High-pressure forming

B1.29.6 HIGH-PRESSURE FORMS OF FAMILIAR OR USEFUL MATERIALS DIAMOND, FLUID METALLIC HYDROGEN, METALLIC OXYGEN, IONIC CARBON DIOXIDE, GALLIUM NITRIDE... 

When the temperature or pressure is decreased very rapidly, high temperature or high pressure phases can be "trapped", and are observed at atmospheric temperature and pressure (diamond, for instance, is a high-pressure form of carbon. It is only metastable at atmospheric pressure the stable form is graphite.)... 

Figure 9.6 Schematic representation and exampies of various chain metasilicates (SiO with repeat distances (in pm) after i, 2,. 7. 9 or 12 tetrahedra (T), ((ht) high-temperature form (hp) high-pressure form].

Many of the high-pressure forms of ice are also based on silica structures (Table 14.9) and in ice II, VIII and IX the protons are ordered, the last 2 being low-temperature forms of ice VII and III respectively in which the protons are disordered. Note also that the high-pressure polymorphs VI and VII can exist at temperatures as high as 80°C and that, as expected, the high-pressure forms have substantially greater densities than that for ice I. A vitreous form of ice can be obtained by condensing water vapour at temperatures of — 160°C or below. 

Ice, high pressure form Solid hydropen, high. 

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

The high pressure form of FeOOH is more compact than any other iron oxide hydroxide, hence it has a higher than usual Neel temperature of 470 K. At room temperature, high pressure FeOOH is antiferromagnetic with a collinear spin arrangement parallel to the c-axis (Fernet et al., 1973). High-pressure FeOOH is completely miscible with CrOOH. Substitution with Cr reduces T to the extent that with 80%... 

One might also point out that while the compound K2S itself has the antifluorite structure (Cl) under normal conditions, Cl often transforms to C23 under pressure, which may be expected to be the (not very) high-pressure form of K2S. Thus (a situation we shall return to) P-K2SO4 may be regarded as an anion-stuffed, high-pressure K2S polymorph. 

Rutile and anatase are used in photocatalysis and as catalyst supports and we describe some important structural features later. The titanium-oxygen system plays a key role in understanding stoichiometric variations. The highest oxide, Ti02, is polymorphic and the four known crystal structures are rutile, anatase, brookite and a high-pressure form similar to a-Pb02. 

As with ceramics and metals, polymer crystals can have multiple crystal forms. Polyethylene has a metastable monoclinic form and a orthohexagonal high pressure form. A list of some of the more common polymers and their corresponding crystal strnctnres is given in Table 1.24. Finally, X-ray diffraction can be used to determine the amorphous to crystalline ratio in semicrystalUne polymers in much the same way that Eq. (1.61) can be used. Figure 1.66 shows a schematic illustration of the X-ray diffraction patterns for semicrystalline and amorphous polyethylene. The estimation of crystalline content is based upon a ratio of the peak areas in the two samples. 

Fig. 2.62 Structure of YbO(OH), (2, 2)HCpYb O (OH) . This is the high-pressure form of YbO(OH). The peaks of trigonal prisms are distorted anti-clockwise, and thereby the coordination number of O or (OH) by Yb becomes seven.

A good example of its use applied to a protein associated with food chemistry is that of P-lactoglobulin. Ikeuchi et al. (2001) used ANS as a probe to follow P-lactoglobulin de-naturation under high pressure and its subsequent renaturation on release of pressure (Fig. B3.6.11). The denaturation was shown to be completely reversible at pH 2 but not at neutral pH, explaining why whey protein isolates subjected to high pressures form a gel at pH 7 but not at acid pH. 

CuMo04 high pressure form. X isostructural with ... 

A very-high-pressure form, ice X, which was predicated to have a H atom located midway between every pair of hydrogen-bonded O atoms, was identified by infrared spectroscopy with a transition at 44 GPa. 

As a third route to LDA, heating or decompression of the high-pressure polymorph ice VIII can be employed, which is the proton-ordered pendant to ice VII. Most high-pressure forms of ice quench-recovered at 77 K experience a... 

Ba,MnF6 Normally Ba2 Fll(MnF4) , but high pressure form has the K2[GeF6] structure... 

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