Struktur und Umwandlungen von Eisphasen in Gegenwart der Gase Helium, Neon und Argon

Abstract

Water in its crystalline phases is an ideal system to investigate the interactions of water molecules with each other as well as with various gases. Condensed water exists in a variety of crystalline phases, the ice phases, and forms with gases ice like structures, so called clathrate hydrates or gas hydrates.In the phase diagram of water some known phases co-exist, and the existence of further metastable phases has been inferred from a number of observations. Therefore it appeared important to find out more about the exact location the phase boundaries and work out reproducible conditions for synthesis. This can be achieved with difference-pressure-analysis (DPA) and in situ diffraction methods. With these methods procedures for the synthesis of a number of ice phases have been established, notably for the metastable ice phase XII.The structure of Ice Ih is influenced in presence of helium and neon by adsorption of these gases in the crystal lattice. Caused by additional interactions of these gases with the water molecules, the ice structure is stabilized to higher pressures due to the formation of a gas hydrate ("stuffed ice"). Moreover, water forms in the presence of neon a clathrate hydrate of which the structure was established here for the first. Likewise, the structure of a neon hydrate was determined, which is topologically identical to ice II.