Amorphous transitions

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and... 

This reaction is irreversible. Epitaxial silicon is deposited. Polycrystalline silicon is obtained in the range of 610-630°C, which is close to the crystalline-amorphous transition temperature. 

When particle impacts with a solid surface, the atoms of the surface layer undergo crystal lattice deformation, and then form an atom pileup on the outlet of the impacted region. With the increase of the collision time, more craters present on the solid surface, and amorphous transition of silicon and a few crystal grains can be found in the subsurface. 

Binary systems of ruthenium sulfide or selenide nanoparticles (RujcSy, RujcSey) are considered as the state-of-the-art ORR electrocatalysts in the class of non-Chevrel amorphous transition metal chalcogenides. Notably, in contrast to pyrite-type MS2 varieties (typically RUS2) utilized in industrial catalysis as effective cathodes for the molecular oxygen reduction in acid medium, these Ru-based cluster materials exhibit a fairly robust activity even in high methanol content environments of fuel cells. 

D.E. Luzzi, M. Meshii, The crystalline to amorphous transition of intermetaUic compounds under electron irradiation -a review, Res. Mechanica 21(3) (1987) 207-247. 

The amorphous transition at —97° C was first detected in measurements of the specific heat. (Furukawa, McCoskey, and King). It is attributed to motions involving small sequences of CFa groups. The dependence of the temperature of this transition on the frequency of oscillation from 0.15 cps to 5 x 10 cps is shown in Fig. 10 (McCrum, 1959b). 

The amorphous transition at 127° C is depressed to lower temperatures in copolymers of tetrafluoroethylene with up to 14 mole per cent hexa-fluoropropylene (McCrum, 1959b). For this reason it is attributed to longer sequences of CF2 groups. 

The concept of a single structural transition in amorphous material, i.e., an amorphous-amorphous transition, was coined in 1985 by Mishima et al. [25] on the example of water. Today, in many respects the nature of pressure- and/or temperature-induced transformations in glasses and amorphous solids remains unclear. In most cases, the pressure treatment of glasses and amorphous solids results in residual densification. In the process, the densified glasses and... 

Although we focus here on the status quo concerning the debated aspect of multiple amorphous-amorphous structural transitions, we want to emphasize that there are numerous reviews on the topic of polyamorphism and single amorphous-amorphous transitions worth reading [37M-8]. 

Low-Density Amorphous Ice (LDA). Upon heating HDA to T > 115 K or very high density amorphous ice (VHDA) to T > 125 K at ambient pressure, the structurally distinct amorphous state LDA is produced. Alternatively, LDA can also be produced by decompressing HDA or VHDA in the narrow temperature range of 139-140 K to ambient pressure [153-155]. The density of this amorphous state at 77 K and 1 bar is 0.93 g/cm3 [152]. These amorphous-amorphous transitions are discussed in Sections III.C and III.D. 

We now turn from the single, possibly first-order like nature of the LDA-HDA transition to the multiple amorphous-amorphous transitions in water. When pressurizing LDA at slow rates at 125 K, a stepwise transition LDA—>HDA—>VHDA is observed [173], The piston displacement and density data as obtained from a piston-cylinder experiment that shows the stepwise nature are depicted in Fig. 11. The upstroke HDA >VHDA densification of 5% takes place in the pressure range 0.80-0.95 GPa and is somewhat less sharp when comparing with the upstroke LDA > HDA densification of 20% at 0.40-0.50 GPa. During fast compression, the LDA — HDA transition is still sharp, whereas the HDA—>VHDA transition is smeared out over a broad pressure range [173]. These findings support the possibility of an LDA-HDA first-order like transition but leave the question open whether a first-order like transition underlies the HDA—>VHDA transition and/or whether the observed second step... 

The question of whether there is a tme glassy nature of amorphous ices is of interest when speculating about possible liquid-liquid transitions in (deeply) supercooled water. For true glasses, the amorphous-amorphous transitions described here can be viewed as the low-temperature extension of liquid-liquid transitions among LDL, HDL, and possibly VHDL. That is, the first-order like LDA <-> HDA transition may map into a first-order LDL HDL transition, and the continuous HDA <-> VHDA transition may map into a smeared HDL VHDL transition. Many possible scenarios are used how to explain water s anomalies [40], which share the feature of a liquid-liquid transition [202, 207-212]. They differ, however, in the details of the nature of the liquid-liquid transition Is it continuous or discontinuous Does it end in a liquid-liquid critical point or at the reentrant gas-liquid spinodal ... 

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... 

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