Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate

Abstract

Visual observations through a microscope and in situ Raman scattering measurements of a synthesized nitrogen hydrate have been performed at pressures up to 6 GPa and 296 K. High-pressure transformations have been found at 0.85 and 1.45 GPa. The cubic structure II (sII) nitrogen hydrate initially transforms to the hexagonal structure (sH) at 0.85 GPa and finally forms the orthorhombic dihydrate (sO) above 1.45 GPa. The sO phase of nitrogen hydrate exists up to at least 6 GPa. A variety of Raman spectra composed of three peaks have been sometimes observed in sII phase below 0.50 GPa, which implies that the guest nitrogen molecules doubly occupy the large hexakaidecahedron cages. Two Raman bands of the guest nitrogen vibrations with nearly equal intensities appearing in sH phase suggest that five nitrogen molecules are filling in extra large icosahedron cages.

Raman study of methane clathrate hydrates under pressure: new evidence for the metastability of structure II

“…43 In both regions, the pressure shift is similar to that observed in our experiments. Thus it seems that in sI and sII hydrates, whichever guest gas is trapped, the H 2 O skeleton has the same Raman behavior under pressure as ice Ih.…”

Ageing and Langmuir Behavior of the Cage Occupancy in the Nitrogen Gas Hydrate

“…It should be noted that the Raman profile fitting with two contributions (LC and SC) does not take into account the slight difference of the Raman stretching frequencies for a single occupancy of the LCs and for a double occupancy of the LCs. Indeed, these two contributions are respectively observed at 2322 cm −1 and 2323.4 cm −1 at 2900 bar and 293 K, i.e., with a frequency difference of 1.4 cm −1 at this high pressure condition [47]. Such Raman bands are difficult to disentangle in the present conditions of measurements: their frequency difference at 1 bar and 77 K might indeed be significantly smaller than the one measured at 2900 bar and 293 K (successful fitting of the present Raman profiles with these contributions could not be achieved).…”

Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations

“…Time steps of 1 fs are used in all simulations. The initial positions of the water oxygen atoms of the hydrate lattice were obtained from the crystallographic structures of sH 29 and sII hydrate. 30 The disordered water hydrogen atoms were arranged about water oxygen sites subjected to the constrains of the Bernal-Fowler ice rules 31 via a unit-cell dipole-moment minimizing Monte Carlo calculation.…”