High-pressure/low-temperature neutron scattering of gas inclusion compounds: Progress and prospects

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   Fig. 1.

   Fitted neutron diffraction pattern of a mixed methane–ethane sI clathrate (82.93 wt% methane + 17.07 wt% ethane) with minor sII phase and D₂O ice. Data are shown as red plus signs, and the green curve is the best fit to the data. Tick marks below the pattern show the positions of allowed reflections (blue, sI phase; red, ice; black, sII phase). P-T, pressure-temperature. (Inset) Neutron patterns of methane clathrate formed from ice and D₂ in situ in the Al pressure cell. Note that formation of methane clathrate was not complete even after 10 h at 500 bars and 270 K, indicating sluggish formation kinetics compared with hydrogen clathrate.

   
   
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   Fig. 2.

   Neutron diffraction patterns of deuterium clathrates produced from water (A and B) and powdered ice (C), at approximately the same pressure–temperature conditions (11). Note that the formation kinetics of D₂ clathrates formed from ice powder is much faster than from water. [Reproduced with permission from ref. 11 (Copyright 2005, American Institute of Physics).]

   
   
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   Fig. 3.

   Structural view of D₂ distribution in the large (Upper) and small (Lower) cages of deuterium clathrate (7). Oxygen atoms are shown as red spheres, deuterium framework atoms are green, and guest D₂ molecules are yellow. Note that the D₂… D₂ separation is the distance between the centers of mass of the two D₂ molecules. Below 50 K, the guest D₂ molecules are localized (Left). With increasing temperature, the D₂ molecules can rotate more freely, yielding a nearly spherical D₂ density distribution (Right). [Reproduced with permission from ref. 7 (Copyright 2004, American Physical Society).]

   
   
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   Fig. 4.

   Neutron diffraction of D₂ sorption in Cu₃[Co(CN)₆]₂. (A) Fitted neutron diffraction pattern of Cu₃[Co(CN)₆]₂·nD₂ at 40 K and 10 MPa of D₂ pressure. Data are shown as red plus signs, and the green curve is the best fit to the data. Tick marks below the pattern show the positions of allowed reflections (first row, Al cell; second row, sample). (Inset) Increased unit-cell volume under 10 MPa of D₂ pressure at 100 K. (B) Crystal structure of Cu₃[Co(CN)₆]₂·9H₂O at ambient condition (brown, Cu; pink, Co; black, C; light blue, N; dark blue, water). (C) Crystal structure of Cu₃[Co(CN)₆]₂·nH₂ (green, H₂) showing H₂ at the (¼, ¼, ¼) site (the relatively large green sphere illustrates free-rotation of H₂ molecules in the cages). (D and E) Difference Fourier nuclear maps showing that the incorporated D₂ molecules mainly occupy the original water site (¼, ¼, ¼) (E) with some possibly associated with Cu (D). The residual intensities at the Co and Cu sites are probably caused by the limited resolution of our data.

   
   
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   Fig. 5.

   Low-frequency neutron vibrational spectrum of Cu₃(BTC)₂ with its crystal structure shown (Inset). Cu₃(BTC)₂ has three H₂ equivalents adsorbed at 40 K. The curve is a maximum entropy reconstruction of the spectrum. It enhances some details that are present in the data (dots) but may be not easy to see.

   
   
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   Fig. 6.

   Structure diagram of a potential hybrid material of the MOF with the CH₄(H₂)₄ molecular compound encapsulated into its cage. MOF consists of metal-oxygen clusters (tetrahedra) on the vertices of the lattice and organic linker molecules (hexagons) along its edges, which largely define the size and shape of the cage. Blue balls in CH₄(H₂)₄ represent CH₄, and red balls represent H₂.

   
   
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   Fig. 7.

   Schematic general view (in the top-left corner) and the enlarged sectional view of the experimental setup designed for in situ high-pressure/low-temperature neutron diffraction at the Los Alamos Neutron Science Center (8). [Reproduced with permission from ref. 8 (Copyright 2005, American Institute of Physics).]