Phase transitions in the hydroquinone hydrogen sulfide clathrate compound

Heat capacities of [C₆H₄(OH)₂]₃·(H₂S)_x were measured between 1 and 15 K. Heat capacity peaks were found at (7.56±0.09) K, (7.61±0.05) K, and (7.65±0.07) K for the compounds withx=0.92, 0.95, and 0.96. A weak anomaly was observed around 6.75 K for the compound withx=0.85. The temperatures of these anomalies are unusually low among the phase transitions of molecular crystals. The decrease of the transition temperature from that of crystalline H₂S (=103.52 K) is a clear indication of the effect of enclathration on the molecular interaction. A comparison of the rotational heat capacity of the trapped hydrogen sulfide molecules with that of crystalline hydrogen sulfide shows that the trapped hydrogen sulfide molecules have a large rotational freedom at low temperatures (13 K). This agrees with the results from far infrared spectroscopic data. The dielectric constant of the clathrate compound obeyed the Curie-Weiss law above 30 K and no significant dielectric loss was found over the whole temperature range. These results showed that the trapped hydrogen sulfide molecules execute free rotation or are orientationally disordered above 20 K.Dedicated to Professor H. M. Powell.