Pressure effects transformations

Depending on the data available, Eqs (6.17)-(6.23) reproduce experimental pressure effects with considerable accuracy in many cases. In particular, Eq. (6.18) can be used to confirm entropy data derived using more conventional techniques and can also provide data for metastable allotropes. Ti again provides a leading example, as pressure experiments revealed that the u -phase, previously only detected as a metastable product on quenching certain Ti alloys, could be stabilised under pressure (Fig. 6.14). Extrapolation of the P/w transus line yields the metastable allotropic transformation temperature at which the / -phase would transform to w in the absence of the a-phase, while die slope of the transus lines can be used to extract a value for the relevant entropy via Eq. (6.18). 

One may then expect that, in materials as complex as the bismuthates, the observed pressure effects are complex. The phase transformations apparently observed in BaPbj.jjB Oj via Mossbauer (65) as a function of temperature can most likely also be driven by pressure. Some indication of these transitions can be observed in the very high pressure room temperature measurements of Clark et al. (66) on BaPbj B Og, and Sugiura and Yamadaya (67) on BaBiOg. 

Tolbert SH, Herhold AB, Brus LE, Alivisatos AP (1996) Pressure-induced structural transformations in Si nanocrystals Surface and shape effects. Phys Rev Letters 76 4384-4387 Wickham JN, Herhold AB, Alivisatos AP (2000) Shape change as an indicator of mechanism in the high-pressure structural transformations of CdSe nanocrystals. Phys Rev Letters 84 4515-4515... 

In the discussion of pressure effects on enatioselectivity one has to differentiate again between pressure and temperature effects. As already shown for diastereo-selective transformations, lowering the temperature may improve the enantioselectivity. Provided that the enantioselective transformation has a large negative AV the reaction can be performed at lower temperature under high pressure at a reasonable rate. However, this again is not a pressure effect on the enantioselectivity but a temperature effect. 

An externally applied electric field is a vectorial perturbation for chemical or orientational distributions involving interacting molecules or molecular organizations. Unlike the isotropic temperature and pressure effects on chemical-conformational transformations, direct sensitivity to electric field forces is bound to certain electrical properties of the chemical structures involved. Major structural-chemical changes in electric fields require the presence of ions, or ionized groups, or permanent or induced dipolar charge configurations, preferably in macromolecular structures. 

Vandaele, A.C., Hermans, C., Fally, S., Carleer, M., Colin, R Meerienne, M.-F., Jenouvrier, A., Coquart, B. High-resolution Fourier transform measuremoit of the NO2 visible and near-infrared absorption cross sections temperature and pressure effects. J. Geophys. Res. 107 (D18), 4348 (2002). doi 10.1029/2001JD000971... 

As will be described below, both pressure and temperature effects can be used to influence chemical transformations. For example, reaction selectivity can be influenced indirectly through a pressure-dependent dielectric constant for a polar SCF solvent (68), and equilibrium constants can be shifted to favor desired products. Combining this manipulation of reaction characteristics through pressure effects with the use of solvents having moderate critical temperatures can... 

Pressure is a fundamental physical property that affects various thermodynamic and kinetic parameters. Pressure dependence studies of a process reveal information about the volume profile of a process in much the same way as temperature dependence studies illuminate the energetics of the process (83). Since chemical transformations in SCF media require relatively high operating pressures, pressure effects on chemical equilibria and rates of reactions must be considered in evaluating SCF reaction processes (83-85). The most pronounced effect of pressure on reactions in the SCF region has been attributed to the thermodynamic pressure effect on the reaction rate constant (86), and control of this pressure dependency has been cited as one means of selecting between parallel reaction pathways (87). This pressure effect can be conveniently evaluated within the thermodynamic framework provided by transition state theory, which has often been applied to reactions in solutions (31,84,88-90). This theory assumes a true chemical equilibrium between the reactants and an activated transition... 

Daoudi et al. (1974) found that rare earth calcium gallates (R= La through Yb) form with the olivine-type structure, and that some of these are then transformed irreversibly to the K2Nip4-type by the action of pressure (R = Eu through Dy) or temperature (R = La through Sm). The pressure effect is accompanied by a volume decrease of some 16%. 

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