Study of the phase transition of a benzil single crystal by far infrared spectroscopy measurement

Abstract
Benzil exhibits a solid-solid phase transition at 83.5 K 1,2. For the first time infrared spectra have been recorded between 1.3 K and 300 K from 8 to 250 cm-1. Frequency dependance of the external modes versus temperature are reported. The soft mode of the high temperature phase decreases from 15 to 8 cm-1 for a sample temperature change from 300 K to 83.5 K. When phase transition occurs each mode of the trigonal form splits and the soft mode gives two components. When temperature decreases each of the two components of the soft mode splits and at 1.3 K we observe four modes.