Polyamorphic

Water is a very structurally versatile molecule. Water exists in all three physical states solid, liquid, and gas. Under extremely high temperature and pressure conditions, water can also become a supercritical fluid. Liquid water can be cooled carefully to below its freezing point without solidifying to ice, resulting in two possible forms of supercooled water. In the solid state, 13 different crystalline phases (polymorphous) and 3 amorphous forms (polyamorphous) of water are currently known. These fascinating faces of water are explored in detail in this section. 

Strong Transition and Polyamorphism in the Energy Landscape of Liquid Silica. 

Our approach based on experimental fact that in conditions enough fast cooling at T < 133K water is able to form amorphous phases [1-4], There is polyamorphism of ice, but for all that ice has only two essentially different amorphous forms. In the field of low pressure the amorphous phase of low density is formed, and at increase of pressure there is an amorphous phase of higher density. Qualitative scheme of this polyamorphic transition in ice is shown on Fig. 1. At change of pressure or temperatures in amorphous phases occur more then 20% jumps of density. 

With the purpose of explanation of such kind polyamorphic transition in ice we suggest to take under consideration some nanostructural models of cryogenic amorphous glasses of water within fundamental approaches thermo field dynamics [5] and quantum field chemistry [6-9]. According to these theories the condensed... 

As a result one obtains that average size of compact supermolecules (H20)n in a cryogenic HDA-ice is equal to 18 nanometers that corresponds to n 105 molecules of water. The same as liquid water under normal conditions LDA-ice has an average size of compact supermolecules (H20)n about 1 nanometer that corresponds approximately n 30 molecules of water. These distinctions at a size of compact multiparticles can serve for explain observable density jump at polyamorphic transition of ice. According to scheme of this transition shown on Fig. 3 one comes to think of it. Indeed, knowing share distributions of amorphous phases it is possible to calculate percentage change of volume ... 

In conclusion of this communication some applications of obtained results to substantiation possibilities to use ice polyamorphism phenomena for adjustable stores hydrogen fuel in the form of CH4 are discussed. 

Notice that transformation from a crystalline phase to presumably metastable amorphous phases is called amorphization. It is very promising to use for making of adjustable stores hydrogen fuel phenomena that is called polyamorphism. This term meaning that the pure material can exist in more than one amorphous state. In principle, the abovementioned mechanism of density jumps at polyamorphic transition of ice allows to obtain reversible accumulation of methane inside cellular nanostructures of cryogenic amorphous ice. It is important that the degree of accumulation can be sharp adjusted by pressure and temperature. 

Stability of the graphite-like phase which appears in the irradiated diamonds as a result of polyamorphic transition with high decrease of density was studied using neutron and x-ray methods. The graphite-like structure was shown to be stable up to 50 kbar from ambient temperature to 1500 K at normal pressure. Simultaneously at rapid heating to 900-1000 K new (apparently metastable) modifications of carbon are formed. The diffraction patterns of the modifications do not coincide with those of known structures of carbon (diamond, lonsdeylite, graphite, chaoite, carbine, fullerene and its derivatives etc). It was shown that density of these structures does not differ much from the density of graphite, and at least one of these phases corresponds to a superstructure based on the bee modification of C8 with modified density [14],... 

Figure 3. Internal energy Uvs. density for a polyamorphic transition at pCI 2.7 g/cm3 ASk is the configuration entropy.

Thus obtained results show that the polyamorphic transitions occur not only at compression (Si02, H20, etc.) but at extension as well (C) in the systems having stable or metastable crystal analogs with a different density and a different coordination number z. At the minimal z=2 (chain structures) the transitions may occurs only at compression, at the maximal z=12 (close-packed structures) - only at extension, at the intermediate z (2structure-sensitive properties change and new metastable phases can appear. Amorphization under radiation (crystal lattice extension) can be associated with a softening of phonon frequencies. The transitions in the molecular glasses consisted from the molecules with unsaturated bonds are accompanied by creation of atomic or polymeric amorphous systems. 

Benmore, C. and Siewenie, J. (2004) Polyamorphism and Extreme Environments on GLAD, Neutron News 15(3), 16-18. 

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]. 

In contrast to what is known about a-Si, much less is understood about polyamorphism in Ge. The authors of most early experiments reported no direct evidence of LDA-HDA transition in Ge [260-262, 270, 271]. Shimomura et al. [260] observed a stepwise drop of the electronic resistance (at 6 and lOGPa) after compression of an -Ge him. This decrease, however, may have resulted from (partial) recrystallization to a metallic high-pressure polymorph under pressure. Tanaka [270] measured X-ray diffraction patterns and optical absorption spectra of -Ge at pressures up to lOGPa. In this experiment, the sample was indeed partly transformed to the (3-tin crystalline phase ( 25% in volume) at 6 GPa. Imai et al. [262] also observed an amorphous to [3-tin crystal transition. Freund et al. [271], in contrast, have observed no sign of crystallization or transition to an HDA form after compression up to 9 GPa. 

Unfortunately, the experimental and theoretical studies described above do not provide a consistent picture of polyamorphic transformations of Ge. The ab initio calculations [273] have predicted a highly densified a-Ge with Nc of 8, which may be categorized as VHDA (a third amorphous form). However, considering the stability of the p-tin structure in a wide pressure range, a p-tin-like HDA form is more likely to be formed in Ge, as suggested in Refs. [272,274, and 276]. Additional experimental evidence is eminently desirable. 

Anomalies of Water and Polyamorphism Hydrogen Bond Network Dynamics of Water Molecules Hydrophobic Hydration and Interaction Ion Hydration... 

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