Structure, high-pressure phase

Steric stabilization 167,173-174 Structure, high-pressure phase 69 SuHane monosuHonic acid 158, 161-162 Sulfate reducing bacteria 169 Sulfide, chemical oxidation 179 -, oxidation 171... 

AlP II Ol Fm3m fee NaCl structure High-pressure phase ... 

Figure 4.15. Shock pressure versus specific volume for calcia and fused quartz indicating three regimes fused quartz, low-pressure regime is fused quartz, mixed phase regime, and high-pressure regime representing stishovite. In the case of calcia, the low-pressure phase is the B1 structure, mixed phase is indicated, and the high-pressure phase regime is in the B2 structure.

Ahrens, T.J., Anderson, D.L., and Ringwood, A.E. (1969), Equation of State and Crystal Structures of High-Pressure Phases of Shocked Silicates and Oxides, Rev. Geophys. 7, 667-707. 

The room temperature transformation of the columbite phase to baddeleyite commences at 13-17 GPa 6, with transition pressure increasing linearly with temperature Direct transition from rutile to baddeleyite phase at room temperature and 12 GPa has also been reported 7. The baddeleyite phase undergoes further transition to an as yet undefined high-symmetry structure at 70-80 GPa. The most likely candidate for the high-pressure phase is fluorite, which is consistent with the general pattern of increasing Ti coordination number from 6 in rutile, to 7 in baddeleyite (a distorted fluorite structure), and to 8 in fluorite. 

P.Y. Simons and F. Dachille, The structure of Ti02 II, a high-pressure phase of Ti02, Acta Crysi. 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

In addition to the three principal polymorphs of silica, three high pressure phases have been prepared keatite [17679-64-0], coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stability relationships of coesite, quartz, tridymite, and cristobalite. A number of other phases, eg, silica O, silica X, silicalite, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a structurally unique fibrous form, silica W. 

Diamond C is the high-pressure phase of carbon, and the C-C bonding is of sp pure covalent nature. The structure has a three-dimensional framework as indicated in Fig. 9.1, and is different from the low-pressure phase, graphite, which has a sheet structure consisting of sp covalent bonds and Van der Waals bonds connecting the sheets. Other polymorphs called lonsdalite, fullerene, and carbon nanotube, which consist of mixed sp and sp bonds, are also known. 

Yamamoto, T., Miyagi, H. and Asai, K. Structure and properties of high pressure phase of polyethylene. Japan. J. Appl. Phys. 16, 1891 (1977)... 

While si, sll, and sH are the most common clathrate hydrates, a few other clathrate hydrate phases have been identified. These other clathrate hydrates include new phases found at very high pressure conditions (i.e., at pressures of around 1 GPa and higher at ambient temperature conditions). Dyadin et al. (1997) first reported the existence of a new methane hydrate phase at very high pressures (500 MPa). This discovery was followed by a proliferation in molecular-level studies to identify the structure of the high pressure phases of methane hydrate (Chou et al., 2000 Hirai et al., 2001 Kurnosov et al., 2001 Loveday et al., 2001, 2003). 

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