Phase Transitions in Channel Clathrates of Thiourea with Hexachloroethane and Tetrachloroethane

An explanation is proposed for the nature of phase transitions in channel clathrates of thiourea with hexachloroethane and tetrachloroethane. Analysis of experimental temperature dependences of specific heat, entropy, and NQR spectra indicates the existence of first‐ and second‐order phase transitions. In the thiourea + hexachloroethane compound, the second‐order phase transition (94.60 K, entropy gain Rln2) implies a transformation of two equiprobable states (low‐temperature phase) into a single new state (high‐temperature phase). The first‐order phase transition corresponds to clathrate channel distortion. In the thiourea + tetrachloroethane compound, the second‐order phase transition (224K, entropy change Rln 6) occurs with transformation of six equiprobable states (low‐temperature phase) into a single new state (high‐temperature phase). The first‐order phase transition corresponds to clathrate channel distortion. The next second‐order phase transition observed in this compound at 248 K leads to disappearance of the four possible orientations of guest molecules in the clathrate channel of hexagonal section and to a transition of guest molecules to chaotic orientation (entropy increment equals Rln 4).