Collapse Of An Ice Clathrate Under Pressure Observed Via Thermal Conductivity Measurements

摘要

Irreversible transformation of the tetrahydrofuran ice clathrate at ～ 130 K was studied by measuring thermal conductivity k with increase in pressure p. Initially, k increases slowly with p up to ～0.75 GPa where it levels off, is roughly constant up to ～0.95 GPa, then decreases up to ～1.05 GPa. Pressure collapses the clathrate structure, plausibly beginning with lattice distortion, and k increases at ～1.05 GPa in a sharp sigmoid-shape manner due to large densification until the transformation is complete at ～ 1.25 GPa. This is the opposite of that found for ice whose k decreases first slowly with increase in p and then rapidly in an inverted sigmoid-shape manner [O. Andersson and H. Suga, Phys. Rev. B 65, 140201 (2002)]. At 1.08 GPa and 131 K, κ increases with time t (s) according to ～exp(t/2945), which is also the opposite of the collapse of ice [G. P. Johari and O. Andersson, Phys. Rev. B 70, 184108 (2004)]. The difference in its behavior is attributed to strong phonon scattering from the tetrahydrofuran guest molecules, κ of the collapsed clathrate is ～30% less than that for the collapsed ice, which is comparable with the 25% lesser k of the tetrahydrofuran-water solution from k of water at ambient pressure. On depressurizing at 130 K, κ decreases progressively more rapidly and k of the collapsed state at ～0.3 GPa is slightly lower than that of the as-made clathrate, showing that its original structure is not recovered.