Temperature effect on vibrational properties of crystalline p-quaterphenyl.

Vibrational properties of the crystalline p-quaterphenyl at low temperature are investigated by Raman scattering. The temperature dependent vibrational behaviors associated with the intra- and intermolecular terms are analyzed in detail. The order-disorder transition makes significant impacts on the energies, widths, and intensities of the vibrational modes, and the drastic splits of the modes mainly result from the sudden decline of the peak widths. Meanwhile, the intensity ratio of the 1280-cm and 1220-cm mode exhibits anomalous tendency around the transition temperature, as well as the frequency separations between them and between the higher energy mode and the lower energy mode around 1600 cm. Thus, they still can well indicate the molecular planarity in the low temperature conditions. However, the intensity ratio of the two modes around 1600 cm is no longer to be a good indicator of this transition due to the bare anomalous behavior around the transition temperature. Our work is of important significance for understanding the internal vibrational properties of p-quaterphenyl, and it also provides considerable supports for the further study of this material.