Phonons and structures of tetracene polymorphs at low temperature and high pressure

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. Application of pressure above 1 GPa yields polymorph II, which is also obtained by cooling the sample below 140 K. However, the conditions for inducing the phase transitions depend on sample preparation and history, and polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. The temperature dependence of the spectra is described correctly. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.