THERMAL HYSTERESIS IN 4,4' DICHLOROBENZOPHENONE STUDIED BY CL-35 NQR

A pulsed NOR study of 4,4' dichlorobenzophenone has been performed to get information about dynamic features in its three crystalline phases. We found in phases I (T > 190 K) and III (T < 180 K) a normal thermal behaviour of the NQR frequency and spin-lattice relaxation time. In the intermediate phase II (180 K < T < 190 K) the behaviour of the NQR parameters could not be explained with a simple Bayer's model of molecular torsional oscillations. This phase has been previously studied and is a nearly incommensurate one. Furthermore, a thermal hysteresis has been observed in both phase transition temperatures. This thermal hysteresis and the temperature range of the intermediate phase II showed a strong impurity concentration dependence.