Pressure-induced phase transformation

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 stmcture at atmospheric pressure, but undergoes soHd-state phase transformations at high pressures (2,3). Because of such transformations, barium exhibits pressure-induced superconductivity at sufftciendy low temperatures (4,5). 

The shock-compression induced structural phase transformation in iron from the low pressure bcc phase to the high pressure hep phase is one of the most visible problems studied in shock-compression science, and its discovery was responsible for widespread recognition of the capabilities of the high pressure shock-compression experiment. The properties of many shock-induced phase transitions are summarized in Duvall and Graham [77D01]. 

Numerous resistance measurements have been carried out under high-pressure shock compression [79D01]. Most of the work has been motivated by the desire to develop stress gauges to measure pressures in shock-compressed materials. Other measurements were undertaken to determine critical pressures to induce phase transformations. Although most of the work is not carried out in sufficient detail to relate resistance observations to defect characterizations, excess resistance at given shock pressures is observed in every case compared to comparably loaded static pressure observations. The presence of residual resistance for times after the loading is removed provides explicit evidence for irreversible changes in resistance due to defects. 

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. 

It is rather likely that pressure-induced phase transformations can also occur in hydrogenated multicomponent industrial titanium alloys. However, there were no available data on the high-pressure behavior of such alloys. 

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. 

Another example of pressure-induced polymorphism is seen in the case of amiloride hydrochloride, where ball-milling Form-B causes a solid-state phase transformation into Form-A [43]. These workers deduced the phase relationship between two different pressure-induced polymorphs of the dihydrate, as well as the alternative route to one of those dihydrate forms that used the anhydrous form as the source material and effected the phase transformation through storage at high degrees of relative humidity storage. 

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed," and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

Fourier-transform infrared (FT-IR) spectroscopy and small-angle X-ray (SAXS) and neutron (SANS) scattering studies have shown the existence of a further pressure induced gel phase in DPPC bilayers." " Pressure dependent NMR studies yielded complementary information on the pressure-induced gel phases. The gel state of DPPC bilayers shows a variation in lineshapes which depend on the particular pressure and temperature. The types of lineshapes seen in a high pressure investigation of dg2-DPPC by Jonas et al. are... 

Pressure-induced phase transformations are known to occur for a wide range of solids. Bulk Si, for example, has the diamond structure at ambient conditions but converts to the 3-tin structure at pressures around lOOkbar. Figure 2.5 shows how it is possible to use the kinds of information we have calculated in this chapter using DFT to predict the existence of pressure-induced phase transformations. It was essentially this idea that was used to make the geologically relevant predictions of the properties of minerals such as MgSi03 that were mentioned as one of the motivating examples in Section 1.2. 

Variable-temperature X-ray diffraction studies of crystalline substances are useful in the study of phase transitions, thermal expansion and thermal vibrational amplitudes of atoms in solids. Similarly, diffraction studies at high pressures are employed to examine pressure-induced phase transitions. Time-resolved X-ray diffraction studies (Clark Miller, 1990) will be of great value for examining reactions and other transformations. 

It is evident that similar sets of multiple-phase transitions are bound to be observed in the respective melts. Such structural studies on the oxide melts have not yet been carried out, which is because of greater experiment complexity. Nevertheless, the chalcogenide melts, which experience structural transformations at more moderate temperatures and pressures, give examples of multiple phase transitions. Thus, the AsS [29] and CdTe [130] melts feature at least two pressure-induced transitions accompanied by a change in the short-range order structure and properties of the liquids. 

Only few direct pressure syntheses have been reported. LiTiMF6 phases (M = Mn—Ni) were prepared from respective binary fluorides at T = 700-1200°C and 15-70 kbar [27]. The products crystallized in Na2SiF6 and PbSb206 type structures. On the other hand, pressure induced phase transitions are common, e.g. KMnF3 transforms from cubic to tetragonal symmetry. However, in contrast to oxide containing perovkites, the transition temperature rises with increasing pressure [28]. 

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