Scientists these days are struggling to understand the basic nature of molecules and are diving into great depths of understanding Atomic and Molecular Physics and also exploring the Chemical behavior. In such an attempt, Scientists have succeeded to form the LOWEST DENSITY ICE CRYSTAL'S which was never earlier done before.
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Water the most commonly used thing has many polymorphs. Seventeen ice polymorphs, including metastable ones, have been found experimentally by researchers. It is uncommon that a pure substance has such a lot of crystal structures. The variety of ice polymorphs arises from the fact that water molecules prefer the tetrahedrally directed network topology due to the hydrogen bonds. Some of the ice polymorphs were discovered very recently. They are of particular interest because they are less dense than normal ice and are supposed to be stable under neagative pressure. Falenty et al. succeeded to produce empty sII clathrate hydrate by vacuum pumping of neon hydrate in 2014, and this ice was named ice XVI. Existence of this porous ice was originally predicted theoretically, and its properties were surveyed by computer simulations. The same methodology was applied to hydrogen-lled ice C0 to produce ice XVII. Many hypothetical ices have been produced by computer simulations. Researchers predicted that empty sH clathrate hydrate is more stable than any other known ice phases under extreme negative pressures. After it, scientists proposed that two ultralow-density ice phases occupy wide areas in the phase diagram of water at negative pressures.They searched for possible stable ice structures by a Monte Carlo packing algorithm and evaluated thermodynamic stabilities of them by the combination of DFT calculations and classical molecular dynamics (MD) simulations with the TIP4P/2005 water force eld. The network structures of the two stable low density ice phases are the same as the zeolite frameworks of RHOand FAU. We call them ice RHO and ice FAU in this paper. The network structures of ice XVI and the empty sH clathrate hydrate are the same as the MTN and DOH zeolite frameworks, and thus we refer to them as ice MTN and ice DOH, respectively.
Although the vapor phase is thermodynamically mores stable than any other phase on approaching to the limit of zero pressure, solid phases can be metastable at low temperatures in the negative pressure region. A survey of metastable ice phases would be helpful to understand the complex phase behavior of water in conned geometries. Consideration on possible metastable phases is also important because they might affect kinetics of phase transitions. Although various porous ice structures have been proposed, the comprehensive knowledge of them is still lacking. We pose a question to disambiguate the problem: what is the most stable solid phase under negative pressure? Hypothetical ice structures derived from zeolite frameworks can be good candidates for the stable ice phases under negative pressure.26 Although more than 200 of zeolite frameworks have been reported, most of them have not been considered in the early studies on porous ices. Some of ice analogues might be more stable than either ice MTN, DOH, RHO, or FAU in certain regions in the phase diagram at negative pressures. There are many clathrate hydrate structures other than several well-knownonessuchassIIandsH,andthey also can be candidates for stable negative pressure ice. In this paper, we examine exhaustively ice structures of zeolite and clathrate hydrate frame works using classical MD simulations. It is found that a zeolitic ice is more stable than either ice MTN, DOH, RHO, or FAU at deeply negative pressures.We also demonstrate that it is possible to design sparse ice structures of arbitrary density from several zeolitic ices, and they can be more stable than any of zeolitic ices and empty clathrate hydrates under negative pressure at low temperatures.
Scientists have examined three types of low-density ices, space fullerene ices, zeolitic ices, and aeroices. The structures of space fullerene ices are the same as real or hypothetical empty clathrate hydrates. It is found that the empty sII hydrate is more stable than any other space fullerene ice. The zeolitic ice structures are prepared from SiO2 zeolite structures given in the zeolite database. Ice ITT, which is an analogue to zeolite ITT, is more stable than ice FAU which was proposed as the most stable phase under deeply negative pressures in a previous paper.12 The third type of low-density ices, aeroices, are obtained from a type of zeolitic ice that consists of prismatic edges and polyhedral vertices by elongation of the prismatic edges. The aeroice structures do not collapse in MD simulations at T = 77 K and p = 1 bar, indicating that they are at least mechanically stable at the liquid nitrogen temperature. Aeroices can be more stable than any zeolitic ices at certain thermodynamic conditions under negative pressure. As the prismatic edges get longer, the density of the aeroice phase approaches zero while the potential energy converges to a certain negative value. As a result, the transition pressure from ice Ih to the aeroice phase becomes a quite small negative value.
Credit : The Journal of Chemical Physics.
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