Local structure of guest molecules in gas hydrates - a model study of Kr and Xe clathrates

Gas hydrates constitute a class of solids in which small molecules occupy cavities inside an ice-like structure. There is enormous scientific and technological interest in understanding the structure, stability and formation mechanism of clathrates. A variable-temperature high-pressure cell for X-rays measurements has been developed and constructed, which allows in situ studies of clathrate formation or decomposition. XAFS and diffraction techniques have been used to study the evolution of the structure during formation and decomposition. Two clathrates structures were studied, structure I (Xe) and structure II (Kr), and the local structure around the guest atom was identified. The rare gas-water complexes that act as precursor to the formation of the crystalline phases were identified. The transformation of the clathrate from structure II to structure I when Xe is added to Kr clathrates was observed.