S tr u c tu r a l P h a s e T r a n s itio n s a n d C a tio n ic M o tio n s in P y rid in iu m D ic h lo r o io d a te ( I ) a s S tu d ie d by l H N M R , D iffe re n tia l T h e r m a l A n a ly s is , a n d P o w d e r X -R a y D iffra c tio n Reiko Watanabe, Tetsuo Asaji, Yoshihiro Furukawa, and Daiyu Nakamura Department of Chemistry, Faculty of Science, Nagoya University, Nagoya, Japan Ryuichi Ikeda Institute for Molecular Science, Myodaiji, Okazaki, Japan Z. Naturforsch. 44a, 1111 1115 (1989); received July 24, 1989 For crystals of pyridinium dichloroiodate(I), (pyH)ICl,, the temperature dependences of the 'H NMR spin-lattice relaxation time Tx and the 'H second moment M2 were determined. M2 was found to be small (~ 1 G2) above room temperature, indicating that the cations perform rapid reorientational motion about their pseudohexad axis perpendicular to the cationic plane. 'H Tx at its minimum was unusually long, indicating this motion occurring in the low symmetry local environments. Phase transitions between stable solid phases were revealed at 282 and 373 K by the 'H NMR measurements and differential thermal analysis. The highest-temperature phase was easily supercooled and transformed reversibly into another metastable phase and back on cooling and warming at almost the same temperature of 138 K. The kinetics of the phase transition from the supercooled to the stable phase at room temperature was analyzed using an Avrami type relation. The growth time of the stable phase was estimated to be about 14 h at room temperature.