  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. A pressure induced phase transition, observed above 1 GPa, leads to
polymorph II, which is also obtained at temperatures below 140 K. 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. All
published x-ray structures, once assigned to the appropriate polymorph, are
also reproduced.
