Polymorphism in $\alpha$-sexithiophene crystals: Relative stability and   transition path
  We present a joint theoretical and experimental study to investigate polymorphism in $\alpha$-sexithiophene (6T) crystals. By means of density-functional theory calculations, we clarify that the low-temperature phase is favorable over the high-temperature one, with higher relative stability by about 50 meV/molecule. This result is in agreement with our thermal desorption measurements. We also propose a transition path between the high- and low-temperature 6T polymorphs, estimating an upper bound for the energy barrier of about 1 eV/molecule. The analysis of the electronic properties of the investigated 6T crystal structures complements our study. 
Bernhard Klett, Caterina Cocchi, Linus Pithan, Stefan Kowarik, Claudia   Draxl
cond-mat.mtrl-sci