzoate could be observed up to 280 MPa and 103 °C. The decyloxy homolog exhibiting all three blue phases at atmospheric pressure looses BPjj at 120 MPa. The dodecyloxy homolog only with BPi shows a pressure-induced BPn already at lower pressures. A correlation between the pre-transitional behavior of the optical activity in the isotropic liquid phase and the phase behavior of the blue phases at high pressures is found. 

The three general states of monolayers are illustrated in the pressure-area isotherm in Fig. IV-16. A low-pressure gas phase, G, condenses to a liquid phase termed the /i uid-expanded (LE or L ) phase by Adam [183] and Harkins [9]. One or more of several more dense, liquid-condensed phase (LC) exist at higher pressures and lower temperatures. A solid phase (S) exists at high pressures and densities. We briefly describe these phases and their characteristic features and transitions several useful articles provide a more detailed description [184-187]. 

The ability to control pressure in the laboratory environment is a powerful tool for investigating phase changes in materials. At high pressure, many solids will transfonn to denser crystal stmctures. The study of nanocrystals under high pressure, then, allows one to investigate the size dependence of the solid-solid phase transition pressures. Results from studies of both CdSe [219, 220, 221 and 222] and silicon nanocrystals [223] indicate that solid-solid phase transition pressures are elevated in smaller nanocrystals. 

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