An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram

Abstract

The recent back-to-back findings of low-density porous ice XVI and XVII have rekindled the century-old field of the solid-state physics and chemistry of water. Experimentally, both ice XVI and XVII crystals can be produced by extracting guest atoms or molecules enclosed in the cavities of preformed ice clathrate hydrates. Herein, we examine more than 200 hypothetical low-density porous ices whose structures were generated according to a database of zeolite structures. Hitherto unreported porous EMT ice, named according to zeolite nomenclature, is identified to have an extremely low density of 0.5 g/cm 3 and the largest internal cavity (7.88 Å in average radius). The EMT ice can be viewed as dumbbell-shaped motifs in a hexagonal close-packed structure. Our first-principles computations and molecular dynamics simulations confirm that the EMT ice is stable under negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen molecules, the EMT ice hydrate can easily outperform the record hydrogen storage capacity of 5.3 wt % achieved with sII hydrogen hydrate. Most importantly, in the reconstructed temperaturepressure (T-P) phase diagram of water, the EMT ice is located at deeply negative pressure regions below ice XVI and at higher temperature regions next to FAU. Last, the phonon spectra of empty-sII, FAU, EMT, and other zeolite-like ice structures are computed by using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm 3 ), both the bending and stretching vibrational modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds. porous ice | ultralow density | EMT ice | reconstructed temperaturepressure phase diagram | record hydrogen storage capacity W ater is a unique form of matter with many intriguing properties. One such physical property is its wide variety of stable and metastable crystal structures. To date, 18 different crystalline ice phases have been established experimentally (1-3). Many more ice phases ranging from 1D to 3D have been predicted from computer simulations (4-10). Another known ice form is ice clathrate hydrates with large internal cavities that can host guest molecules. Clathrate natural gas hydrates have received considerable attention because they are an enormous energy source on Earth. Indeed, the amount of carbon in natural gas hydrates is estimated to be at least twice that in all other fossil energies combined (11). Clathrate hydrogen hydrates have also received growing attention as they are a renewable and carbon-free energy source (12).In previous computer simulation studies, guest-free clathrate hydrates of type sII were independently predicted to be a stable phase at negative pressures by Jacobson et al. (13), Conde et al. (14), and Huang et al. (8). Remarkably, the guest-free clathrate hydrate of type sII was recently produced in the laboratory by Falenty et al. (2) by pumping off guest Ne atoms from the cavities of sII clathrate hydrate, confirming earlier ...

Hydrogen Inter-Cage Hopping and Cage Occupancies inside Hydrogen Hydrate: Molecular-Dynamics Analysis

“…The calculated diffusion coefficients at the different temperatures for each of the simulated systems are presented in Figure 2; for the H 2 and D 2 systems, the values are tabled in Tables S3 and S4, respectively (see SI). The clathrate hydrates are of special interest because they are less dense than regular ice and, under negative pressure, are expected to be stable [29,[40][41][42][43][44][45]; the density of the clathrate hydrate is also less in low temperatures, and the density increases with the temperature, as shown in the Table S4 (see SI). The diffusion coefficient is a physical constant dependent on the molecule size and other properties of the diffusing substance, as well as on temperature and pressure.…”

Computational Prediction of Novel Ice Phases: A Perspective

“…The s-H clathrate entails the smallest unit cell, which is composed of three 5 12 cages, two 4 2 5 5 6 4 cages, and one 5 12 6 8 cage with 34 water molecules. Moreover, numerous guest-free porous ices have been predicted from computer simulations, namely, s-III, s-IV, ice ITT, sL, and EMT . These guest-free hydrate ices are unlikely to be thermodynamically stable at ambient pressure because their porous structures are largely stabilized by the gas molecules entrapped in the cages.…”

“…Prediction of Porous Ice at Negative Pressure from Computer Simulations . Computer simulations have indicated that some low-density porous ice phases are stable or metastable at negative pressure, and several porous ices have been synthesized in experimental studies. − , An example is the empty structure of the s-II clathrate, known as ice XVI . Recently, using an extensive Monte Carlo packing algorithm and dispersion-corrected density-functional theory optimization, Huang et al predicted two porous ice phases, one stable and one metastable, that may occupy regions in the phase diagram of ice under negative pressure. , As shown in Figure , the network structures of the two porous ice phases are the same as the zeolite frameworks of RHO (Figure a) and FAU (Figure b), termed ice RHO and ice FAU.…”

Formation of porous ice frameworks at room temperature

“…4E) appears always to be the WOF I_f, suggesting that the phase transition between ice I h and I_f would occur prior to those between ice I h and the other WOFs. In addition, we constructed a P-T phase diagram based on the Gibbs free energies computed using the Einstein molecule method (5,7,28,29) and the TIP4P/ 2005 water potential. As depicted in SI Appendix, Fig.…”

“…Recently, the structures of ices XVI and XVII were confirmed in the laboratory, whereas ice XVI is only stable at negative pressures between approximately −0.4 and −1 GPa (3), while ice XVII is a low-density porous ice containing spiral internal channels (4). Numerous zeolite-like porous ices have also recently been predicted to be metastable at negative pressures (5)(6)(7)(8)(9)(10)(11). Like many known porous materials, such as zeolites, metal-organic frameworks (MOFs) (12), and covalent organic frameworks (COFs) (13), porous ices may be applicable to gas storage, purification, and separation (14,15), or even potentially possess medical applications owing to their lack of toxicity.…”