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 (ordered) phase is associated with a lowering of the symmetry. As a consequence, a ferroelastic and an antiphase domain structure coexist in the ordered phase. The formation of the ferroelastic domain structure in tetracene doped and undoped p-terphenyl was observed under polarization microscopy to occur in a narrow range around the phase transition temperature. The shape and size of domains were found not to depend on the guest concentration but on the crystal quality. On the other hand, experiments with differential scanning calorimetry show that even small guest concentrations (10−3 mol/mol) may decrease the phase transition temperature, indicating that tetracene guests favour conformational disorder in the triclinic phase.