Compression amorphous ices

The pressure-density isotherms of various water models in supercooled region, obtained in simulations, can be directly compared with the available experimental isotherms, showing transformations between amorphous ices upon compression." Experimental measured isotherms are unavoidably affected by the transformation kinetics and by the strong hysteresis. These two effects may be reduced by using slow compression rates and higher temperatures, respectively. Therefore, the equilibrium isotherms obtained in simulations we compare with the experimental pressurization curves," which were obtained under the slowest compression rate and at highest temperature. 

Isobaric compression of ice Ih at 165 other phases (e.g., ices II and III) [40] and neither liquid nor amorphous ice can be formed in pure ice experiments. This is the main problem in understanding the relationship between the ice Ih melting line, at 7 > 250K, and the amorphization line, at 7 77K. One way to avoid the transformation of ice Ih to other crystalline forms is to use emulsified ice [13]. In this emulsion, water is mixed with different solutes and cooled at low temperature. The resulting ice emul slon consists of ice Ih domains confined in droplets with radius of 1-10 )U.m. Such small volumes suppress the transformation of ice Ih to other crystalline forms upon isothermal compression and the melting and amorphization lines obtained upon isobaric compression of emulsified ice Ih can be traced at all temperatures [37]. 

The evolution of the piston displacement upon compressing the LDA sample is shown in Figure 5. For comparison, the results obtained upon PIA of ice Ih are included. The LDA-to-HDA transformation occurs at f 0.6 GPa, as indicated by the sudden change in d(/. This pressure is lower than the pressure at which ice Ih transforms to HDA ( 1 GPa). Still, the LDA-to-HDA transition is at least as sharp as the ice Ih-to-HDA transition and, thus, it also resembles a first-order transition in its volume change. We note that the density of HDA at 1 bar and T — 77K is, within error bars, the same density of the HDA samples obtained from PIA of ice Ih, 1.17 g cm . Moreover, the X ray diffraction patterns of HDA, obtained from ice Ih and LDA, are also very similar to each other [62]. Therefore, the HDA form obtained from LDA is apparently the same amorphous ice that results from PIA of Ih at r = 77K [24,62]. If the LDA to HDA transformation is indeed a true first-order transition, then one would expect to observe that HDA transforms back to LDA upon decompression. Otherwise, the LDA to HDA transformation could be interpreted as a simple relaxation effect of LDA. In this case, there would be a single amorphous phase of water (LDA), and HDA, instead of being a new amorphous phase different from LDA, would be a relaxed version of LDA [63]. Figure 5 shows... 

Figure 6 shows the volume of an LDA sample compressed to 1.2 GPa, followed by decompression to OGPa [64]. During the compression/decompression process, the temperature of the sample increases slowly. The temperature in these experiments is in the range 7 130-140K. Since the compression/decompression temperature is close to LDA s glass transition temperature (Tg 136K) at which translational mobility increases (Section IX), it is easier for the amorphous ices to evolve from one phase to the other. At 130-140K, the LDA-to-HDA transition occurs at T 0.3 GPa and is even sharper than the corresponding transformation at T = 77K (trace a in Figure 6). The sharpness of this transition suggests that there...

The interpretation that VHDA is still a distinct amorphous material and should be considered the third amorphous ice phase is supported by the data shown in Figure 11. Figure 11 shows the density of amorphous ices obtained by compressing recovered HDA at T = 77K to different pressures, followed by annealing to temperatures just below the corresponding crystallization temperature. The data can be fitted quite nicely by two straight lines. The first straight line in the pressure... 

Neutron scattering methods have been used in the past primarily to explore both the structural and dynamic properties of bulk water. One example is a study in which the two phases of the water polymorphism were described, that is, the LDL and the HDL [42]. These experiments were on compressed water in a temperature regime in which the anomalous properties of water are most visible, that is, close to the ice I/ice III triple point (T = 251K, P = 209 MPa). The 00, OH, and HH partial structure factors and the site site radial distribution function between distinct atoms were extracted from the diffraction data. If we assume that the structure of water can be represented as a linear combination of the structures of the end points, that Is, the HDL and LDL structures, we obtain two values for the densities /Ohdl = L20 g cm (0.0402 molecules A ) and pldl = 0.88 g cm (0.0295 molecules/A ). These values are close to the reported densities of high-density and low-density amorphous ice [97]. 

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... 

The samples were desiccated in a vacuum oven at 110°C for 24 hr. They were then compression molded on a Pasadena press at 280°C and 30,000 psi for 1 min. They were then immediately quenched in the ice-water mixture and melted again in the press without pressure for another minute. They were then immediately quenched in the ice-water mixture. A set of these amorphous samples was used in the determination of the Tg by DSC. Other samples were crystallized in a fluidized bed at 150°, 130°, 110°, and 90°C for 15, 30, 60, 120, 180, 240, 300, 600, 900, and 1,800 seconds, following which they were quenched again. 

In July 1984,1 compressed LDA at 77K, and found that its volume decreased by 20% suddenly, rapidly, and discontinuously at 0.6 GPa (Fig. 7). The sharpness of the transition contrasted remarkably with the known dullness of the pressure-induced densification of general glasses [16], The transition pressure of 0.6 GPa was much lower than that of 1 GPa of ice Ih, which hinted that LDA was truly amorphous. Then, the halo pattern of LDA before the transition and the halo pattern... 

The blend sample was compression molded into a thin film with 25-pm thickness at 300°C mold temperature and then was quickly quenched in ice water. The film specimen prepared in this procedure is amorphous. 

A film of 2b made by standard compression molding was typically opaque, indicative of some degree of crystallinity. A film molded at 370 C and quenched immediately in ice-water was amorphous and showed a Tg at 147 C, a crystallization exotherm at 232 C (16 J/g), and a broad melting endotherm at 300 -340 C (14 J/g). Annealing the amorphous film at 250 C for 15 min brought about crystallization. Assuming that the heat of crystallization of the PEK block of 2b is... 

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