ON THE PHASE TRANSITION IN SYM-TRIOXANE

Abstract

Polarized Raman spectra of sym-trioxane single crystals have been recorded in the Intra-and Intermolecular region. Three phonons were observed at room temperature at 59.6, 64.3, and $84.8 cm^{-1}$, belonging to E symmetry. Temperature dependent Raman Spectra were also recorded which showed the following important features; a) polarization data could not be obtained below Ca. 65K; b) the phonon at $85 cm^{-1}$ (300K) splits Into two modes at 63K and the splitting grows continuously to 1.6K, the lowest temperature reached in this study; c) the phonons at 60 ad $65 cm^{-1}$ (300K) show a constant-slope linear increase in energy from 300 to 65K at which point the slope changes considerably; and d) linewidths of these modes evidence discontinuous changes at 63K. Temperature dependence of the observed E phonon splitting follows approximately an equation of the form \begin{equation} n(T)/n(1.6K) = |(T_{c} - T)/T_{c}|^{1/4}. \end{equation} The phase transition is discussed in terms of various mechanisms and models which could be applicable in the absence of a distinct soft mode feature.