Pressure-induced transitions

Present in the next sections are the LDA results for equilibrium structure, pressure-induced transitions and electronic properties of various polymorphs, and the comparative analysis of the results for rutile and anatase that were obtained using LDA and GGA forms of the exchange-correlation potential. 

Finally, the use of the constant pressure minimization algorithm allows searching for phenomena that can be considered as precursors of pressure-induced transitions. For example, the predicted behaviour of the anatase cell constants as a function of pressure shows that the a(P) and c(P) plots are only linear for P<4 GPa, the value that is close to both the theoretical and experimental transition pressures. At higher pressures the a constant starts to grow under compression, indicating inherent structural instability. In the case of ratile there is a different precursor effect, nami y at 11 GPa the distances between the titanium atom and the two different oxygens, axial and equatorial, become equal. Once again, the pressure corresponds closely to the phase transition point. 

An interesting aspect of Au oxidation states is provided by the investigation of the pressure-induced transition from the mixed-valence state of Au(l)/Au(lll) to the single valence state of Au(ll) as described for M2[Au(l)X2][Au(lll)X4] (M = Rb, Cs X = Cl, Br, 1) [385, 386]. The valence states of Au(l) and Au(lll) at ambient pressure were clearly distinguishable. With increasing pressure, the doublets gradually increase their overlap. Finally, the Au Mbssbauer spectrum of CS2AU2I6 shows, at 12.5 GPa of applied pressure, only one doublet which was associated with Au(II). 

When the atoms are forced to move closer by the exertion of pressure, their interaction increases and the bands become wider. At sufficiently high pressures the bands overlap again and the properties become metallic. The pressure-induced transition from a non-metal to a metal has been shown experimentally in many cases, for example for iodine and other nonmetals. Under extremely high pressures even hydrogen should become metallic (metallic hydrogen is assumed to exist in the interior of Jupiter). 

The peak height ratio between the antisymmetric and symmetric CHj stretching modes H-2880/H-2850 is plotted as a function of pressure in Figure 7. This parameter has been shown to reflect the magnitude of interchain interactions in systems that contain polymethylene chains [20]. An abrupt increase in this ratio, and thus an increase in the interchain interactions, is observed at the pressure-induced transition from the micellar to the coagel phase. Furthermore, it is also evident from Figure 7 that the transition has a considerable pressure hysteresis. Interestingly, pressure [76] and temperature [79] have different effects on the H-2880/H-2850 ratio in the micellar phase of... 

Also observed in Fig. 13 is a change in mechanical stress-strain behavior that occurs for a change in ambient pressure between p = 4 kb and p = 5 kb. The authors speak of this as a pressure-induced transition. Another interpretation of... 

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. 

Fig. 3.9. W vs. T for a PPy-PF6 sample in the critical regime. The pressure-induced transition to the metallic regime and the field-induced transition to the...

A variety of solid materials, including metals, semiconductors, molecular solids, and ionic compounds, show a remarkable deviation from the simple Hooke s law in the pressure dependence of volume at 1-lOGPa. The deviation is quite often a prelude to a pressure-induced transition to a denser structure. - " For example, Fe metal changes its strac-ture from cubic (a-phase) to hexagonal (e-phase) at 11 GPa... 

N2O4known at ambient pressure [87, 88]. Figure 10 summarizes the pressure-induced transitions between phases with different molecular geometries. 

Vidugiris, G. J. A. and Royer, C. A. 1998, Determination of the volume changes for pressure-induced transitions of apomyoglobin between the native, molten globule, and unfolded states. Biophysical Journal 75, 463-470. 

Following the pioneering work of Jamieson [51], traditional understanding of structural systematics in elemental and binary semiconductors was based on the notion of pressure-induced transitions to the high-symmetry structures of increasing coordination, from covalent fourfold (cubic diamond, zincblende, wurtzite) to metallic quasisixfold ( -tin). In the octet semiconductors with increased ionicity (>0.45), this transformation path included an intermediate sixfold-coordinated rocksalt (NaCl) structure, with the zincblende (wurtzite) -> rocksalt transformation pressure continuously decreasing until the rocksalt structure becomes thermodynamically stable at ambient conditions in the mostly ionic compounds (Fig. 31) [136,137]. The universality of the zincblende rocksalt — j8-tin tran-... 

Figure 2.15 W versus temperature plot for a PPy-PFg sample in the critical regime pressure-induced transition to the metallic side and the field-induced transition to the insulating side (Reproduced by permission from R. Menon, GO. Yoon, D. Moses, Y Cao and A.J. Heeger, Synthetic Metals, Elsevier Science SA, 1995, 69, 1-3,329)...

In the SIC-LSD formalism, this happens when localization scenarios of different/" configurations become close in energy. In the present T = 0 K theory, only discontinuous pressure-induced transitions can be described, while experiments often, but not always, observe continuous transitions, signalled by anomalous softening of the pV-curve. 

I. Noda, G.M. Story, C. Marcott, Pressure-induced transitions of polyethylene studied by two-dimensional infrared correlation spectroscopy, Vib. Spectrosc. 19 (1999) 461. 

The phenomenology associated to glass polymorphism is very rich and involves many transformations that, for space limitations, we cannot cover in detail in this chapter (see, e.g.. Refs [21,50,64,73]). For example, in addition to the LDA-HDA pressure-induced transitions discussed so far, LDA can be obtained by heating HDA at low pressure. In Refs [12,13], it is shown that decompression of HDA... 

Figure 8. Left panel phase diagram of ice T> T (P)) and transition lines corresponding to the ice Ih-to-HDA, LDA-to-HDA, and HDA-to-LDA transformations T

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