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The evidence for a metastable phase in norbornadiene: X-ray and quasi-elastic neutron scattering studies

The dynamics of the norbornadiene molecule is studied over the temperature range 190-265 K. Measurements below T=202 K and above T=254 K correspond to the ordered phase and to the liquid state, respectively. In the range 202-254 K, two plastic phases can exist, depending on the cooling conditions. On the timescale accessible to time-of-flight neutron spectroscopy (10-11-10-12 s), an elastic incoherent structure factor is extracted in the different phases. From its analysis, quasi-elastic spectra are described on the basis of a model associating 60 degrees jump rotations with a displacement of the centre of mass of the molecules. The same model holds for the two plastic phases, no difference in the dynamics of the molecules being evident. The temperature dependence of the broadening yields a unique Arrhenius law with a rather small value of the activation energy. In the liquid state, in addition to this rotational motion, a long-range diffusion by a jump mechanism was evidenced.