Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit

Abstract

The lattice constants of hydrogenated and deuterated CH4-, CO2-, Xe- (clathrate structure type I) and N-2-hydrates (clathrate structure type II) from 10 K up to the stability limit were established in neutron-and synchrotron diffraction experiments and were used to derive the related thermal expansivities. The following results emerge from this analysis: (1) The differences of expansivities of structure type I and II hydrates are fairly small. (2) Despite the larger guest-size of CO2 as compared to methane, CO2-hydrate has the smaller lattice constants at low temperatures, which is ascribed to the larger attractive guest-host interaction of the CO2-water system. (3) The expansivity of CO2-hydrate is larger than for CH4-hydrate which leads to larger lattice constants for the former at temperatures above similar to 150 K; this is likely due to the higher motional degrees of freedom of the CO2 guest molecules. (4) The cage occupancies of Xe-and CO2-hydrates affect significantly the lattice constants. (5) Similar to ice Ih, the deuterated compounds have generally slightly larger lattice constants which can be ascribed to the somewhat weaker H-bonding. (6) Compared to ice Ih, the high temperature expansivities are about 50% larger; in contrast to ice Ih and the empty hydrate, there is no negative thermal expansion at low temperature. (7) A comparison of the experimental results with lattice dynamical work, with models based on an Einstein oscillator model, and results from inelastic neutron scattering suggest that the contribution of the guest atoms' vibrational energy to thermal expansion is important, most prominently for CO2-and Xe-hydrates. (C) 2016 AIP Publishing LLC.