Observation of an incommensurate phase in the stable phase sequence of deuterated thiophene by powder neutron diffraction

Deuterated thiophene has been studied by powder neutron diffraction spectroscopy from ∼ 60 K to melting temperature. Preliminary DSC measurements have shown that the thermodynamic behaviours of hydrogenated and deuterated thiophene are almost identical, so that the conclusions drawn from the present structural studies of C₄D₄S are valid for both compounds. The structures of stable phases IV and V have been observed for the first time: phase V lattice is obtained by multiplying by 2 parameter c of orthorhombic phase III and has been assigned space group P2₁ma; phase IV also corresponds to a superstructure of phase III, but, as expected because of the quasi-fivefold molecular symmetry and of its reorientational dynamics, it can be thought to be an incommensurate phase.