Phase changes of filled ice Ih methane hydrate under low temperature and high pressure

Pressure-regulated rotational guests in nano-confined spaces suppress heat transport in methane hydrates

Materials with low lattice thermal conductivity are essential for various heat-related applications like thermoelectrics, and usual approaches for achieving this rely on specific crystalline structures. Here, we report a strategy for thermal conductivity reduction and regulation via guest rotational dynamics and their couplings with lattice vibrations. By applying pressure to manipulate rotational states, we find the intensified rotor-lattice couplings of compressed methane hydrate MH-III can trigger strong phonon scatterings and phonon localizations, enabling an almost three-fold suppression of thermal conductivity. Besides, the disorder in methane rotational dynamics results in anharmonic interactions and nonlinear pressure-dependent heat transport. The overall guest rotational dynamics and heat conduction changes can be flexibly regulated by the rotor-lattice coupling strength. We further underscore that this reduction mechanism can be extended to a wide range of systems with different structures. The results demonstrate a potentially universal method for reducing or controlling heat transport by developing a hybrid system with tailored molecular rotors.

Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations

Understanding physicochemical properties of amorphous gas hydrate systems is of great significance to reveal structural stabilities of polycrystalline gas hydrate systems. Furthermore, amorphous gas hydrates can occur ordinarily in the nucleation events of gas hydrate systems. Herein, the mechanical properties of amorphous carbon dioxide hydrates are examined by means of all-atom classical molecular dynamic simulations. Our molecular simulation results reveal that mechanical strengths of amorphous carbon dioxide hydrates are evidently governed by temperatures, confining pressures, and ratios of water to carbon dioxide molecules. Notably, under compressive loads, amorphous carbon dioxide hydrates firstly exhibit monotonic strain hardening, followed by an interesting distinct phenomenon characterized by a steady flow stress at further large deformation strains. Furthermore, structural evolutions of amorphous carbon dioxide hydrates are analyzed on the basis of the N-Hbond DOP order parameter. These important findings can not only contribute to our understanding of the structural stabilities of amorphous gas hydrate systems, but also help to develop fundamental understandings about grain boundaries of gas hydrate systems.

Structure, Luminescence, and Magnetic Properties of Crystalline Manganese Tungstate Doped with Rare Earth Ion

The precursor prepared by co-precipitation method was sintered at various temperatures to synthesize crystalline manganese tungstate (MnWO₄). Sintered MnWO₄ showed the best crystallinity at a sintering temperature of 800 °C. Rare earth ion (Dysprosium; Dy³⁺) was added when preparing the precursor to enhance the magnetic and luminescent properties of crystalline MnWO₄ based on these sintering temperature conditions. As the amount of rare earth ions was changed, the magnetic and luminescent characteristics were enhanced; however, after 0.1 mol.%, the luminescent characteristics decreased due to the concentration quenching phenomenon. In addition, a composite was prepared by mixing MnWO₄ powder, with enhanced magnetism and luminescence properties due to the addition of dysprosium, with epoxy. To one of the two prepared composites a magnetic field was applied to induce alignment of the MnWO₄ particles. Aligned particles showed stronger luminescence than the composite sample prepared with unsorted particles. As a result of this, it was suggested that it can be used as phosphor and a photosensitizer by utilizing the magnetic and luminescent properties of the synthesized MnWO₄ powder with the addition of rare earth ions.

Effect of Ammonia on Methane Hydrate Stability under High-Pressure and High-Temperature Conditions

High-pressure experiments were conducted to investigate the stability and phase transition of methane hydrate (MH) in the water-methane-ammonia system at room-to-high temperatures employing Raman spectroscopy and synchrotron X-ray powder diffraction, in combination with an externally heated diamond anvil cell. The results revealed that, at room temperature, MH undergoes phase transitions from MH-I to MH-II at ∼1.0 GPa and from MH-II to MH-III at ∼2.0 GPa. These transition behaviors are consistent with those in the water-methane system, which indicates that ammonia has a negligible effect on a series of phase transitions of MH. Contrarily, a sequential in situ Raman spectroscopy revealed that ammonia affects the stability of MH-III under high pressure and high temperature: the dissociation temperature of MH-III was more than 10 K lower in the water-methane-ammonia system than in the water-methane system. These findings aid in improving the internal structural models of icy bodies and estimating the origin of their atmospheric methane.

