THE PHASE-TRANSITION IN 2-METHYL-1,3-CYCLOHEXANEDIONE CRYSTAL

The phase transition in 2-methyl-1,3-cyclohexanedione crystal was inve stigated by Brillouin scattering, nuclear magnetic resonance (NMR), ca lorimetry and dielectric permittivity measurements. Thermal anomalies related to the second-order phase transition were observed at T-c = 24 3 K. The transition entropy was found to be Delta S = 5.7 J mol(-1) K- 1. NMR results are interpreted assuming two models of molecular reorie ntation, whereas dielectric studies indicate an activation process aro und T-c. The elastic constants measured in the Brillouin scattering ex periment have been related to the structure; its temperature dependenc e confirms that the phase transition has slow dynamics like in an orde r-disorder transformation without any softening of modes. Considerable differences can be noted between the phase transitions and molecular dynamics in the crystals of 2-methyl-1,3-cyclohexanedione and the anal ogous 1,3-cyclohexanedione.