Order-disorder phase transition in p-terphenyl and p-terphenyl:tetracene doped crystals as studied by Raman spectroscopy

Phase transition in crystalline pure p-terphenyl and p-terphenyl:tetracene doped crystals was studied with Raman spectroscopy, for temperatures from 295 to 10 K. In particular, the torsional Raman vibrational mode with a ''hard-core frequency'' of 230.8 cm(-1) was investigated in its frequency and bandwidth dependence upon temperature. The results were analyzed based on an order-disorder model allowing the determination of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and triclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, whereas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appears to affect the activation energy and the orientational correlation times in a different way in each phase.