Structural evolution of methane hydrate under pressures up to 134 GPa

High-pressure experiments were performed to understand the structural evolution of methane hydrate (MH) up to 134 GPa using x-ray powder diffraction (XRD) and Raman spectroscopy with diamond anvil cells. XRD revealed the distinct changes in the diffraction lines of MH owing to phase transition from a guest-ordered state phase [MH-III(GOS)] to a new high-pressure phase (MH-IV) at 33.8-57.7 GPa. MH-IV was found to be stable up to at least 134 GPa without decomposition into solid methane and high-pressure ices. Raman spectroscopy showed the splits in the C-H vibration modes ν₃ and ν₁ of guest methane molecules in filled-ice Ih (MH-III) at 12.7 GPa and 28.6 GPa, respectively. These splits are caused by orientational ordering of guest methane molecules contained in the hydrate structure, as observed in a previous study. These results suggest that the structural evolution of the filled-ice structure of MH is caused by successive orientational ordering of guest methane molecules, thereby inducing changes in the host framework formed by water molecules.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Observation of methane filled hexagonal ice stable up to 150 GPa

Gas clathrates in which water cages enclose the guest gas molecules naturally form on our planet, for instance on ocean floors. Related methane hydrates are also expected to be present under very high pressures (10 to 200 GPa) in giant planetary interiors such as Uranus or Neptune. However, the stability of such structures at these pressures is currently debated. Joint Raman spectroscopy and ab initio simulations show the stability of a high-pressure methane hydrate phase up to at least 150 GPa, the highest pressure explored to date for such compounds. The structure of this phase and the complex transition mechanism from the known MH-III phase are detailed and shown to be in accordance with all known experimental observations.

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH₄ molecules are located within H₂O or D₂O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH₄–D₂O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D₂O network is isomorphic with ice Ih, forms at ∼40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III → MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

Salt- and gas-filled ices under planetary conditions

In recent years, evidence has emerged that solid water can contain substantial amounts of guest species, such as small gas molecules-in gas hydrate structures-or ions-in salty ice structures-and that these 'filled' ice structures can be stable under pressures of tens of Gigapascals and temperatures of hundreds of Kelvins. The inclusion of guest species can strongly modify the density, vibrational, diffusive and conductivity properties of ice under high pressure, and promote novel exotic properties. In this review, we discuss our experimental findings and molecular dynamics simulation results on the structures formed by salt- and gas-filled ices, their unusual properties, and the unexpected dynamical phenomena observed under pressure and temperature conditions relevant for planetary interiors modelling. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

In situ Raman and X-ray diffraction studies on the high pressure and temperature stability of methane hydrate up to 55 GPa

High-temperature and high-pressure experiments were performed under 2-55 GPa and 298-653 K using in situ Raman spectroscopy and X-ray diffraction combined with externally heated diamond anvil cells to investigate the stability of methane hydrate. Prior to in situ experiments, the typical C-H vibration modes of methane hydrate and their pressure dependence were measured at room temperature using Raman spectroscopy to make a clear discrimination between methane hydrate and solid methane which forms through the decomposition of methane hydrate at high temperature. The sequential in situ Raman spectroscopy and X-ray diffraction revealed that methane hydrate survives up to 633 K and 40.3 GPa and then decomposes into solid methane and ice VII above the conditions. The decomposition curve of methane hydrate estimated by the present experiments is >200 K lower than the melting curves of solid methane and ice VII, and moderately increases with increasing pressure. Our result suggests that although methane hydrate may be an important candidate for major constituents of cool exoplanets and other icy bodies, it is unlikely to be present in the ice mantle of Neptune and Uranus, where the temperature is expected to be far beyond the decomposition temperatures.

Time-resolved X-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate

The mechanisms by which methane hydrate transforms from an sI to sH structure and from an sH to filled-ice Ih structure were examined using time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device camera observation under fixed pressure conditions. The XRD data obtained for the sI-sH transition at 0.8 GPa revealed an inverse correlation between sI and sH, suggesting that the sI structure is replaced by sH. Meanwhile, the Raman analysis demonstrated that although the 12-hedra of sI are retained, the 14-hedra are replaced sequentially by additional 12-hedra, modified 12-hedra, and 20-hedra cages of sH. With the sH to filled-ice Ih transition at 1.8 GPa, both the XRD and Raman data showed that this occurs through a sudden collapse of the sH structure and subsequent release of solid and fluid methane that is gradually incorporated into the filled-ice Ih to complete its structure. This therefore represents a typical reconstructive transition mechanism.