Phase transitions in some clathrate compounds¹

Abstract

Low temperature properties of clathrate compounds of hydro-quinone and thiourea and of clathrate hydrates are discussed using experimental results from calorimetry, neutron scattering and nuclear resonance experiments. All of them undergo phase transitions of order-disorder type, involving orientational ordering of the guest molecules in the first two and ordering of both the guest and host molecules in the third. Even though the basic situation as to the motional freedom of the guest molecule in the cavity is the same in the three clathrate systems, the time scales of the reorientational motion range from 1 ps to more than hours even at a fixed temperature of 20 K. In the slowest case, the substance behaves as a glass. For clathrate hydrates, not only the host lattice determines the motion of the guest molecule, but the guest molecule controls ordering of the orientation of the water molecules in the host lattice.