CURIOUS PHASE-TRANSITION IN 4,4'-DICHLOROBENZOPHENONE AS STUDIED BY CL-35 NQR

A curious phase transition in the crystals of 4,4'-dichlorobenzophenone has been found to occur at 331 +/- 2 K by DTA and Cl-35 NQR measurements. A single NQR line observed at room temperature splits into two lines above 331 K. The frequency difference between these two lines (DELTA-nu) becomes large as temperature increases and the relation DELTA-nu is-proportional-to (T-320) holds in the temperature region up to the melting point (418 K). A quasi-continuous nature of the transition reflects on this behavior of DELTA-nu. The symmetry in the high-temperature phase is lower than that in the room-temperature phase. It is probable that the transition accompanies the symmetry lowering of the molecular structure in the high-temperature phase of the crystal.