THERMODYNAMIC PROPERTIES OF DEUTERATED HEXAMETHYLBENZENE AND OF ITS SOLID-SOLUTIONS WITH THE HYDROGENATED ANALOG - A LARGE ISOTOPE EFFECT ON THE PHASE-TRANSITION AT THE TEMPERATURE 117-K

The heat capacities of hexamethylbenzene-d18 (HMB-d18) were measured at temperatures T between 4 K and 300 K. The III-to-II transition occurs at T = 132.4 K and the molar entropy of transition is 13.1 J·K-1·mol-1, which is 1.41 times as large as that of HMB-h18. The heat capacities of the solid solutions with n(D)/n(H) = 1/9 and n(D)/n(H) = 1 were also measured between T = 14 K and 300 K. Thermodynamic properties of the transition change continuously with the composition. Crystal structures of HMB-h18, HMB-d18, and their solid solutions are the same in phase II and phase III. The large isotope effect in the entropy of transition can be attributed to differences in the torsional modes of the methyl groups. Hysteresis phenomena for HMB-d18 and HMB{n(D)/n(H) = 1} and the memory effect for HMB{n(D)/n(H) = 1 } in relation to the III-to-II transition were studied. The temperature dependence of the Raman spectrum is also reported. © 1992 Academic Press Limited All rights reserved.