RESUMO
We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C0 at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move. It has a network that is unrelated to any experimentally known ice, silica, or zeolite network but is instead related to two Zintl compounds. Both hydrates are found to be stable in electronic structure calculations, with hydration ratios in very good agreement with experiment.
RESUMO
We present results from a first-principles study on the stability of noble gas-water compounds in the pressure range 0-100 kbar. Filled-ice structures based on the host water networks ice-Ih, ice-Ic, ice-II, and C0 interacting with guest species He, Ne, and Ar are investigated, using density functional theory (DFT) with four different exchange-correlation functionals that include dispersion effects to various degrees: the non-local density-based optPBE-van der Waals (vdW) and rPW86-vdW2 functionals, the semi-empirical D2 atom pair correction, and the semi-local PBE functional. In the He-water system, the sequence of stable phases closely matches that seen in the hydrogen hydrates, a guest species of comparable size. In the Ne-water system, we predict a novel hydrate structure based on the C0 water network to be stable or at least competitive at relatively low pressure. In the Ar-water system, as expected, no filled-ice phases are stable; however, a partially occupied Ar-C0 hydrate structure is metastable with respect to the constituents. The ability of the different DFT functionals to describe the weak host-guest interactions is analysed and compared to coupled cluster results on gas phase systems.