The solid state vibrational spectra of <i>s</i>‐triazine and <i>s</i>‐triazine‐<i>d</i>₃ and the monoclinic to rhombohedral phase transition

Abstract

Abstract Infrared and Raman spectra of polycrystalline s ‐triazine and s ‐triazine‐ d 3 have been recorded over a wide range of temperatures. The spectra of the high temperature phase are in agreement with predictions based on the reported D 3 d 6 rhombohedral crystal structure. The spectra of the low temperature monoclinic phase are compatible with a unit cell of symmetry C 2h with two molecules in C s or C 2 sites. Possible space groups are C 2 h i ; i =1,2,3,4, or 6. The higher order nature of the rhombohedral to monoclinic phase transition is indicated by the gradual increase in splitting of degenerate modes in the infrared and Raman spectra of both C 3 N 3 H 3 and C 3 N 3 D 3 as the temperature was lowered. From studies of these splittings up to the phase change, the transition temperature was found to be 200±5K for C 3 N 3 H 3 and 190±5K for C 3 N 3 D 3 .