Studies on Molecular Motionsand Phase Transition in Pyridinium Tetrachloroiodate(III)Crystals by the Measurements of the Temperature Dependences of Chlorine Nuclear Quadrupole. Resonance Frequencies and Relaxation Times

Abstract

Abstract To elucidate the phase transition of pyridinium tetrachloroiodate(III) at Tc = 217 K, the temperature dependences of the chlorine NQR frequencies and corresponding NQR spin-lattice and spin-spin relaxation times (T1Q and T2Q) have been measured in the range 77-370 K. Above ca. 370 K and between 206 and 223 K no chlorine NQR frequency could be observed, the chlorine relaxation times becoming extremely short (ca. 100 μs). Except for near Tc, the T1Q values below ca. 310 K can be explained by fluctuations of the electric field gradient at the chlorine nuclei due to in-plane reorientations of the cations about their pseudohexad axis. The increase of T1q -1 in the vicinity of Tc suggests a critical slowing down of the orientational fluctuations of the cations and a distortion of the complex anions. The decrease of T1Q above 310 K is considered to be mainly due to the in-plane reorientation of the complex anions. An activation energy of 64 kJ mol-1 is evaluated for that motion.