Amorphization high pressure

There are no crystalline forms known for the low atomic molecules S2 to S5 although these molecules are present in the gaseous and liquid phase [49, 59]. Since the cyclic molecules Sie, S17, S19, and S (n>21) have not yet been prepared, no molecular and crystal structure data are available. However, a mixture of large sulfur rings Sx( s25) was observed as an unstable residue during the preparation of S12 (see above) [43]. The Raman spectrum of this mixture resembles that of a high pressure amorphous sulfur form as well as that of polymeric sulfur, often called (see the section on high-pressure forms of sulfur below). 

Elemental phosphoms is produced and marketed in the a-form of white or yellow phosphoms, the tetrahedral ahotrope. A small amount of ted amorphous phosphoms, P, is produced by conversion from white phosphoms. White phosphoms as the element is characterized by its combustion in air to form phosphoms pentoxide. Consequentiy, white phosphoms is generally stored and handled under water. Elemental white phosphoms is also highly toxic, and suitable precautions ate requited by those who manufacture or handle it. The black phosphoms modification prepared under high pressure does not have commercial importance. 

At least five high-pressure allotropes of sulfur have been observed by Raman spectroscopy up to about 40 GPa the spectra of which differ significantly from those of a-Sg at high pressures photo-induced amorphous sulfur (a-S) [57, 58, 109, 119, 184-186], photo-induced sulfur (p-S) [57, 58, 109, 119, 184, 186-191], rhombohedral Se [58, 109, 137, 184, 186, 188, 191], high-pressure low-temperature sulfur (hplt-S) [137, 184, 192], and polymeric sulfur (S ) [58, 109, 119, 193]. The Raman spectra of two of these d-lotropes, a-S and S, were discussed in the preceding section. The Raman spectra of p-S and hplt-S have only been reported for samples at high-pressure conditions. The structure of both allotropes are imknown. The Raman spectrum of Se at STP conditions is discussed below. 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

The behavior of cristobalite PON has been studied as a function of pressure. No in situ evidence for pressure-induced amorphization was noticed. Whereas cristobalite Si02 displays four crystalline phases up to 50 GPa (195), PON remains in a cristobalite phase (193, 196). By using Raman spectroscopy and synchrotron X-ray diffraction, Kingma et al. (193, 197) observe a displacive transformation below 20 GPa to a high-pressure cristobalite-related structure, which then remains stable to at least 70 GPa. The high value of the calculated bulk modulus (71 GPa) (196) is indicative of the remarkable stiffness of the phase. 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

The product obtained from the high-pressure reaction of benzene has been identified as amorphous [309]. The amorphous character of the sample prevents the obtainment of the Raman spectra. Other physical-chemical properties of the reaction product are the following refractive index n = 1.75 density p = 1.39 g/cm elastic constant Bq = 80 GPa optical gap 2.5 eV. These values must to be considered only as typical values of the properties because, as described above, the reaction product is reported to change according to the... 

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