The Raman spectrum and phase transition in s y m-triazine

Abstract

The complete Raman spectrum of polycrystallines y m‐triazine has been observed at different low temperatures. The Raman active molecular vibrations have been assigned on the basis of the published normal coordinate analysis of the free molecule, while the external Raman active vibrations at 293 °K have been assigned in terms of molecular librations and translations, based on a model lattice calculation of frequencies and Raman intensities. Accurate temperature dependent Raman measurements of the splitting of the internal mode at 1550 cm−1 coupled with differential thermal calorimetric observations confirm the existence of a phase transition of second order character at T 0=196±1 °K. The transition proceeds continuously below T 0 as shown by sizable shifts of certain phonon frequencies with temperature. The temperature dependences are consistent with the transition displacements proposed by Coppens and Sabine, but do not show the existence of a soft Cochran type phonon.