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.
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