Calorimetric, dielectric, and neutron diffraction studies on phase transitions in ordinary and deuterated acetone crystals

Heat capacities of acetone (CH₃)₂CO and its deuterated analogue (CD₃)₂CO have been measured with an adiabatic calorimeter in the temperature range 13-300 K. A broad peak in the heat capacity attributable to a phase transition was observed at 127 K in (CH₃)₂CO and at 132 K in (CD₃)₂CO. The accompanying excess entropy ((CH₃)₂CO 2.04 J K^-1 mol^-1, (CD₃)₂CO 2.08 J K^-1 mol^-1) was small, suggesting that the transition is not of the order-disorder type. In both samples, the magnitude and shape of the heat capacity peaks were affected not by cooling rates but rather by the temperature to which the samples were cooled before the measurements. From these data, hysteresis loops (the extent of the transition vs temperature) were determined for the cooling and heating processes. The dielectric permittivity of (CH₃)₂CO has also been measured in the frequency range 100 Hz to 1 MHz over the temperature range 87-270 K. Both high- and low-temperature phases exhibit small permittivities and no dielectric dispersion, indicating that the dipole moments of the acetone molecules are ordered in both phases. A hysteresis effect quite similar to that in the calorimetric measurement was observed. The transition properties are similar to those of a martensitic transition more typically observed in metals. Preliminary neutron powder diffraction data have been recorded for (CD₃)₂CO as a function of temperature and confirm unusual structural behavior. Both high- and low-temperature structures are orthorhombic (at 5 K V_c = 1469.45 ų and at 140 K V_c = 1535.47 ų). There is no evidence to suggest a change of space group between the phases.