Gas clathrate hydrates or gas hydrates are made of H-bonded water molecules forming cages, within which gaseous (guest) molecules are encapsulated. The formed clathrate structures, which may be metastable, depend on the nature and on the partitioning of the guest molecules in the water cage. This work focuses on the structural and vibrational properties of nitrogen hydrate in its two clathrate forms (namely, SI and SII) in the thermodynamic ranges 50−200 bar and 150−270 K, together with a comprehensive analysis of the transformation from SI to SII of this gas hydrate. The thermal expansion of both structures has been measured at 1 bar, and the melting of the nitrogen hydrate has been measured at ca. 210 K at 1 bar. Moreover, the SI structure is metastable in the studied pressure region: from time-dependent neutron powder diffraction analysis, it is shown that the SI structure transforms over time to the SII structure with a rate of (1.37 ± 0.17) × 10 5 s −1 at 100 K and at 1 bar. The transformation is also characterized by an induction time (i.e., the lifetime of the pure SI structure) of 0.49 day. We have also investigated the guest partitioning of the nitrogen hydrate using highresolution Raman scattering. Vibrational bands of nitrogen molecules encapsulated in large cages are measured at lower wavenumbers than the one associated with encapsulation in small cages (by 1.1 cm −1 in SI and 0.8 cm −1 in SII). In the case of the thermodynamically stable SII phase, the dependence of the guest partitioning has been characterized as a function of the pressure−temperature conditions. Variation of the relative cage filling is demonstrated. While the small cages remain singly occupied according to previous neutron diffraction analysis, this variation is attributed to large cages of the nitrogen hydrate that easily catch or release nitrogen guest molecules. This study thus provides new opportunities for preparing nitrogen gas hydrates with a "targeted" structure and relative cage filling not only by varying the pressure and temperature but also by playing with the structural metastability.

Gas hydrates are inclusion compounds composed of a H-bonded water network forming cages, inside of which gaseous (guest) molecules are encapsulated. Depending on the nature and partitioning of the guest molecules, various types of clathrate structures may be formed. In this work we have elucidated the guest partitioning of the CO hydrate, using high-resolution Raman microspectroscopy, and investigated the impact of pressure−temperature (P−T) conditions on this partitioning. For the first time, vibrational signatures of CO molecules encapsulated in a large cage and small cage are identified. It is also shown that the large cages of the CO hydrate have the ability to easily catch or release CO guest molecules, while the small cages remain singly occupied. Moreover, the study of the P−T dependence of the Raman signature demonstrates not only the CO stretching frequency dependence with the cage filling but also the tuning effect of the cage filling by the P−T conditions of treatment.