RAMAN STUDY OF THE THREE SOLID-STATE PHASE-TRANSITIONS IN DIAMANTANE

A Raman study of the three-solid-state phase transitions in diamantane is presented. The low-temperature transition at Tc1=35K is characterised by a zone-centre soft librational mode; this mode has been fitted to a damped harmonic oscillator function, and the damping and static susceptibility of the soft mode obtained as a function of temperature. These results indicate that this transition is almost pure second order, a particularly rare example of such an event in a molecular crystal. The behaviour of a lattice mode at approximately 50 cm-1 which splits into two at Tc1 indicates a change in the space group. At Tc2=407K, the Raman-active lattice modes are replaced by a broad, overdamped mode which softens rapidly as the plastic phase transition at Tc3=440K is approached from below; the plastic phase gives the characteristic spectrum associated with rapid, isotropic molecular re-orientation.