Pressure transformations

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

Temkin S. I., Suvernev A. A., Burshtein A. I. Pressure transformation of the Q-branch of the CARS spectrum of spherical molecules. Opt. ... 

Titanium, zirconium and hafnium in normal conditions crystallize in the hexagonal close-packed structure (a modification) with a c/a slightly smaller than the ideal one c/a = 1.587 (Ti), 1.593 (Zr) and 1.581 (Hf). At high temperature they have the bcc W-type structure ((3 modification). High-pressure transformations are known (Tables 5.21-5.23). 

Figure 28. Comparison of the IR spectra of the (a-C H) obtained by CVD preparation and from the high-pressure transformation of benzene. Asterisks indicate the absorption bands of unreacted benzene.

In some respects this high-pressure transformation is similar to that of olivine spinel there is a large volume decrease ( 11% cf. 10% for olivine - spinel), but no change in primary coordination number (OZr2Si, ZrOg, Si04). We have already considered the two structure types involved (Sect. 3.4 and Sect. 3.6). 

Over time, the organic matter was covered with layers of mud and sediments. As layer upon layer built up, heat and tremendous pressure transformed the sediments into shale and the organic matter into solid, liquid, and gaseous materials. These materials are the fossil fuels—coal, oil, and natural gas—that society depends on today. (See Figure 13.2.)... 

In all silicate minerals formed under crustal conditions silicon is coordinated to four oxygen atoms. In high-pressure transformations, silicon commonly increases its coordination number. The longer- the Si—O distances in tetrahedral silicates the higher the pressure transformations to phases with octahedral silicon. The average Si—O bond distance for the pressure transformation is 159 pm. This distance is achieved at room temperature at pressures in all measured silicates and may be a minimum for tetrahedryl Si—O bonds 300 kbar is an upper pressure limit for the silicon tetrahedron and SOkbar is a lower pressure limit for octahedral silicon. Temperature has little effect on Si—O bond distances in either tetrahedra or octahedra... 

High pressure transformations of compositions prepared at atmospheric pressure. 

Two solid forms of white phosphorus are known, with the cubic a-form converting to the jS-form at 197 K. The basic molecular structure of both consists of the P4 tetrahedron (1). Recent work has raised the intriguing suggestion that liquid phosphorus, consisting of the P4 molecules, undergoes a high-pressure transformation to a polymeric form. Indications are... 

The corresponding arsinieacid (p. 164), suspended in hydrochloric acid, is saturated with sulphur dioxide in the presence of a little iodine. The chloride separates as an orange-red oil, wluch is isolated by the aid of carbon tetrachloride. Attempts to distil the product at 10 mm. pressure transform it completely into 6-chlorophenosayarsine. Combination of the chloride in alkaline solution with methyl iodide yields o-phenoceyphenyl-rnethyliodoarsine, which may be converted into the oxide, and then into the chloride. The latter is a brownish-yellow, viscous oil. ... 

As mentioned in 1.1, the conduction band of the DCNQI-Cu system is composed of the d (d y) orbital of Cu and the LUMO of DCNQI. This system exhibits a variety of physical properties depending on the chemical modification or pressure. Figure 10 is a schematie phase diagram for the DCNQI-Cu system. The electronic states of this system are classified into three types according to transport properties. The type I state is metallic down to the lowest temperature. In the metallie state, the Cu ion is in the mixed-valence state and the valenee of Cu is elose to 4/3 -1-. Therefore, the one-dimensional organic pir band interacts with the Cu 3d orbital. An applieation of pressure transforms the type I state into the type II state. The type II state exhibits a sharp first-order metal-insulator (M-I) transition. The M-I transition of this system is accompanied by a CDW formation... 

Wurtzite (wBN) can be prepared by various static and dynamic compression methods , depending on the relative amounts of hBN and rBN in the initial sample, and the T-P conditions of the consolidation. Phase stability data for wBN is available . hBN can be converted to cBN and wBN at pressures from 12 to 40 GPa and temperatures between 300 and 1200 K , also with the use of Mg as a catalyst. A mixture of hBN, H2O, and an alkaline solution (e.g., NaOH) may be subjected to a shock wave at or above 10 GPa to prepare high quality wBN . wBN is transformed to zBN by shock compression at pressure above 100 GPa , while static high pressure transforms zBN to wBN . However, the reverse transformations are also possible". Additional references on wBN are given in physics abstracts. ... 

Boffa-Ballaran T, Angel RJ, Carpenter MA (2000) High-pressure transformation behaviour of the cummingtonite-grunerite sohd solution. Eur J Mineral (in press)... 

Strictly speaking, the AH in Eq. (7.72) is the AH for the constant-volume transformation. To convert it to the appropriate value for the constant-pressure transformation, we must add to it the enthalpy change of the process ... 

Consider the constant-pressure transformation of a solid from the absolute zero of temperature to some temperature T below its melting point ... 

Pre-prepared Pt hydrosols stabilized by surfactants can be used as precursors for heterogeneous hydrogenation catalysts active in the selective high-pressure transformation of 3,4-dichloronitrobenzene to the corresponding aniline (Fig. 5). The catalytic performance of the new systems was evaluated in batch and continuous tests and the results were compared with those obtained from conventional Pt/C systems. 

The potential energy-osmotic pressure transformation formula. The measurement of the osmotic pressure of colloidal dispersions, in principle, provides a relatively straightforward method for determining the distance dependence of the steric repulsion. As Ottewill (1980) has pointed out, the relatively small osmotic pressures involved render the actual measurements difficult to accomplish. None the less, it is worth establishing how to transform osmotic pressures into potential energies of interaction between the particles involved and vice versa. 

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