Crystallization at High Pressure

The shaded region in Fig. 7 labeled Ai 3, with pressure from about 4.5 GPa to 9 GPa and temperature from about 1475 K to 2475 K, gives the conditions under which the 

The graphite-diamond equilibrium expression in this region, following Berman and Simon [14] is 

2175 K at this pressure even after treatments lasting 8 h or more. These findings are consistent with the current theories of natural diamond genesis. 

The two regions both marked N in Fig. 7 are intended to illustrate the conditions under which natural diamond formed in the earth s crust, probably over geological time scales (about 3.3 bilUon years). There are two environments linked to natural diamond formation, the peridotitic (depicted by the lower pressure N region on Fig. 7) and the eclogitic (at higher pressures and about 200 K hotter, also marked N). 

The pressure given by the N positions are estimated from the accepted correspondence of 0. 1 GPa pressure to 3 km depth into the earth crust. The lower pressure limit of N is given by the boundary zone between the upper region lithosphere and the mobile asthenosphere at about 150 km in the earth s mantle. 

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Our results [104,117] have shown that the extended chains of ethyl cellulose liquid crystal at high pressures can act as the nucleus of PE ECC and induce the formation of ECC. (The details will be introduced in Section IX.)... 

The extended chain is another conformation which has been discussed often and proved to exist—e.g., in linear polyethylene crystallized at high pressures. It has also been assumed for amorphous polymers in the form of strands of parallel molecules [kink model (57)]. The maximum length L (x-ray) of a sequence of vinyl monomers of basic molecular weight M0 is 2.52 A. Straight strands of molecules can be... 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

Shibata, T., Takahashi, E., Ozima, M. (1994) Noble gas partition between basaltic melt and olivine crystals at high pressure. In Noble Gas Geochemistry and Cosmochemistry, J. Matsuda, Ed., pp. 343-54. Tokyo Terra Scientific Publishing Co. 

Fig. 5 Mode Griineisen parameters for IR- and Raman-active intramolecular modes in C60 and C70. Squares - Raman data for C60, dots - IR data for C70, open circles - Raman data for C70. Reprinted with permission from KP Meletov, AA Maksimov, and II Tartakovskii, Energy spectrum and phase transitions in C-70 fullerite crystals at high pressure , J. Exp. Theor. Phys. (JETP) vol. 84 (1997) 144-50 [40]. Copyright 1997 American Institute of Physics...

Because structural phase transitions are often ferroelastic or coelastic in character it is essential to have a well-defined stress applied to the crystal at high pressures. In effect, this means that a hydrostatic pressure medium must be used to enclose the crystal. A 4 1 mixture by volume of methanol ethanol remains hydrostatic to just over 10 GPa (Eggert et al. 1992) and is convenient and suitable for many studies. If the sample dissolves in alcohols, then a mixture of pentane and iso-pentane which remains hydrostatic to 6 GPa (Nomura et al. 1982), or a solidified gas such as N2, He, or Ar can be employed. Water appears to remain hydrostatic to about 2.5 GPa at room temperature, just above the phase transition from ice-VI to ice-VII (Angel, unpublished data). The solid pressure media such as NaCl or KCl favoured by spectroscopists are very non-hydrostatic even at pressures below 1 GPa and have been shown to displace phase transitions by at least several kbar (e g. Sowerby and Ross 1996). Similarly, the fluorinert material used in many neutron diffraction experiments because of its low neutron scattering power becomes significantly non-hydrostatic at -1.3 GPa. Decker et al. (1979) showed that the ferroelastic phase transition that occurs at 1.8 GPa in lead phosphate under hydrostatic conditions is not observed up to 3.6 GPa when fluorinert was used as the pressure medium. At pressures in excess of the hydrostatic limit of the solidified gas and fluid... 

H. Arashi, T. Suzuki, ad S.-I. Akimoto, Non-destructive phase transformation of Zr02 single crystal at high pressure,./. Mater. Sci. Lett. 6, 106-108 (1987). 

Kolesnik, E.N., Goryainov, S.V., Boldyreva, E.V. Different behavior of L- and DL-serine crystals at high pressures phase transitions in L-serine and stability of the DL-serine structure, Doklady Chem. 404(2005) 61-64 (Rus.), or 169-172 (Engl.). 

High molecular mass, crystallized at high pressure. Very slow measurement to avoid superheating. 1.30... 

A similar trend exists for the halogens. The structure of crystalline iodine is shown in Figure 13.9. The ratios r2/r for crystalline CI2 (s), Br2 (s), and I2 (s) are 1.68, 1.46, and 1.29, respectively. Although each of the halogens is classified as a nonmetal, the percent metallic character increases down the series. Iodine is the most metallic of the group, a fact that is supported by its grayish color as a solid and its metallic reflectance. In fact, iodine can even adopt a distorted cubic closest-packed structure similar to many metallic solids when it is crystallized at high pressure. 

S Webster, DN Batchelder. Raman spectroscopy of poly[bis(p-toluene sulphonate) of 2,4-hexadi-yene-l,6-diol] crystals at high pressure. Macromol Symp 87 177-185, 1994. 

Melting/Crystallization at High Pressures (Hydrostatic Effect)... 

Nuclear magnetic resonance (NMR) is one of the major experimental tools in structural chemistry and biochemistry. The prediction of NMR shifts from ab initio calculations has been demonstrated for isolated molecules (see NMR Chemical Shift Computation Ab Initio), but the development of a practical ab initio approach for the calculation on NMR shifts in solids has been accomplished only quite recently. Based on DFT-LDA and a pseudopotential plane wave approach, these authors have presented an approach which promises to be useful in the investigation of NMR shifts in crystalline solids as well as in amorphous materials and liquids. As a demonstration of this approach, Mauri et al. have calculated the H NMR shifts of LiH and HF in the state of isolated molecules and in a crystal. In the case of LiH the results show very little change between the free molecule (a = 26.6 ppm) and the crystal (cr = 26.3 ppm). However, a significant change is found for the crystal at high pressures (65 GPa), where the chemical shift increases to 31.2 ppm. A quite different picture is obtained for the HF molecule, where the theory predicts a shift of 28.4 ppm in remarkable agreement with the experimental value of 28.4 ppm. For the HF crystal, a shift of... 

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