The structure and cage filling of gas hydrates as established by synchrotron powder diffraction data

Abstract: Synchrotron powder diffraction data were measured on various gas hydrates prepared at different fugacities and recovered to low temperatures. The data extend to sin {\theta}/{\lambda} values of at least 1.01 {\AA}-1, in some cases up to 1.48 {\AA}-1, i.e. at least 3 times further in reciprocal space than the best single crystal data obtained so far. Here the results of CO2-hydrate are presented in detail. Structural information is obtained from Rietveld refinements to unprecedented precision. All H-bonded O-O distances are found to be close to 2.75-2.76 {\AA}, i.e. very close to the values in ice Ih; differences between a hydrogenated and deuterated host lattice are insignificant. Cage occupancies were obtained for the first time in free refinements together with positional parameters of the guest molecule and their atomic displacement parameters. There is good agreement between the experimental cage occupancies and prediction results from CSMGem.