HOST STRUCTURE AND GUEST OCCUPANCY OF N₂-AND O₂-CLATHRATES AS A FUNCTION OF PRESSURE BY NEUTRON POWDER DIFFRACTION. W. F. Kuhs^*, B. Chazallon^*, F. Pauer ^{*deg., *}Mineralogisch-Kristallographisches Institut, Universität Göttingen, Goldschmidtstr.1, D-37077 Göttingen, ^deg.Alfred-Wegener-Institut für Polar- und Meeresforschung, Columbusstraße, D-27568 Bremerhaven

Air-clathrates crystallize (at least at lower pressures) in the Stackelberg type II clathrate structure as do the pure O₂- and N₂- clathrates and they occur naturally in deeper parts of the arctic and antarctic ice shields. In order to elucidate the thermodynamic and structural properties of natural air-clathrates we have synthesized the pure N₂- and O₂-clathrates and analyzed their structure and composition by neutron diffraction in the range of 150 to 2500 bar nitrogen and 120 to 330 bar oxygen pressure respectively. The main question is whether or not the filling follows a Langmuir isotherm corresponding to an ideal solid solution behaviour and how important the differences are between the nitrogen and the oxygen filling at given pressures in the pure phases. Diffraction is the appropriate technique allowing for the determination of the filling behaviour vs. gas pressure (fugacity) for the different types of cages in the clathrate structure (which is not possible with standard physico-chemical studies); neutron diffraction permits in-situ work under high gas pressures. The experiments were performed at the Institute Laue-Langevin in Grenoble with high pressure cells developed by us for the use in standard "orange" cryostats.

We will give details of the sample preparation and the results of the crystal structure refinements by Rietveld refinement techniques for a variety of samples at different pressures and temperatures, and as a function of reaction time. We have determined the degree of filling for the small and large cages as a function of pressure. For the first time we have evidence for a double occupancy of the large cages, a fact which up to now has not been considered in theoretical calculations of the clathrate filling behaviour. We also have observed the (already predicted) occurence of a Stackelberg type I clathrate at higher gas pressures as well as their back-transformation to a type II structure on cooling. Finally the relevance of these results for the work on air-clathrate inclusions in ice cores will be discussed.