Fast Molecular Transport in Hydrogen Hydrates
by High-Pressure Diamond Anvil Cell NMR
Properties of molecular materials change with pressures. At several to several tens of gigapascals, chemical bonding may transform its fundamental nature. Water ices and gas hydrates are typical examples that involve flexible hydrogen-bonding phenomena and, therefore, particularly sensitive to the pressure. Their structure and transportation properties at this pressure regime are not only important by itself but also giving implications for internal structure and material transport of icy giant planets and icy satellites in the solar system. We investigated hydrogen hydrates which are typical examples of these ice materials, using a novel method to analyze both molecular structure and motion at high pressure - high-resolution diamond-anvil-cell nuclear magnetic resonance spectroscopy [1].
A distinct feature of H2 is its smallest size among the molecules. It is smaller than intrinsic interstices of some ice polymorphs. At pressures higher than 0.4 GPa, H2 spontaneously dissolve into these ice frameworks to produce dense hydrogen hydrates [2]. These hydrates have the structures of ice II and Ic , hence they are called as filled ice hydrates. The filled-ice II hydrate (C1) is stable to 3 GPa with a composition H2:H2O = 1:6. The filled-ice Ic hydrate (C2) is stable to at least 60 GPa with H2:H2O = 1:1.
Fig. 1. Solid-echo powder NMR spectra at 300 MHz and optical micrograph of the hydrogen hydrates synthesized within the diamond anvil cell at high pressures. For each figure, the upper part shows the observed spectrum and its fitting curve, the middle part shows model fitting peaks, and the lower part shows the residual. (a) The filled-ice II phase. (b) The mixture of filled-ice Ic and pure ice VII phases. The L1 peak shows highly-mobile H2 guest molecules enclosed within the ice frameworks. The L2 and G1 peaks show moderately mobile and static H species within the H2O framework.
In situ proton NMR spectra of these filled-ice hydrogen hydrates at pressures reveal fast translational motion of the H2 molecules enclosed within the ice frameworks. The NMR spectra to 3.6 GPa gave sharp resonances of the H2 guests, revealing liquid-like mobile nature of the guest H2 (Fig. 1). Pressure effects on T1-1 and T2-1 of the H2 indicate that molecular rotation and diffusion contribute together to the spin relaxation, from which two motional correlation times Ąrot and Ądif of the guest H2 were determined separately (Fig. 2). A liquid-like large diffusion coefficient of the H2 with little pressure sensitivity was deduced from Ądif, indicating that the ice framework allows active guest translation even in extensively compressed states.
Fig. 2. Two motional correlation times (Ąrot and Ądif) and diffusion coefficient (DG) of the guest H2 molecules. They are determined from pressure and frequency dependences on T1-1 and T2-1 of the guest H2.
References
[1] T. Okuchi, Phys. Earth Planet. Inter., 143-144, 611 (2004).
[2] W. L. Vos, L. W. Finger, R. J. Hemley, and H. K. Mao, Phys. Rev. Lett. 71, 3150 (1993).
[3] T. Okuchi, M. Takigawa, J. Shu, H. K. Mao, R. J. Hemley, and T. Yagi, Phys. Rev. B., 75, 144104 (2007).