Brillouin and Rayleigh scattering studies of the phase transition in chloranil

Abstract

The nonferroic (constant rotational symmetry) phase transition at ∼93 K in chloranil (C6Cl4O2) is investigated by Brillouin and Rayleigh light scattering. The phase transition is driven by a zone boundary soft mode at the (00 1/2) point which, through various coupling mechanisms, can affect the zone center strains. The major conclusions of these investigations are: (1) the LA-phonon mode propagating along the [010*] direction exhibits a step function anomaly at Tc with an associated transition region in the low temperature phase and a linewidth anomaly. The frequency of this LA mode in the transition region decreases below its low temperature phase value near Tc; (2) the lowest frequency QTA phonon propagating around [010*] has a negative temperature coefficient in the high temperature phase which goes to zero in the low temperature phase; (3) external stress induced relaxation processes can be observed by correlation scattering spectroscopy which are highly polarization, experimental scattering geometry, and stress direction dependent; (4) the relaxation time and intensity of scattered light behave anomalously in the vicinity of the phase transition; and (5) a similar intensity anomaly without a relaxation time anomaly is observed in the absence of external stress applied to the sample. These data are analyzed using a theory developed by Yao, Cummins, and Bruce for improper ferroelastic–ferroelectric phase transitions. The negative temperature slope of the QTA mode in the high temperature phase is thought to be associated with an incipient, unrealized phase instability driven by an optical mode at 16 cm−1 with similar temperature dependence.