Novel Hydrogen Clathrate Hydrate

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal $\mathit{R}3\mathit{c}$ or $\mathit{R}\overline{3}\mathit{c}$ space group (proton ordered or disordered, respectively) and the composition of ${({\mathrm{H}}_2\mathrm{O})}_6{\mathrm{H}}_2$. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase $C_1^{\prime }$, distinct from the well-known ordered $C_1$ clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.

Figure 1

Tentative $P\text{−}T$ phase diagram for ${\mathrm{H}}_2\text{−}{\mathrm{H}}_2\mathrm{O}$ compounds superimposed on those of pure ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{H}}_2$. Black solid lines are the ${\mathrm{H}}_2\text{−}{\mathrm{H}}_2\mathrm{O}$ phase equilibrium lines proposed in Ref. [9], while gray dashed lines correspond to phase lines for pure water (e.g., Ref. [18]) and dark cyan dashed line is for pure ${\mathrm{H}}_2$ [19]. Dark blue open diamonds represent experimentally determined $P\text{−}T$ conditions in our work for the $C_0$ phase. A new $C_1^{\prime }$ phase (open red circles and red triangles) appears upon pressure release of the $C_1$ phase at 295 K (open red triangle down) and on pressure increase from fluid (crossed filled red triangles up). The new $C_1^{\prime }$ clathrate transforms to the known $C_1$ phase (open blue squares) on pressure increase at 295 K and upon cooling down with a simultaneous pressure increase.

Figure 2

Raman spectra of hydrogen hydrates $C_1$ and $C_1^{\prime }$ and ice II. The roton modes of molecular ${\mathrm{H}}_2$ are labeled; the vibron mode at about $4200{\mathrm{cm}}^{-1}$ (not shown here) is present for $C_1$ and $C_1^{\prime }$ and is absent for ice II (Figs. S1, S2). The arrows mark the unique lattice modes of the $C_1$ phase, which are not observed in the $C_1^{\prime }$ phase, while most of them have counterparts in ice II. Note the broadening of the lattice modes of $C_1^{\prime }$ compared to $C_1$. The spectra are shifted vertically for clarity. The vertical scale is different for the left (lattice vibrations) and the right (O-H stretching modes) panels.

Figure 3

(a) SCXRD 2D image collected from a single crystal of $C_1^{\prime }$ phase at 1.3 GPa upon rotation of $\pm 35°$ of the diamond anvil cell demonstrating high quality of a single crystal (Fig. S3); squares mark reflections with the given $hkl$ indices; strong unlabeled reflections are from diamond; weak unlabeled peaks could originate from other smaller single crystal(s), which could not be refined but their contribution to the intensities collected from the large crystal can be definitively singled out by a variety of algorithms (see Supplemental Material [20]); (b) $H0L$ slice of reciprocal space visualized by crysalispro software package [39] with attributed diffraction spots. Note the apparent absence of $(003{)}_{hkl}$ equivalent reflections; (c) Powder XRD data (the x-ray wavelength is 0.291 Å) integrated in 1D pattern (gray plus symbols present the data and the red solid line is the Le Bail fit), the short red ticks correspond to $\mathit{R}\overline{3}\mathit{c}$ space group with lattice parameters $a=13.102(4)\AA$, $b=6.263(7)\AA$. The blue ticks also represent XRD of the $C_1$ $\mathit{R}\overline{3}$ structure (the same space group as ice II) with the same lattice parameters. Inset shows an image of the 2D powder XRD data of the $C_1^{\prime }$ phase.

Figure 4

The structure of $C_1^{\prime }$ clathrate in $\mathit{R}\overline{3}\mathit{c}$ setting, where the major structural motif (${\mathrm{H}}_2\mathrm{O}$ rings) is shown in different projections to demonstrate two different disordering schemes. Orange spheres correspond to the centers of mass of the hydrogen molecules. Bluish spheres of different shades correspond to partially occupied positions for hydrogen atoms in the water cages. The solid thin light cyan and blue lines connect the duplicated hydrogen positions, generating “cyclic” (in an armchairlike virtual plane formed by six oxygen atoms) and “acyclic” (outside the plane) types of disorder, respectively.