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

Abstract

In a combination of neutron and synchrotron diffraction the lattice constants and expansivities of hydrogenated and deuterated CH4-, CO2-, Xe- (structure type I) and N2-hydrate (structure type II) from 10 K up to the stability limit under pressure were established. Some important results emerge from our analysis: (1) Despite the larger guest-size of CO2 as compared to methane, CO2- hydrate has the smaller lattice constants at low temperatures which we ascribe to the larger attractive guest-host interaction of the CO2-water system. (2) The expansivity of CO2-hydrate is larger than for CH4-hydrate which leads to larger lattice constants for the former at temperatures above ~ 150 K; this is likely due to the higher motional degrees of freedom of the CO2 guest molecules. (3) The cage filling does not affect significantly the lattice constants in CH4- and CO2-hydrate in contrast to Xe-hydrate for which the effect is quantitatively established. (4) Similar to ice Ih, the deuterated compounds have slightly larger lattice constants for all investigated systems which can be ascribed to the somewhat weaker H-bonding; the isotopic difference is smallest for the Xesystem,in which the large Xe atoms lead to an increase of averaged H-bond distances. (5) Compared to ice Ih the high temperature expansivities are about 50% larger; in contrast to ice Ih, there is no negative thermal expansion at low temperature.