Type II clathrate hydrates (CHs) were studied by thermal and dielectric measurements. All CHs amorphize, or collapse, on pressurization to 1.3 GPa below 135 K. After heating to 160 K at 1 GPa, the stability of the amorphous states increases in a process similar to the gradual high density to very high density amorphous ice (HDA to VHDA) transition. On a subsequent pressure decrease, the amorphized CHs expand partly irreversibly similar to the gradual VHDA to expanded HDA ice transformation. After further heating at 1 GPa, weak transition features appear near the HDA to low density amorphous ice transition. The results suggest that CH nucleation sites vanish on heating to 160 K at 1 GPa and that a sluggish partial phase-separation process commences on further heating. The collapsed CHs show two glass transitions (GTs), GT1 and GT2. GT1 is weakly pressure-dependent, 12 K GPa−1, with a relaxation time of 0.3 s at 140 K and 1 GPa; it is associated with a weak heat capacity increase of 3.7 J H2O-mol−1 K−1 in a 18 K range and an activation energy of only 38 kJ mol−1 at 1 GPa. The corresponding temperature of GT2 is 159 K at 0.4 GPa with a pressure dependence of 36 K GPa−1; it shows 5.5 times larger heat capacity increase and 4 times higher activation energy than GT1. GT1 is observed also in HDA and VHDA, whereas GT2 occurs just above the crystallization temperature of expanded HDA and only within its ∼0.2–0.7 GPa stable pressure range.
To approximately calculate the density of the ultimately densified CHs at 1 GPa, we use experimental results39,40 and an estimate that the compressibility of a CH is ∼1.15 times that of ice Ih.44 The density of ice Ih increases ∼10% on pressurization from 1 bar to 1 GPa at 125 K43 by a short extrapolation from the transition pressure of 0.85 GPa, which yields an ∼12% density increase for the CH. The density of CHs at 130 K and 1 bar is approximately 1.00 g cm−3, which is calculated from the cell constant of ∼17.15 Å.41,42 Accounting for the density increase at the collapse (19% at 77 K10) and the subsequent densification on heating, which is observed for both ice Ih5 and CHs,38 the density of the ultimately densified collapsed CHs would be ∼1.4 g cm−3 at 1 GPa with an uncertainty of approximately 5%. (If the density of the collapsed CHs would be 7%–8% higher than collapsed, and ultimately densified, ice Ih, or VHDA, as for the crystalline phases at 1 atm, then the density would be 1.45 g cm−3.)
The transition coordinates were taken from the temperature and pressure where the change in the thermal conductivity exceeded 5% of the total change during the transition.