DOPING INFLUENCE ON THE DOMAIN-STRUCTURE IN P-TERPHENYL CRYSTALS - MICROSCOPY AND CALORIMETRY STUDY

The order-disorder phase transition of p-terphenyl from the monoclinic high-temperature (disordered) to the triclinic low-temperature (order ed) phase is associated with a lowering of the symmetry. As a conseque nce, a ferroelastic and an antiphase domain structure coexist in the o rdered phase. The formation of the ferroelastic domain structure in te tracene doped and undoped p-terphenyl was observed under polarization microscopy to occur in a narrow range around the phase transition temp erature. The shape and size of domains were found not to depend on the guest concentration but on the crystal quality. On the other hand, ex periments with differential scanning calorimetry show that even small guest concentrations (10(-3) mol/mol) may decrease the phase transitio n temperature, indicating that tetracene guests favour conformational disorder in the triclinic phase. (C) 1997 Elsevier Science B.V.