Lattice constants and thermal expansion coefficient of air clathrate hydrate in deep ice cores from Vostok, Antarctica

We measured the temperature-dependent lattice constant of air hydrates in d eep ice cores from Vostok, Antarctica, using single-crystal X-ray diffracti on. Thus the thermal expansion coefficient was determined, the first for a structure II clathrate hydrate containing small guest molecules. Despite th eir high dissociation pressures, measurements were done at atmospheric pres sure because the air hydrates had stabilized during their original residenc e at high pressure. At 200 K, the lattice constant and thermal expansion co efficient were 17.20 Angstrom and 44 (+/-9) x 10(-6) K-1, respectively. Thi s thermal expansion coefficient is approximately equal to that from tetrahy drofuran hydrate, another structure II hydrate, but the lattice constant is 0.3% smaller. Crystal structure refinement determined that the total cage occupancy by air molecules was 0.9.