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.