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:tetra cene doped crystals was studied with Raman spectroscopy, for temperatu res from 295 to 10 K. In particular, the torsional Raman vibrational m ode 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 determinat ion of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and tr iclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, wher eas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appea rs to affect the activation energy and the orientational correlation t imes in a different way in each phase. (C) 1997 Elsevier Science B.V.