Phase Transitions in 4 , 4 ' - Dichlorobenzophenone as Studied by 3 SC 1

The temperature dependence of 35C1 NQR frequencies in 4,4'-dichlorobenzophenone was mea­ sured between 9.3 and 372 K by the pulse Fourier-transform method. Successive phase transitions were observed at 189 and 194 K. Concerning these phase transitions, the curious thermal hysteresis phenomenon found in a previous NQ R experiment was not reproduced in the present study. It also follows that NQ R indicates another phase transition around 220 K, although no thermal anomaly was detected there by DTA. Tentative explanations for these three phase transitions are presented in relation to the incommensurability between 189 and 220 K. In addition, a novel phase transition was found to occur at 331 K according to both DTA and 35C1 NQR. A single N Q R line observed at room temperature splits into two components above 331 K, suggesting that the symmetry above 331 K is lower than that at room temperature. This is the behavior of re-entrant phase transition, and it reveals the quasi-continuous nature.