NEUTRON DIFFRACTION OF CLATHRATE HYDRATES

Clathrate hydrates are a group of crystalline inclusion compounds consisting of ice-like frameworks of hydrogen-bonded H₂O molecules with gas molecules trapped inside the cages. Natural clathrate hydrates are found in oceanic floor sediments and permafrost as well as in the outer solar system, and they contain primarily methane and minor amounts of other gases. Clathrate hydrates can potentially be used for storage and separation of various gases (such as hydrogen and carbon dioxide) for technological applications. To understand the natural formation process of clathrate hydrates and to realize their practical applications, it is essential to study their crystal structures, thermodynamic stability, and kinetics of formation and decomposition at relevant pressure-temperature (high-P and/or low-T) conditions. Because neutrons are sensitive to hydrogen (and its isotopes) and other light elements, high-P low-T neutron diffraction is a powerful tool for such studies. In this presentation, I will give an overview of neutron diffraction studies of clathrate hydrates including examples of our own work on carbon oxide hydrates. Focuses will be on illustrating the mechanisms of interactions between gas molecules and hydrate framework and their effects on thermodynamics and kinetics of clathrate formation. Lastly, I will offer a perspective on future neutron scattering research, especially, on (semi-)clathrate hydrates that can be stabilized at close-to-ambient conditions for practical applications.