Crystal structures at high pressure

There are four principal methods of determining crystal structures at high pressures employing powders or single-crystals, using X-rays or neutrons. Here 1 will give a brief review of the relevant diffraction techniques and analysis methods used with each technique, focusing on recent developments. 

Once Te-III was identified as incommensurate, subsequent analysis was conducted on the previously-collected powder-diffraction data using the formalism of 4D superspace [234], and the JANA2000 software for structure refinement [235]. The Rietveld refinement of the incommensurate Te-III diffraction profile is shown in Fig. 9, and the modulated structure is shown in Fig. 10. Tellurium was only the second element found to have a modulated crystal structure at high-pressure, the... 

Many substances have quite different crystal structures at high pressure. Uusually these structures exist only under pressure, and their study necessarily requires high-pressure cameras. Maximum pressures reached in such cameras are of the order of 100 kilobars (1 bar = 10 N/m = 14.5 Ib/in a I atmosphere). 

The experimental determination of crystal structures at high pressures and the measurement of the physical properties of these phases is difficult and limited by instrumental constraints. Since these constraints do not exist for theoretical approaches and the computational effort and expected accuracy are similar to calculations for systems at ambient pressure, the prediction of properties of solids at high pressures is a splendid example for the power of computational approaches. As an example the high pressure phases of ice will be discussed here. ... 

Whenever the polymer crystal assumes a loosely packed hexagonal structure at high pressure, the ECC structure is found to be realized. Hikosaka [165] then proposed the sliding diffusion of a polymer chain as dominant transport process. Molecular dynamics simulations will be helpful for the understanding of this shding diffusion. Folding phenomena of chains are also studied intensively by Monte Carlo methods and generalizations [166,167]. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

Pure barium is a silvery-white metal, although contamination with nitrogen produces a yellowish color. The metal is relatively soft and ductile and may be worked readily. It is fairly volatile (though less so than magnesium), and this property is used to advantage in commercial production. Barium has a bcc crystal structure at atmospheric pressure, but undergoes solid-state phase transformations at high pressures (2,3). Because of such transformations, barium exhibits pressure-induced superconductivity at sufficiendy low temperatures (4,5). 

Since X-ray absorption, especially EXAFS, is sensitive to the immediate environment of atoms, it can be used in ordered or disordered systems (amorphous solids or liquids) and, in particular, to study the disordered state which occurs at a first-order phase transition between two crystal structures under high pressure. 

High-pressure studies have a special relevance to synthetic polymers. In addition to the normal interest in the way familiar materials alter as the pressure is raised, it has become evident that some important polymers form novel structures at high pressures—usually high temperatures are also required. This is a consequence of their long-chain constitution, which promotes orientational order so that stable phases exist which are orientationally, but not fully spatially, ordered, in the manner of liquid crystals. The name columnar crystals has been suggested. ... 

High-pressure crystal chemistry stuffed framework structures at high pressure. In ... 

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. 

T.L. Boggs et al, AIAA J 8 (2), 370-72 (1970) CA 72, 113371 (1970) Scanning electron microscopy is used to study the surface structure of solid proplnts, prepd from AP (1) and polyurethane or caiboxylated polybutadiene. Polyurethane proplnts are self-extinguish-ing at high pressure due to the flow of molten binder over I crystals. I crystals formed a thin surface melt with gas liberation in the molten phase... 

At high pressures, solid II can be converted (slowly) to solid III. Solid III has a body-centered cubic crystal structure. Line bd is the equilibrium line between solid II and solid III, while line be is the melting line for solid III.P A triple point is present between solid II, solid III, and liquid at point b. Two other triple points are present in this system, but they are at too low a pressure to show on the phase diagram. One involves solid II, liquid, and vapor while the other has solid I, solid II, and vapor in equilibrium. 

Annealing drawn PE hydrostatically at high pressure, generates a wide spectrum of crystal thicknesses varying from the common oriented chain folded to the chain-extended structures — where folds and ties tend to disappear63 —. This range of crystal thicknesses coupled with the chain axis orientation, offers a suitable model in... 

According to this rule, rutile and, at high pressures, the modification with the oc-Pb02 structure are the most stable forms of Ti02. Numerous compounds crystallize in the rutile type and some in the oc-Pb02 type, whereas scarcely any examples are known for the brookite and the anatase structures. 

M. McMahon, R. Nelmes, Incommensurate crystal structures in the elements at high pressures. Z. Kristallogr. 219 (2004) 742. 

The authors further tested the Pt(l 11) and Pd(l 10) surfaces [71, 72] using in situ STM and SXRD. All these single crystals show a similar kinetic behavior in CO oxidation. The gradual roughening of the surface corresponds to the formation of surface oxides and a higher CO oxidation rate. The structure insensitivity observed at high pressure is in contrast with the results obtained in UHV, where the reactivity shows a strong orientational dependence. 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

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