Unstable phase

Figure 49. Possible phase relations in systems with variable Fe content. Chi = chlorite ML = mixed layered illite-montmorillonite I = illite. a) montmorillonites and mixed layered phase stable b) mixed layered phase unstable.

Mg. In vapour phase, unstable Cd2 molecules have been detected (bond energy about 7.4KJmol ). Cd exhibits a certain stability when exposed to dry air, but in the presence of moisture it is slowly covered with an oxide layer. It is softer and more malleable than Zn and can be easily laminated, extruded, drawn, and processed. A characteristic sound (a sort of scream) is generated on bending a bar of the metal. Cd shows a great resistance to corrosion, a low-melting point, and excellent electrical conduction. Unlike zinc, Cd has... 

The crystallization behaviour has been described by Oyumi et al. (1987a). An amorphous phase, unstable at room temperature, can be prepared under kinetic conditions by rapid removal of solvent (acetone or acetonitrile) or rapid cooling of the melt. It initially appears as a transparent waxy material that transforms into a crystalline material over about an hour. It can also be prepared on a DTA instrument by cooling from the melting point of the crystalline material (406 K) to the temperature range 333-290 K. On the other hand, crystals suitable for single crystal structure determination (carried out by the same authors) can be grown by slow evaporation from the same solvents. 

In panel (a) the value of 0 in the third (y) phase is computed along the 2- 1 Path that is traced by the coexistence of the a and j8 phases. At the intersection of the 0- 1 lines all three phases are equally stable, and a triple-point heralds the emergence of the phase. Subsequent integration in the 2- 1 plane must be done in a way that traces coexistence of the phase with either the a or j8 phase. Either may be taken, but one should be careful to continue 1 in the direction that finds the third phase unstable with respect to the two newly coexisting phases. 

Sensitive to oxidation, light and pH, astringent and bitter taste, slightly soluble in water Low solubility in aqueous and lipid phases, easily degraded by sunlight and susceptible to react with dissolved Oj, hitter taste Scarse solubility in water and oil phases, unstable at neutral-basic pH values... 

Aqueous phase — — Unstable No Immediately precipitated and slowly formed very large agglomerates... 

Figure A2.5.6. Constant temperature isothenns of redueed pressure versus redueed volume for a van der Waals fluid. Full eiirves (ineluding the horizontal two-phase tie-lines) represent stable situations. The dashed parts of the smooth eurve are metastable extensions. The dotted eurves are unstable regions.

Figure A2.5.7. Constant temperature isothenns of reduced Helmlioltz free energy A versus reduced volume V. The two-phase region is defined by the line simultaneously tangent to two points on the curve. The dashed parts of the smooth curve are metastable one-phase extensions while the dotted curves are unstable regions. (The isothenns are calculated for an unphysical r = 0.1, the only effect of which is to separate the isothenns...

Figure A2.5.9. (Ap), the Helmholtz free energy per unit volume in reduced units, of a van der Waals fluid as a fiinction of the reduced density p for several constant temperaPires above and below the critical temperaPire. As in the previous figures the llill curves (including the tangent two-phase tie-lines) represent stable siPiations, the dashed parts of the smooth curve are metastable extensions, and the dotted curves are unstable regions. See text for details.

For T shaped curves, reminiscent of the p, isothemis that the van der Waals equation yields at temperatures below the critical (figure A2.5.6). As in the van der Waals case, the dashed and dotted portions represent metastable and unstable regions. For zero external field, there are two solutions, corresponding to two spontaneous magnetizations. In effect, these represent two phases and the horizontal line is a tie-line . Note, however, that unlike the fluid case, even as shown in q., form (figure A2.5.8). the symmetry causes all the tie-lines to lie on top of one another at 6 = 0 B = 0). 

Figure A3.3.2 A schematic phase diagram for a typical binary mixture showmg stable, unstable and metastable regions according to a van der Waals mean field description. The coexistence curve (outer curve) and the spinodal curve (iimer curve) meet at the (upper) critical pomt. A critical quench corresponds to a sudden decrease in temperature along a constant order parameter (concentration) path passing through the critical point. Other constant order parameter paths ending within tire coexistence curve are called off-critical quenches.

A homogeneous metastable phase is always stable with respect to the fonnation of infinitesimal droplets, provided the surface tension a is positive. Between this extreme and the other thennodynamic equilibrium state, which is inhomogeneous and consists of two coexisting phases, a critical size droplet state exists, which is in unstable equilibrium. In the classical theory, one makes the capillarity approxunation the critical droplet is assumed homogeneous up to the boundary separating it from the metastable background and is assumed to be the same as the new phase in the bulk. Then the work of fonnation W R) of such a droplet of arbitrary radius R is the sum of the... 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

Although modem laser teelmiques ean in prineiple aehieve mueh narrower energy distributions, optieal exeitation is frequently not a viable method for the preparation of exeited reaetive speeies. Therefore ehemieal aetivation—often eombined with (laser-) flash photolysis—still plays an important role in gas-phase kmeties, in partieular of unstable speeies sueh as radieals [ ]. Chemieal aetivation also plays an important role in energy-transfer studies (see chapter A3.13). 

Matsen M W and Sehiek M 1994 Stable and unstable phases of a dibloek eopolymer melt Phys. Rev. Lett. 72 2660... 

Cluster research is a very interdisciplinary activity. Teclmiques and concepts from several other fields have been applied to clusters, such as atomic and condensed matter physics, chemistry, materials science, surface science and even nuclear physics. Wlrile the dividing line between clusters and nanoparticles is by no means well defined, typically, nanoparticles refer to species which are passivated and made in bulk fonn. In contrast, clusters refer to unstable species which are made and studied in the gas phase. Research into the latter is discussed in the current chapter. 

The C-C linkage in tire polymeric [60]fullerene composite is highly unstable and, in turn, tire reversible [2+2] phototransfonnation leads to an almost quantitative recovery of tire crystalline fullerene. In contrast tire similarly conducted illumination of [70]fullerene films results in an irreversible and randomly occurring photodimerization. The important aspect which underlines tire markedly different reactivity of tire [60]fullerene polymer material relative to, for example, tire analogous [36]fullerene composites, is tire reversible transfomration of tire fomrer back to the initial fee phase. 

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