Amorphization pressure-induced

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. 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

Very High Density Amorphous Ice (VHDA). By annealing HDA to T > 160 K at pressures > 0.8 GPa, a state structurally distinct from HDA can be produced, which is called VHDA ice [152]. The structural change of HDA to a distinct state by pressure annealing was first noticed in 2001 [152]. Even though VHDA was produced in experiments prior to 2001 [170], the structural difference and the density difference of about 10% at 77 K, and 1 bar in comparison with HDA remained unnoticed. Powder X-ray diffraction, flotation, Raman spectroscopy, [152] neutron diffraction [171], and in situ densitometry [172, 173] were employed to show that VHDA is a structural state distinct from HDA. Alternatively, VHDA can be prepared by pressurization of LDA to P > 1.1 GPa at 125 K [173, 174] or by pressure-induced amorphization of hexagonal ice at temperatures 130 K < T < 150 K [170]. The density of this amorphous state at 77 K and 1 bar is 1.26 g/cm3 [152]. 

Raman spectra for the sample were conducted in a compression-decompression cycle. In this experiment, the crystalline diffraction began to disappear above 7-8 GPa during compression, and pressure-induced amorphization was indicated by the Raman spectra above 13 GPa (Fig. 14). The resultant HDA Si exhibits the Raman spectrum that differs from the spectrum of normal -Si (LDA Si). Rather, the characteristics of the spectrum for HDA Si resemble those of the (3-tin crystal, which indicates that HDA Si has a (locally) analogous structure to the (3-tin structure. The synthesis of the HDA form of Si by Deb et al. [263] has a strong resemblance to that of water (ice) by Mishima et al. [149, 196]. Whereas compression induced amorphization that was almost completed at 13-15 GPa, decompression induced an HDA-LDA transition below 10 GPa, which is clearly shown in the Raman spectra (Fig. 14). This is the first direct observation of an amorphous-amorphous transition in Si. The spectrum at 0 GPa after the pressure release exhibits the characteristic bands of tetrahedrally coordinated -Si (LDA Si). Based on their experimental findings Deb et al. [263] discussed the possible existence of liquid-liquid transition in Si by invoking a bond-excitation model [258, 259]. They have predicted a first-order transition between high-density liquid (HDL) and low-density liquid... 

Pressure-induced amorphization was reported for the first time in 1984 for hexagonal ice." Since then, a number of fundamental questions have been raised and are still now under investigation. Here, typical experimental data for Ge02 will be outlined in brief" ... 

The pressure-induced amorphous phase should be discriminated from thermally vitrified glass by the difference in density and also by different behavior under pressure compression of the glass up to 60 GPa does not yield rutile, and germaiuum completely reverts to tetrahedral coordination on decompression. 

The physical and chentical properties of these new pressure-induced amorphous phases may be quite interesting. 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Arora, A. K. (2000) Pressure-induced amorphization versus decomposition. 

The compressibility of a protein may also be obtained from fluorescence line-narrowing spectroscopy at 10 K low temperatures. Under these conditions one does expect the hydrational changes not to play a very prominent role. Nevertheless the compressibilities that are obtained under such conditions are of the same order of magnitude as those obtained at ambient conditions [34]. This points to important contributions from the cavities to the compressibility and the thermal expansion. The observed pressure-induced amorphization in inorganic substances [35], liquid crystals [36], synthetic polymers [37,38] and starch [9] also support this hypothesis. 

Although most of the amorphous materials modelled by MD have been prepared by melting either a crystalline or random structure and quenching the resulting melt to generate the appropriate glassy structure, other methods of preparation have also been used such as pressure induced amorphization, defect induced amorphization, and radiation induced amorphization. Examples will be considered below. 

Mechanical Stability of Quartz Under Pressure The Origin of Pressure Induced Amorphization... 

The high pressure Raman spectroscopy study of Ba24Siioo by Shimizu et al. [107] showed evidence for two structural transitions. The first occurs between 3.9 and 6.5 GPa and is characterized by the splitting of the 126 cm Raman mode along with a general decrease of the intensity of the modes associated with vibrations of Ba atoms. The second transformation at 20.7-23.2 GPa is associated with the disappearance of all the observable Raman features, and this was initially attributed to a pressure-induced amorphization. However, subsequent X-ray diffraction studies showed that the transition at 23 GPa in fact corresponds a volume collapse similar to Type I silicon clathrates MgSi4g [108]. 

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