High-Pressure Allotropes

Even at the lowest temperatures, a substantial pressure is required to soHdify helium, and then the soHd formed is one of the softest, most compressible known. The fluid—soHd phase diagrams for both helium-3 and helium-4 are shown in Eigure 1 (53). Both isotopes have three allotropic soHd forms an fee stmeture at high pressures, an hep stmeture at medium and low pressures, and a bcc stmeture over a narrow, low pressure range for helium-4 and over a somewhat larger range for helium-3. The melting pressure of helium-4 has been measured up to 24°C, where it is 11.5 GPa (115 kbar) (54). 

Abstract Molecular spectroscopy is one of the most important means to characterize the various species in solid, hquid and gaseous elemental sulfur. In this chapter the vibrational, UV-Vis and mass spectra of sulfur molecules with between 2 and 20 atoms are critically reviewed together with the spectra of liquid sulfur and of solid allotropes including polymeric and high-pressure phases. In particular, low temperature Raman spectroscopy is a suitable technique to identify single species in mixtures. In mass spectra cluster cations with up to 56 atoms have been observed but fragmentation processes cause serious difficulties. The UV-Vis spectra of S4 are reassigned. The modern XANES spectroscopy has just started to be applied to sulfur allotropes and other sulfur compounds. 

Fig. 29 Raman spectrum of p-S at high pressure and room temperature [109]. The wavenumbers indicated are given for the actual pressure. No signals of other allotropes have been detected. The line at 48 cm (ca. 25 cm atp 0 GPa) may arise from lattice vibrations, while the other lines resemble the typical pattern of internal vibrations of sulfur molecules...

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]. 

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. 

Fig. 30 Raman spectra of high-pressure low-temperature sulfur at two different pressures [184]. The peaks marked by asterisks were reported as originating from another high-pressure allotrope (presumably p-S). The intensities of the spectra at lower wavenumbers (below ca. 300-350 cm ) have been magnified (factor has not been reported)...

The phase hplt-S appeared in Raman spectra of sulfur at about 12 GPa and above, and this allotrope could be observed up to about 40 GPa [137, 184]. The high-pressure sample was evidently a mixture as the Raman spectra consisted of lines of both Se and p-S. Raman spectra at two different pressures are shown in Fig. 30. Three peaks were observed in the stretching... 

Rhombohedral Se was found as a high-pressure allotrope of sulfur above 9-10 GPa by several groups [58, 137, 150, 184, 186, 188, 191]. The pressure dependence of frequencies [137, 150, 184] as well as the kinetics of the transition from p-S to Ss [186] have been investigated systematically by Raman spectroscopy. The pressure dependent frequency shifts of chemically prepared Ss and of high-pressure Ss have been found to be identical [137, 150]. 

Thus, from the measurement of stretching wavenumbers it is possible to estimate the bond lengths in sulfur molecules of unknown structure, e.g., in newly synthesized rings or in high-pressure allotropes like p-S. This has clearly been demonstrated in the case of cyc/o-nonasulfur, for example [156, 157]. 

Various names for this allotropes were in use intermediate phase sulfur (ip-S) [184], high-pressure unknown phase (HPU phase) [191], and photo-sensitive or, more recently, photo-induced sulfur (p-S) [57, 58, 119, 186], respectively. We are inchned to make use of the term photo-induced since it has clearly turned out that the formation of this allotrope is induced when employing laser light of suitable power density and wavelength [1]... 

The only element that was discovered in body fluids (urine). This is plausible, as P plays a main role in all life processes. It is one of the five elements that make up DNA (besides C, H, N, and 0 evolution did not require anything else to code all life). The P-O-P bond, phosphoric acid anhydride, is the universal energy currency in cells. The skeletons of mammals consists of Ca phosphate (hydroxylapatite). The element is encountered in several allotropic modifications white phosphorus (soft, pyrophoric P4, very toxic), red phosphorus (nontoxic, used to make the striking surface of matchboxes), black phosphorus (formed under high pressures). Phosphates are indispensable as fertilizer, but less desirable in washing agents as the waste water is too concentrated with this substance (eutrophication). It has a rich chemistry, is the basis for powerful insecticides, but also for warfare agents. A versatile element. 

These non-existent allotropes, which are impurity-stabilized phases, are fee Sc, fee Y-Ce, the bcc Ho, Er, Tm and Lu and fee phases of Nd, Sm, Gd and Dy, some of which have been described as formed at room temperature during mechanical milling. A number of fee high-pressure polymorphs, for instance, are actually compounds, with a structure related to the NaCl-type, formed by reaction with O, N and/or H during mechanical milling (see also Alonso et al. 1992). 

At high pressure various allotropic forms of Hg are known. 

Further study of the high pressure allotropes of americium seems to be required under these circumstances. An additional argument for further invesigation is the discrepancy in volume change between experimental results and calculations. Figure 2a... 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

It is possible to manufacture the different allotropes of carbon. Diamond is made by heating graphite to about 300 °C at very high pressures. Diamond made by this method is known as industrial diamond. Graphite can be made by heating a mixture of coke and sand at a very high temperature in an electric arc furnace for about 24 hours. 

Elemental phosphorus itself exists in several polymeric forms. If the white allotropic form, which consists of P4 tetrahedral molecules, is put under high pressure, preferably at elevated temperatures, it can be catalytically converted to other modifications.41 It first becomes red, then violet, then black as the degree of polymerization increases. These materials are very difficult to characterize because of branching and the formation of cyclics. In the extreme limit, the structure approaches that of graphite, and shows good electrical conductivity.42 No evidence exists at all for the formation of high-molecular-weight, linear chains of elemental phosphorus. 

A substance that can exist in more than one crystalline form is said to exhibit allotropy, and the different forms are called allotropes. Figure 9 is the high-pressure part of the phase diagram of water and shows that water has a number of allotropes. The crystalline forms of water in the allotropes that melt are... 

The two allotropcs of carbon with particularly well defined properties are hexagonal graphite, as thermodynamically stable modification at ambient conditions, and its high-pressure, high-temperature allotrope. cubic diamond. Although both wcll-cryslalliscd forms with only very rarely be encountered in catalytic systems, it is important to recall some details about their prop-... 

Sulfur melts at approximately 120 °C so it can be melted by high-pressure steam, which allows sulfur to be obtained by the Frasch process described earlier. Solid sulfur exists in several allotropic forms. At room temperature, the rhombic crystalline form is stable, whereas above 105 °C, a monoclinic form is stable. These two crystalline forms are shown in Figure 15.2. Other forms of sulfur include a plastic or amorphous form that can be obtained by rapidly cooling molten sulfur. For example, pouring molten sulfur at 160 °C into water produces the amorphous form. 

However, even though this transformation is thermodynamically favored, the diamond allotrope still exists at high pressures and over long time periods. That is, if a particular phase transformation is predicted as spontaneous, the acmal rate of that process will depend on the kinetics of the transformation. Since the sp carbon bonds in diamond are extremely strong, the kinetics governing the migration of carbon atoms between diamond-graphite is extremely slow at normal temperatures and... 

Several other allotropic forms of the eletnent are known. One of these, black phosphorus, is fonned from white phosphorus under high pressure. It is still less reactive than red phosphorus. 

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