Structure determination and phase transition behaviour of mesitylene

The crystal structures of three low-temperature phases of perdeuteromesitylene, 1,3,5-trimethylbenzene (CD₃)₃C ₆D₃, have been determined using high-resolution neutron powder diffraction. The equilibrium phase I structure is cubic, in the space group P2₁3, with the molecule situated on a 3-fold symmetry axis. Phase II, obtained by freezing over-cooled liquid, is monoclinic, in the space group P2₁/c, and comprises one molecule in the asymmetric unit with disordered methyl groups. The phase III structure is triclinic and is reached through a displacive transition at ca. 100 K from the phase II structure. Complementary X-ray single crystal studies confirm the phase II crystal structure and also confirm a fourth and previously unreported phase.