Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran-CO₂ Binary...

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO₂ below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO₂ guests in small pentagonal dodecahedral 5¹² host water cages, not previously observed in the simple CO₂ hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I4₁/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.