THERMODYNAMIC STUDY OF ICE AND CLATHRATE HYDRATES

Although glass transition phenomenon has been believed to occur in an undercooled liquid, evidence has accumulated in recent years to indicate that some orientationally disordered crystals also exhibit behavior which is phenomenologically similar to the glass. The freezing-in disordered states of orientational degree of freedom, keeping the translational symmetry with respect to the center of mass, are designated as glassy crystals. The existence of glassy crystals requires an extension of the concept of glass transition. Ice and clathrate hydrates turned out to belong to one category of glassy crystals in which the reorientational motions of molecules freeze out owing to prolonged relaxation time before the samples reach their hypothetical transition temperatures on cooling. Particular impurity incorporated into their lattices was found to relax locally the ice rules which severely constrain the reorientational motion in these hydrogen-bonded systems, This article reviews the discovery of orientational ordering processes in ice and clathrate hydrates doped with small amounts of alkali hydroxides. Both of them have similar geometrical arrangements of the hydrogen-bonded networks with orientational disorder of water molecules at high temperature. The ordering transition removes substantially the entropy corresponding to the disorder and thus confirms the validity of the third law of thermodynamics. The apparent deviation of these crystals from the law is merely due to a limited time of our laboratory experiment.